Hans-Klaus Roth, Klaus Heinemann (Ed.) 5 International Symposium

Hans-Klaus Roth, Klaus Heinemann (Ed.)  5 International Symposium
Hans-Klaus Roth, Klaus Heinemann (Ed.)
5th International Symposium
Technologies for Polymer Electronics - TPE 12 -
Special Thanks go to Mrs. BIANCA KÄMMER.
She helped us again most conscientiously to cope with the wealth of Abstracts
and showed also remarkable patience in preparing this proceedings of TPE 12.
Proceedings
5th International Symposium Technologies for
Polymer Electronics
- TPE 12 Thuringian Institute of Textile and
Plastics Research, Rudolstadt
and
Ilmenau University of Technology
22. - 24. May 2012, Rudolstadt/Germany
ed. by
Hans-Klaus Roth, Klaus Heinemann
Universitätsverlag Ilmenau
2012
Impressum
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Technische Universität Ilmenau/Universitätsbibliothek
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www.tu-ilmenau.de/universitaetsverlag
Herstellung und Auslieferung
Thüringisches Institut für Textil- und Kunststoff-Forschung e.V.
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07407 Rudolstadt
www.titk.de
ISBN 978-3-86360-022-8 (Druckausgabe)
urn:nbn:de:gbv:ilm1-2012100098
Preface
The previous TPE Symposia in Rudolstadt took place in 2004, 2006, 2008 and 2010. On each
occasion, more than 100 participants from over 20 countries attended, giving many excellent
lectures and poster contributions that are still available on CD.
TPE 12 aims at bringing together researchers and developers who area active in the area of
organic electronics, with manufacturers, future users, and other interested parties. A special
focus will be placed on technology approaches for the large scale fabrication of polymer
electronic products, on recent material developments, and on emerging applications of plastic
electronics including Organic Photovoltaics (OPV).
The scientific programme is subdivided into the following sections:
I
II
III
IV
V
VI
VII
VIII
IX
X
Introductions
Materials and technologies 1
OFETs, Sensors and related
OLEDs and ECDs
Materials and technologies 2
Solar cells / OPV 1
Materials and technologies 3
Solar cells / OPV 2
Materials and technologies 4
Posters
The Symposium is organised by both the technology oriented polymer research institute
TITK Rudolstadt and the Ilmenau University of Technology (TU Ilmenau). The research
institute TITK is a self-supporting research institute at the TU Ilmenau. This University has a
long tradition of outstanding education in the technological sectors of electrical and
mechanical engineering, nanoelectronics, information science, all fields of modern
optoelectronics and others.
Regarding TPE 12 the abbreviation TITK stands for
T
I
T
K
Trends in R & D of polymer electronics and solar cells
Innovations in fabrication of organic or polymer solar cells (OPV), OLEDs,
OFETs, IPCs and polymer
Technologies for production of organic or plastic electronics
Know-how exchange and transfer
The symposium will be a forum of scientific discussion about corresponding actual results of
research and development as well as on application fields.
Hans-Klaus Roth
Symposium Chair
Symposium Board
International Advisory Committee
Prof. Dr. G. v. Assche (VU Brüssel, B)
Prof. Dr. R. Baumann (TU Chemnitz, D)
Prof. Dr. Ch. Brabec (Uni Erlangen, D)
Prof. Dr. V. Dyakonow (Uni Würzburg, D)
Prof. Dr. D. Fichou (UPMC Paris, F)
Prof. Dr. G. Gobsch (TU Ilmenau, D)
Prof. Dr. E. v. Hauff (Uni Freiburg, D)
Prof. Dr. G. Horowitz (Polytechnique, Palaiseau, F)
Prof. Dr. O. Inganäs (Uni Linköping, S)
Prof. Dr. G. E. Jabbour (Adv. PV. C., Uni Arizona, USA)
Prof. Dr. R. Janssen (TU Eindhoven, NL)
Dr. S. Kirchmeyer (Heraeus, Leverkusen, D)
Dr. Th. Kugler (CDT, Cambridge,UK)
Prof. Dr. D. Lupo (TU Tampere, FIN)
Dr. A. Lux (Novaled AG, Dresden, D)
Prof. Dr. N. Martin (Uni Madrid, E)
Dr. D. Müller (Merck, Darmstadt, D)
Prof. Dr. J.-M. Nunzi (Queen´s Uni, Ontario, CDN)
Prof. Dr. V. F. Razumov (ICP Chernogolovka, RUS)
Prof. Dr. H.-K. Roth (TITK, Rudolstadt, D)
Prof. Dr. N. S. Sariciftci (Uni Linz, A)
Prof. Dr. U. S. Schubert (Uni Jena, D)
Dr. M. Shkunov (Uni Surrey, UK)
Prof. Dr. H. Sirringhaus (Plastic Logic, Cambridge, UK)
Dr. B. Stadlober (Joanneum Research, Weiz, A)
Prof. Dr. L. Torsi (Uni Bari, I)
Prof. Dr. D. Vanderzande (Uni Hasselt, B)
Local Conference Committee
Dr. R.-U. Bauer (TITK, Rudolstadt)
Prof. Dr. G. Gobsch (TU Ilmenau)
Prof. Dr. K. Heinemann (TITK, Rudolstadt)
Dr. H. Hoppe (TU Ilmenau)
Mrs. B. Kämmer (TITK, Rudolstadt)
Prof. Dr. H.-K. Roth (TITK, Rudolstadt)
Prof. Dr. P. Scharff (TU Ilmenau)
Dr. S. Scheinert (TU Ilmenau)
Dr. M. Schrödner (TITK, Rudolstadt)
Dr. S. Sensfuss (TITK, Rudolstadt)
CONTENTS
Introductions
Hummelen, J. C.; Koster, L. J. A.; Shaheen, S.; Zou, W.; Pshenichnikov, M.; Groningen (NL)
Theoretical and experimental pathways to a new efficiency regime for molecular solar cells
1
Kirchmeyer, S.; Leverkusen (D)
Organic and printed electronics: status, opportunities and challenges
3
Nunzi, J.-M.; Liu, F.; Ontario (CDN)
Enhanced organic light emitting diode and organic solar cell performances by silver
nanoparticles
6
Materials and technologies 1
Inganäs, O.; Linköping (S)
Alternating copolymers, alternative device geometries and processing for polymer photovoltaics
9
Scherf, U.; Wuppertal (D)
Polymer synthesis as a key tool in the development of improved (opto) electronic materials
10
Balzer, F; Schiek, M.; Osadnik, A.; Lützen, A.; Rubahn, H.-G.; SØnderborg (DK)
Crystalline organic nanofibers
11
Rauh, J.; Würzburg (D)
Electronic trap states in organic polymer-fullerene solar cells
16
OFET´s, Sensors and related
Stadlober, B.; Scheipl, G.; Zirkl, M.; Sawatdee,A.; Helbig, U.; Krause, M.; Kraker, E.;
Andersson Ersman, P.; Nilsson, D.; Platt, D.; Bodö, P.; Bauer, S.; Domann, G.;
Mogessie, A.; Hartmann, P.; Weiz (A)
Smart system integration and all-printed active-matrix sensors
19
Scarpa, G.; München (D)
Organic biosensors based on biocompatible solution-processable materials
30
Georgakopoulos, S.; Sparrowe, D.; Meyer, F.; Shkunov, M.; Guildford (UK)
Polymer Schottky Barrier Transistors
31
Scheinert, S.; Hörselmann, I.; Ilmenau (D)
Cutoff frequency of organic field effect transistors: a simulation study
33
Paasch, G.; Scheinert, S.; Grobosch, M.; Hörselmann, I.; Knupfer, M.;
Bartsch, J.; Dresden (D)
The influence of hole injection barriers on organic field-effect transistors: connection
with photoemission dataa
37
Lupo, D.; Lilja, K.; Heljo, P.; Tuukkanen, S.; Li, M.; Tampere (FIN)
Printed diodes: physics and applications
42
Mateo-Alonso, A.; Freiburg (D)
Curved and Flat Aromatics: Multitask Components in Molecular Machines and
Electronic Materials
49
OLEDs and ECDs
Lüssem, B.; Dresden (D)
Highly Efficient Organic Light Emitting Diodes for Lighting Applications
58
May, C.; Mogck, S.; Dresden (D)
Roll-to-roll processing of flexible oled for lighting applications
65
Edman, L.; Umea (S)
Realizing novel and functional light-emitting electrochemical cells
72
Nazmutdinova, G.; Schache, H.; Schroedner, M.; Raabe, D.; Rudolstadt (D)
Multicolored electrochromic modules for ecd applications
74
Materials and technologies 2
Siebbeles, L. D. A.; Delft (NL)
Photogeneration and ultrafast dynamics of excitons and charges in polymer/
fullerene/quantum dot blend films
81
Sensfuss, S.; Schache, H.; Eisenhawer, B.; Andrae, G.; Pietsch, M.; Shokhovets, S.;
Himmerlich, M.; Klemm, E.; Kroll, M.; Pertsch, T.; Rudolstadt (D)
Polymer solar cells blended with silicon nanowires
84
Shkunov, M.; Opoku, C.; Guildford (UK)
Printed electronics based on solution processable nanowires
89
Fahlmann, M.; Braun, S.; Andersson, L. M.; Sehati, P.; Zhan, Y. Q.; de Jong, M. P.; Linköping (S) 95
Organic and hybrid organic heterojunctions in organic electronics and spintronics applications
Solar cells / OPV 1
Vanderzande, D.J.M.; van Mierlo, S.; Marini, L.; Verstappen, P.; Hadipour, A.; Spijkman, M.J.;
van den Brande, N.; Ruttens, B.; Kesters, J.; D´Haen, J.; van Assche, G.; De Leeuw, D. M.;
Aernouts, T.; Manca, J.; Lutsen, L.; Maes, W.; Hasselt (B)
Narrow bandgap copolymer derivatives based on 4H-cyclopenta(2,1-B; 3,4-B)dithiophene
units: synthesis and photovoltaic performance
Hoppe, H.; Ilmenau (D)
Imaging methods for quality control and degradation analysis of organic solar cells
98
104
Troshin, P. A.; Levchenkova, E. D.; et al; Chernogolovka (RUS)
Novel methods for controlling the quality and evaluation of the degradation profiles
of conjugated polymers designed for photovoltaic applications
105
Schrödner, M.; Blankenburg, L.; Schultheis, K.; Schache, H.;
Sensfuss, S.; Rudolstadt (D)
Highly efficient flexible plastic solar cells made by a reel-to-reel coating process
106
Materials and technologies 3
Halik, M.; Erlangen (D)
Interface engineering and electronic functionality via molecular self-assembly
113
Langa, F.; Caballero, R.; Aljarilla A.; Lopez-Arroyo, L.; Urbani, M.; Pelado, B.;
115
De La Cruz, P.; Toledo (E)
Functional Oligothienylenevinylene-based Materials for optoelectronics
Schmid, G.; Wemken, J.H.; Maltenberger, A.; Petrukhina, M. A.; Dobbertin, T.;
Jaeger, A.; Erlangen (D)
Inorganic and Organometallic Low Cost Dopants for Transport Layers in Organic
Electronic Devices
117
Irimia-Vladu, M.; Glowacki, E.; Voss, G.; Leonat, L.; Monkowius, U.; Schwabegger, G.;
Bozkurt, Z.; Sitter, H.; Bauer, S.; Sariciftci, N. S.; Linz (A)
Hydrogen-bonded indigoids and acridones: highly ordered semiconductors for high
performance organic electronics
119
Kolbusch, T.; Dormagen (D)
Production technologies for large area printed flexible electronics
121
Solar cells / OPV 2
Da Como, E.; Tautz, R.; Feldmann, J.; Scherf, U.; von Hauff, E.;
Structural correlations in the generation of polaron pairs in copolymers for photovoltaics
122
Presselt, M.; Herrmann, F.; Runge, E.; Shokhovets, S.; Hoppe, H.; Gobsch, G.;
Ilmenau (D)
124
Origin of Sub-Bandgap Absorption in P3HT: PCBM Solar Cells
Moons, E.; Anselmo, A. S.; Dzwilewski, A.; Rysz, J.; Budkowski, A.; Svensson, K.;
van Stam, J.; Karlstad (S)
Polymer Solar cells – Visualizing vertical phase separation in solution-processed films of
polymer fullerene blends
125
Materials and technologies 4
Perelaer, J.; Schubert, U. S.; Jena (D)
Low temperature sintering of inkjet printed silver tracks
130
von Hauff, E.; De Sio, A.; Tunc, A. V.; Da Como, E.; Parisi, J.; Freiburg (D)
Improving the photovoltaic performance of polymer based solar cells with molecular doping
132
Brabec, Ch. J.; Matt, G. J.; Bednorz, M.; Glowacki, E. D.; Scharber, M.;
Fromherz, T.; Erlangen (D)
Silicon/organic hybrid hetero–junction infrared photodetector operating in the telecom regime
134
Deibel, C.; Förtig, A.; Lorrmann, J.; Gorenflot, J.; Wagenpfahl, A.; Rauh, D.; Rauh, J.;
Dyakonov, V.; Würzburg (D)
Impact of trap-assisted recombination on the performance of polymer-fullerene bulk
heterojunction solar cells
135
Posters
Bartelt, A. F.; Strothkämper, Ch.; Eichberger, R.; Berlin (D)
Charge separation dynamics at ZnPc: C60 bulk hetero-junctions using time-resolved terahertz
spectroscopy
138
Bergqvist, J.; Arwin, H.; Inganäs, O.; Linköping (S)
In Situ Reflectance imaging of organic thin film formation from solution
143
Hörselmann, I.; Scheiner, S.; Ilmenau (D)
Multi-Frequency Transconductance Technique on OFET’s
146
Schiek, M.; Trautwein, N.; Osadnik, A.; Jensen, J.; Beverina, L.; Lützen, A.; Borchert, H.;
Parisi, J.; Balzer, F.; SǛnderborg (DK)
Different approaches for improving organic solar cells
151
Mukhacheva, O. A.; Troshin, P. A.; Goryachev, A. E.; Sariciftci, N. S.;
Egbe, D. A. M.; . Razumov, V. F.; Chernogolovka (RUS)
Photovoltaic performance of PPV-PPE copolymers: effect of the fullerene derivative
153
Van Assche, G.; Van den Brande, N.; Demir, F.; Bertho, S.; Vanderzande, D.;
Van Mele, B.; Brussels (B)
Advanced Calorimetry for annealing studies of organic photovoltaics
154
Susarova, D. K.; Goryachev, A. E.; Troshin, P. A.; Razumov, V. F.; Chernogolovka (RUS)
A comparative study of bisfunctionalized [60]fullerene derivatives as electron acceptor
materials for organic solar cells
158
Turkovic, V.; Engmann, S.; Gobsch, G.; Hoppe, H.; Ilmenau (D)
Influence of various stress types on the degradation of polymer/fullerene films
159
Engmann, S.; Turkovic, V.; Hoppe, H.; Gobsch, G.; Ilmenau (D)
Revealing bulk heterojunction blend morphology by spectroscopic ellipsometry
161
Rösch, R.; Eberhardt, K.-R.; Gobsch, G.; Hoppe, H.; Ilmenau (D)
Polymer solar cell lifetime: dependence on metal back electrode and encapsulation
162
Seeland, M.; Rösch, R.; Gobsch, G.; Hoppe, H.; Ilmenau (D)
Qualitative and Quantitative Characterization of Polymer Solar Cells by Laterally
Resolved Detection of Luminescence
163
Muhsin, B.; Herrmann, F.; Singh, C.-R.; Gobsch, G.; Presselt, M.;
Hoppe, H.; Ilmenau (D)
Influence of Organic Acids on Device Performance of P3HT : PCBM Solar Cells
164
Synooka, O.; Kretschmer, F.; Hager, M. D.; Schubert, U. S.; Gobsch, G.;
Harald, H.; Ilmenau (D)
Optimization of organic solar cells based on BTD/DPP copolymers
165
Part I:
Introductions
Theoretical and experimental pathways to a new
efficiency regime for molecular solar cells
L.J.A. Koster1, S. Shaheen2, W. Zou3, M. Pshenichnikov1, and J.C.Hummelen1,3*
1. Molecular Electronics, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG Groningen,
The Netherlands.
2. Dept. of Physics and Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, CO 802086900, USA
3. Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The
Netherlands. E-mail: [email protected]
In the first part, we present three different theoretical approaches to identify pathways to
organic solar cells with power conversion efficiencies in excess of 20%. We take off after reasoning
why the Shockley-Queisser limit is applicable to OPV. A radiation limit for organic solar cells is
introduced that elucidates the role of charge-transfer (CT) state absorption. Provided this CT action is
either sufficiently weak or present in its maximized form throughout the active layer material, organic
solar cells can be as efficient as their inorganic counterparts.
Next, a model based on Marcus theory of electronic transfer that also considers exciton
generation by both the electron donor and electron acceptor
is used to show how reduction of the reorganization
energies can lead to substantial efficiency gains.
Third, we introduce the dielectric constant as a
central parameter for efficient solar cells. We analyze how
the dielectric constant influences every fundamental step in
OPV. We analyze and model the case of the 2009 world
record PTB7:[70]PCBM cell of 7.4%, using a driftdiffusion model. Based on the model and based on the fact
that the exciton binding energy diminishes with increasing
dielectric constant of the medium, we find that efficiencies
of more than 20% are within reach upon increasing the
Fig1: (black line) K( PTB7:[70]PCBM) as function of
dielectric constant Hr of the material to 10 (Figure 1).
H; (blue line) exciton binding energy Eb as
(Adv. Energy Mater. 2012, in print)
function of ;(red line) K of OPV with Eb
optimized for each value of H.
A completely different approach towards high efficiency PV is based on photon management.
We report on our recent discovery of efficient broadband near-IR light up-conversion. Photon
upconversion of the (near)infrared (NIR) photons is a promising way to overpass the ShockleyQueisser limit that sets up the maximal efficiency of 32% to a single junction solar cell. However, the
practical applicability of the most efficient upconversion materials known to date at moderate light
intensities (i.e. below 1000 Suns) is limited by the extremely weak and narrow-band (N)IR absorption.
Here we, inspired by a natural design of photosynthetic complexes, introduce a novel concept of an
upconversion material in which an organic (N)IR dye is used as an antenna for ȕ-NaYF4:Yb,Er
nanoparticles (NPs) where the upconversion occurs. The overall upconversion efficiency of the dyesensitized NPs is dramatically enhanced (by a factor of ~3300; see Fig. 2) as a result of increased
absorptivity and overall broadening
of the absorption spectrum of the
upconverter. The proposed concept
can be readily extended to cover any
desired part of the solar spectrum by
applying a set of dye molecules with
overlapping absorption spectra acting
as an extremely broadband antenna
system, connected to a suitable
upconverter.
Fig 2. Upconversion action spectrum of IR806-coated ȕ-NaYF4:Yb,Er NPs (orange line).
Insets: cartoon of dye-coated NPs and
amplified action spectrum showing the
original NP upconversion.
1
BIOGRAPHIC DATA OF PROFESSOR KEES HUMMELEN
Kees (J.C.) Hummelen was born in Groningen, The Netherlands. He
received his MSc in Chemistry and a cum laude doctorate degree in
Science under the mentorship of Hans Wynberg at the University of
Groningen in 1979 and 1985, respectively. After four years of
playing jazz (piano) and art video production, he spent two years as
a post-doctoral fellow with Fred Wudl at UCSB. He was appointed
full Professor in Chemistry in 2000 in Groningen. At present, he is
the Scientific Director of the Stratingh Institute for Chemistry at the
University of Groningen. The last 15 years, his main research activities were in fullerene
chemistry and the development of organic photovoltaics. Other research topics are materials
for molecular field effect transistors and single-molecule electronics. The Web of Science
presently reports 173 papers of him with an average of 105 citations per paper.
In 2011, he was ranked as the #7 scientist in materials science worldwide by Thomson
Reuters.
2
ORGANIC AND PRINTED ELECTRONICS: STATUS,
OPPORTUNITIES AND CHALLENGES
S. Kirchmeyer*, Heraeus Precious Metals GmbH & Co. KG, Conductive Polymers Division,
Chempark, Building B 202, 51368 Leverkusen, Germany
Abstract
The technology of plastic electronics is believed to open a field of low-cost electronics with
new areas of application. Studies have predicted a multibillion dollar market which has
generated a significant attention in research and industry. Few commercial applications have
been realized, the largest sales are currently organic luminescent diodes with approximately 2
bn. US$ sales. Every technology passes through a cycle with a phase of initial high
expectation followed by a phase of disillusion until finally a state of productivity is reached.
At this point there is still belief that printed electronics have a high technological potential but
many voices emphasize the need to convert this technology into significant business.
When technology limitations become visible, the phase of disillusion is the most critical phase
of any emerging technology. The OE-A is an association which acts as a global strategic
platform to build a bridge between science, technology and applications in order to grow an
industry of printed electronics. Being aware of the so called hype cycles the OE-A strives to
facilitate the transformation of technology into a worldwide business. In this regard the OE-A
offers various tools among the most efficient being its roadmap and its demonstrator
activities.
Every two years the OE-A compiles into one document a technology roadmap with input
from its almost 200 members that compiles the technology status, requirements and the “Red
Brick Walls” of printed electronics for each main application. The OE-A roadmap has been
proved to be highly useful to guide for R+D activities and has worldwide a high reputation.
“Red Brick Walls” listed in the road map are defined as “basic limitations to enter mass
markets” and resemble technology challenges to be overcome for commercialization. Basic
challenges comprise the circuit design and system integration, the resolution, registration and
process stability of structuring processes and current limitations in of materials such as
semiconductors, conductors and barrier materials.
3
A totally different challenge is the question how potential end users can be attracted to apply
this technology. Potential end users such as companies in the retail, pharmaceutical or
packaging business without a background in electronics will need a clear and realistic
explanation of potentials and limits of printed electronics. On the other hand the development
of printed electronic devices is currently dominated by pushing this technology rather than by
a demand from the various potential markets. Demonstrators, meaning full working devices,
demonstrate function and capability of printed electronics to potential end users, generate an
understanding of potential and limitations, and facilitate a dialogue between the developers
and the users which may finally result in new realistic applications. On the other hand the
design and production of demonstrators require many partners along the value chain to
cooperate and therefore helps to build a technology network.
Among the numerous “give-away” demonstrators OE-A members have built are the solar
powered printed torch, a book with electronic function, solar power model airplanes, medical
and packaging devices - the latter originating from its student competition.
Some examples of a successful product commercialization will be discussed which reveal that
even very simple devices generate a number of technology challenges although they to be
appear quite unattractive as object for scientific studies. From these examples it can be
predicted that one of the next applications for printed electronics are touch screens made from
materials like conductive polymers that replace indium tin oxide in their function as a
transparent conductor.
4
BIOGRAPHIC DATA OF DR STEPHAN KIRCHMEYER
Stephan Kirchmeyer was born in 1957 in
Empelde near Hannover, Germany. From
1978 to 1984 he studied chemistry at the
University of Hamburg and at the University of
Southern California in Los Angeles. After his
thesis he went to San Jose/USA for a
postdoctoral fellowship with IBM.
He joined Bayer’s Central R&D 1988 and
Bayer’s silicone division in 1996. 1997 he transferred to the electronic chemicals
group to develop the conductive polymer PEDOT.
In 2002 the business was transferred to H.C. Starck GmbH, at that time an affiliate
company of Bayer. For H.C. Starck he worked subsequently as R&D manager, plant
site manager, global business manager and as a director for production and
technology. In 2010 the business of conductive polymers was sold to Heraeus. In
Heraeus he is currently the Head of the Business Unit Functional Coatings. In June
2011 he was elected as the chair of the board of the Organic Electronics Association
(OE-A).
5
ENHANCED ORGANIC LIGHT EMITTING DIODE AND SOLAR CELL
PERFORMANCES BY SILVER NANOPARTICLES
F. LIU 1; J.M. NUNZI* 1, 2
1
Department of Chemistry, Queen’s University, Kingston, ON
2
Department of Physics, Queen’s University, Kingston, ON
Surface plasmons are known as collective oscillations of the conduction electrons at a
metallic interface. The phenomenon has been extensively studied in the fields of surface
enhanced Raman spectroscopy, metal enhanced fluorescence, and non-linear optics.1-2
However, only limited research has been dedicated to its application in organic light emitting
diode (OLED). Here we have fabricated a plasmonic OLED by incorporating silica coated
silver nanoparticles (NPs) into the emitting layer of a phosphorescent organic light emitting
diode (PHOLED), as shown in the schematic diagram Fig. 1. As a result, the luminescence
efficiency of the PHOLED is significantly improved under low charge carrier injection level
due to surface plasmon enhanced exciton formation probability. In contrast, the incorporation
of uncoated bare silver NPs greatly suppresses luminescence of the PHOLED due to metal
NPs induced luminescence quenching. A silica shell with thickness 13 nm or above coated on
Ag NPs surface can avoid the luminescence quenching of the emitting molecules caused by
Ag NPs.
Fig. 1. Configuration of a PHOLED doped with silica coated Ag NPs
So far metal nanoparticles (NPs) have been used in thin film silicon solar cell research
successfully as the substitute of traditional inverted pyramid surface texture for light trapping.
6
However, the application of metal NPs in organic solar cell research is not as successful as
that in silicon solar cell. Although many works have been dedicated to organic solar cells
incorporating NPs, none explicitly isolates the optical function of NP from its electronic
function in organic solar cells due to the multiple roles played.
We designed a polymer solar cell with configuration shown in Fig. 2 from which we
can investigate solely the optical functions of metal NPs in the solar cell. The incorporation of
NPs underneath the active layer results in degraded performance due to massive light loss
caused by Ag NPs scattering and absorption. In contrast, the incorporation of Ag NPs above
active layers can harvest more sun light due to the fact that the surface plasmon of Ag NPs
enhances the extinction coefficient of the active polymer layer and the back-scattering from
Ag NPs improves the optical absorption path length in the active layer, thus solar cell power
conversion efficiency (PCE) is improved. However, a direct contact of Ag NPs with active
polymer results in exciton quenching, which compromises its enhancement effect on PCE.
The best position to incorporate Ag NPs into the solar cell appears to be above the active
layer, with a spacer layer, which is PEDOT in our prototype.
Fig. 2. Schematic diagram of an inverted plasmonic solar cell.
References
1. F. Liu, G. Aldea, and J. M. Nunzi, J. Lumin. 130, 56 (2010).
2. J. M. Nunzi and D. Ricard, Appl. Phys. B 35, 209 (1984).
7
BIOGRAPHIC DATA OF PROFESSOR JEAN-MICHEL NUNZI
Jean-Michel Nunzi is a Tier 1 Canada Research Chair, crossappointed with the department of Physics and the department of
Chemistry at Queen’s University since 2006. He spent his early
carrier at the Atomic Energy Commission in Saclay (France)
where he has been the head of the Organic Devices Group
during which he was the PI of the 1st European Project on
Organic Solar Cells in 1996.
He joined the University of
Angers in 2000, where he built a Technical Research Team on
plastic solar cells co-supported by the Ministry of Education
and Research and Total (France). He is the author of above 200 research papers and 11
patents. He is a member of the French network NANORGASOL and the Photovoltaic
Innovation Network in Canada.
8
Part II:
Materials and
technologies 1
ALTERNATING COPOLYMERS, ALTERNATIVE DEVICE
GEOMETRIES, AND PROCESSING FOR POLYMER
PHOTOVOLTAICS
Olle Inganäs
Biomolecular and organic electronics,
Center of Organic Electronics,
IFM, Linköpings Universitet
Routes towards high speed printing of photovoltaic donor/acceptor materials on flexible
supports are necessary to realize the cost advantages of polymer photovoltaics. Formation of
desired nanostructure from blends in solutions drying onto electrodes is necessary for optimal
photocurrents. Correlating the photocurrent generation, nanostructure formation and
processing when coating electrodes with active materials requires new tools for in situ
monitoring and for process control. These can be imaging methods with low or high
resolution, but also follow the formation of nanostructures through their emission signatures.
We report on concurrent development of optical metrology, processing and module
production using alternating copolymers.
9
POLYMER SYNTHESIS AS A KEY TOOL IN THE DEVELOPMENT
OF IMPROVED (OPTO)
ELECTRONIC MATERIALS
Ullrich Scherf
Bergische Universität Wuppertal, Macromolecular Chemistry Group (buwmakro) and Institut
für Polymertechnologie, Gauss-Str. 20, D-42097 Wuppertal, Germany, Fax:+49-202-4393880,
Tel: +49-202-4393871; E-mail: [email protected]
The lecture presents some recent synthesis driven examples towards systematic control on
morphology and (opto)electronic properties of conjugated oligomers and (co)polymers. The
examples include conjugated (co)polymers for application in organic solar cells, [1,2]
all-conjugated rod-rod block copolymers and their self-assembly in solvent mixtures and in
the bulk, [1,3-5] as well as hyperbranched, multichromophoric conjugated polymers.[6]
References
[1] J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger, G. C.
Bazan, J. Am. Chem. Soc. 2011, 133, 8416.
[2] P. Li, O. Fenwick, S. Yilmaz, D. Breusov, D. J. Caruana, S. Allard, U. Scherf, F. Cacialli,
Chem. Commun. 2011, 47, 8820.
[3] Tu, G., Li, H., Forster, M., Heiderhoff, R., Balk, L. J., Sigel, R., Scherf, U.; Small 2007, 3,
1001.
[4] U. Scherf, A. Gutacker, N. Koenen, Acc. Chem. Res. 2008, 41, 1086.
[5] A. Gutacker, S. Adamczyk, A. Helfer, L. E. Garner, R. C. Evans, S. M. Fonseca, M.
Knaapila, G. C. Bazan, H. D. Burrows, U. Scherf, J. Mater. Chem. 2010, 20, 1423.
[6] J.-M. Koenen, S. Jung, A. Patra, A. Helfer, U. Scherf, Adv. Mater. 2012, 24, 681, special
issue: International Symposium on Electronic/Optical Functional Molecules, Shanghai, 2012
(ISEOFM2012).
10
CRYSTALLINE ORGANIC NANOFIBERS
F. BALZER1*; M. SCHIEK1,2; A. OSADNIK2; A. LÜTZEN2; H.-G. RUBAHN1
1
NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, DK-6400
Sønderborg, Denmark
2
Kekulé-Institute of Organic Chemistry and Biochemistry, Rheinische Friedrich-WhilhelmsUniversity of Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany
Organic semiconductors from small molecules such as para-phenylenes, thiophenes, or
squaraines promise a vast application potential as the active ingredient in electric and
optoelectronic devices [1]. Their self-organization into – sometimes crystalline – organic
nanowires or “nanofibers” adds a peculiar attribute, making, e.g., nanolasers [2] or
waveguides [3] relatively straightforward to accomplish. Functionalization of the molecules
allows the customization of optical and electrical properties, allowing the easy formation of
highly effective frequency-doubling nanoaggregates [4] or the design of light-harvesting
devices such as photovoltaic cells.
(a)
10 μm
(e)
25 μm
(b)
(c)
10 μm
(f)
5 μm
(g)
20 μm
25 μm
(d)
10 μm
(h)
10 μm
Fig. 1: AFM images, (a)-(d), and optical microscope images, (e)-(g) fluorescence microscopy and (h) optical
phase contrast microscopy of PPTPP (a,e), PPTTPP (b,f), p-6P (c,g), and SQOH (d,h) deposited by organic
molecular beam deposition of mica.
Different methods for nanowire growth have been pursued in the past such as filling of
nanoporous templates, organic molecular beam deposition (OMBD) eventually leading to
epitaxial growth, and precipitation from solution. In Fig. 1 examples for such nanofibers
grown by OMBD are presented, both by atomic force microscopy (AFM) images, upper
panel, as well as by fluorescence and phase contrast optical microscopy (lower panel).
11
The phenylene-thiophene co-oligomers PPTPP and PPTTPP and para-hexaphenylene p-6P
form aggregates which are due to their strong fluorescence suitable for light-light generating
devices, whereas the non fluorescing but broadly light-absorbing hydroxyl-squarylium SQOH
is aimed to be part of a photovoltaic cell [5,6]. The different nanowire directions are
determined by a combination of epitaxial growth, substrate surface symmetry, and packing of
the molecules into fibers.
Molecule orientations within the nanofibers as well as with respect to substrate lattice
directions are determined via polarized (fluorescence) microscopy. From this conditions for
single-crystalline growth are easily identified. An example for that is shown in Fig. 2:
whereas p-6P fibers on muscovite are polycrystalline due a possible twinning, the PPTPP
fibers formed on KCl are single crystalline.
(a)
(b)
Fig. 2: Polarization microscope images of OMBD grown nanofibers from (a) p-6P on muscovite mica and (b)
from PPTPP on KCl. The color codes the polarization direction of the emitted fluorescence after UV excitation.
The p-6P fibers on muscovite mica are clearly polycrystalline, whereas PPTPP fibers on KCl are single
crystalline.
The morphological stability of such nanowires with time, in the presence of various gases,
and under thermal load is of major importance for their use in any device. Simple aging
experiments under ambient conditions by atomic force microscopy already reveal substantial
morphological changes by Ostwald ripening due to water vapor. Thermal annealing of
nanowire samples leads to even more pronounced morphology changes, such as a strong
decrease in nanowire number density, a strong increase in nanowire height, and the formation
of new types of crystallites. All these experiments emphasize the need of encapsulation for
any device application.
Finally, the electrostatic properties of nanofiber samples probed by Kelvin Probe Force
Microscopy (KPFM) are discussed. For typical phenylene or thiophene based nanofibers the
contact potential difference (CPD) is strongly related to the sample morphology.
12
However, for squaraine nanofibers with their high photoconductivity the CPD on the surface
is much more delocalized. Illuminating the SQOH nanofibers with visible light from the
microscope lamp leads to a strong increase in CPD by several hundred Millivolts, hinting to a
strong photovoltage generation on the surface and being of major importance for lightharvesting devices.
[1] M. Schiek, F. Balzer, K. Al-Shamery, J. Brewer, A. Lützen, H.-G. Rubahn. Organic
Molecular Technology. Small 4 (2008) 176.
[2] F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer, H.-G. Rubahn. One
Dimensional Random Lasing in a Single Organic Nanofiber. J. Phys. Chem. B 109 (2005)
21690.
[3] F. Balzer, V. Bordo, A. Simonsen, H.-G. Rubahn. Optical Waveguiding in Individual
Nanometer-Scale Organic Fibers. Phys. Rev. B 67 (2003) 115408.
[4] J. Brewer, M. Schiek, H.-G. Rubahn. Nonlinear optical properties of CNHP4 nanofibers:
Molecular dipole orientations and two photon cross-sections. Opt. Commun. 283 (2010)
1514.
[5] F. Balzer, M. Schiek, A. Lützen, H.-G. Rubahn. Self organized growth of organic
thiophene-phenylene nanowires on silicate surfaces. Chem. Mater. 21 (2009) 4759.
[6] F. Balzer, M. Schiek, A. Osadnik, A. Lützen, H.-G. Rubahn. Organic Nanofibers from
Squarylium Dyes. Proc. SPIE 8258 (2012) 82580O.
13
CURRICULUM VITAE DR HABIL. FRANK BALZER
born:
05.08.1966 in Biedenkopf, Germany
address:
University of Southern Denmark,
Mads Clausen Institute, NanoSyd, Alsion 2,
DK-6400 Sønderborg, Denmark
e-mail:
[email protected]
Education
01/2009
Habilitation in Experimental Physics at Humboldt-University Berlin,
Germany. Topic: “Organic Nanoaggregates”
02/1998
Ph.D. in physics, University of Göttingen, Germany. Topic: “Linear and
nonlinear optics at alkali island films”
02/1994
Diploma in physics, University of Göttingen, Germany. Topic: “Time
resolved investigations of laser induced processes near surfaces”
Employment
Since 05/2007
Associate Professor, Mads Clausen Institute, NanoSyd, University of
Southern Denmark, Sønderborg, Denmark
08/2006 – 04/2007 Fellow of the "Hanse Institute for Advanced Study", Delmenhorst,
Germany
08/2001 – 08/2006 Research assistant, Physics Department, Humboldt-University Berlin,
Germany
06/2000 – 07/2001 Gaesteforsker, Physics Department, University of Southern Denmark,
Odense, Denmark
06/1998 – 05/2000 Postdoctoral research associate, Chemistry Department, Stanford
University, Stanford, CA, USA
03/1994 – 05/1998 Research assistant, MPI für Strömungsforschung, Göttingen, Germany
Research Experience
Since 2007
2001 – 2006
2000 – 2001
self-organized growth of nanowires from organic semiconductors,
(polarized) optical properties of organic nanoaggregates; scanning probe
microscopy; metallic nanoparticles in food; graphene as electrode
material for organic electronics
ion scattering from metallic alloys, AFM and spectroscopy on organic
nanowires, Forschergruppe 463 – “Innovative pharmaceuticals and
carrier systems”
organic nanowires on dielectric surfaces
14
1998 – 2000
1994 – 1998
1988 – 1994
molecule surface scattering, resonance enhanced multiphoton ionization,
catalytic reactions on surfaces
linear and nonlinear optics of alkali clusters, laser materials treatment,
photodesorption, self assembled monolayers
study of physics, University of Göttingen
Publications (h-index: 18)
47 peer-reviewed papers, 24 conference proceedings, 2 chapters in monographs, 1 book, and
2 patent applications.
15
ELECTRONIC TRAP STATES IN ORGANIC POLYMER-FULLERENE
SOLAR CELLS
J. RAUH*1
1
Experimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians University
of Würzburg, Am Hubland, 97074 Würzburg, Germany
Trap states can have a significant influence on the performance of organic solar cells, as they
lower the mobility, disturb the internal field distribution and affect the recombination
dynamics. We investigated the trap states in the polymer poly(3-hexylthiophene) (P3HT) as
well as in fullerene derivatives commonly used as electron acceptors in organic bulkheterojunction solar cells, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester),
PC71BM and bisPC61BM, by thermally stimulated current measurements. This technique is
based on optical trap filling at very low temperatures and subsequent heating of the sample,
resulting in a thermal release of the trapped charge carriers, which is detected as a current
flow. This current yields information about the activation energies and the lower limit of the
trap density. Hereby, broad quasi-continuous trap distributions and trap densities in the order
of 1016 cm-3 were revealed for all investigated materials (Figure 1).[1–3] Furthermore, PC71BM
and bisPC61BM exhibited significantly deeper traps compared to PC61BM.[2] This can be
explained by the fact that PC71BM and bisPC61BM are isomeric mixtures, with the different
isomers yielding different LUMO energies, where the lowest can act as trap states.
15
-3
trap density [10 cm ]
6
P3HT:PC61BM
PC61BM
P3HT
5
4
3
2
1
0
100
200
300
400
activation energy [meV]
Figure 1: Trap distributions of P3HT, PC61BM and their blend, as obtained by thermally
stimulated current measurements. All samples exhibit broad trap distributions.
In addition to the pure materials also the trap states in polymer-fullerene blends were studied,
revealing that the traps in the blend are a superposition of those in the neat materials and
16
additional deeper trap states.[3] The ratio of these deeper traps strongly depends on the
preparation conditions of the solar cells, i.e. with or without a thermal annealing step, and
thus on the morphology of the solar cells.
Our investigations are complemented by current-based deep level transient spectroscopy,
yielding additional information about the emission rates of the traps. These measurements
reveal that even trap states with similar activation energies exhibit emission rates that can
differ more than one order of magnitude. These findings are of fundamental importance to
describe charge carrier dynamics, e.g. in transient experiments.
[1] J. Schafferhans, A. Baumann, C. Deibel and V. Dyakonov, Trap distribution and the
impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene), Appl.
Phys. Lett. 93, 093303, (2008)
[2] J. Schafferhans, C. Deibel and V. Dyakonov, Electronic Trap States in Methanofullerenes,
Adv. Energy Mater. 1, 655, (2011)
[3] J. Schafferhans, A. Baumann, A. Wagenpfahl, C. Deibel and V. Dyakonov, Oxygen
doping of P3HT:PCBM blends: Influence on trap states, charge carrier mobility and solar
cell performance, Org. Electron. 11, 1693, (2010)
17
BIOGRAPHIC DATA OF DR JULIA RAUH
Personal Information
Name:
Work Address:
Julia Rauh, née Schafferhans
Julius-Maximilians-University of
Würzburg
Experimental Physics VI
Am Hubland, D-97074 Würzburg
Academic Education
10/2002–09/2004
10/2004–03/2005
04/2005–11/2007
01/2008–06/2011
Physik (Diplom): Vordiplom
University of Bayreuth, Germany
Physik (Diplom)
University of Hamburg, Germany
Physik (Diplom):
Julius-Maximilians-University of Würzburg, Germany
Thesis: Untersuchung elektronischer Störstellen in
ungeordneten organischen Halbleitern
PHD in Natural Sciences (Physics)
Chair of Experimental Physics VI (Energy Research),
Prof. Vladimir Dyakonov
Julius-Maximilians-University of Würzburg, Germany
Thesis: Investigation of defect states in organic semiconductors:
Towards long term stable materials for organic photovoltaics
Academic Career
since 07/2011
Group Leader
Chair of Experimental Physics VI (Energy Research),
Prof. Vladimir Dyakonov
Julius-Maximilians-University of Würzburg, Germany
Scholarships/Awards
05/2008–12/2010
12/2008
05/2009
12/2009
Postgraduate scholarship (Graduiertenstipendium nach dem
Bayrischen Eliteförderungsgesetz)
Wilhelm-Conrad-Röntgen Student Award
Zonta Award
Nominee: Best-Poster-Award (MRS Fall Meeting)
18
Part III:
OFETs, Sensors
and related
SMART SYSTEM INTEGRATION AND ALL-PRINTED ACTIVE
MATRIX SENSORS
G. Scheipl1, M. Zirkl1, A. Sawatdee2, U. Helbig3, M. Krause4, E. Kraker1, P.
Andersson Ersman2, D. Nilsson2, D. Platt2, P. Bodö2, S. Bauer4, G.
Domann3, A. Mogessie5, P. Hartmann1, and B. Stadlober1*
1
Institute of Surface Technologies and Photonics, Franz-Pichler-Strasse 30, 8160 Weiz,
Austria
2
Acreo AB, Box 787, 60117 Norrköping, Sweden
3
Fraunhofer-Department for Silica-Research ISC, Neunerplatz 2, 97082 Würzburg, Germany
4
Soft Matter Physics, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria
5
Department of Earth Science, Universitätsplatz 2, 8010 Graz, Austria
ABSTRACT
In organic and large-area electronics smart sensors for detection of physical forces are very
promising with respect to novel user interface applications. Recently, there have been several
reports on different types of active matrix sensor networks, however in most of these
components the large number of process steps, substrates and materials involved make their
fabrication complex, costly and hard to control, with expectedly small yield. Hence,
developing an active-matrix sensor technology that is fabricated exclusively by printing and
uses as few materials as possible constitutes an advance towards low-cost and large-area
sensing films. Here we demonstrate the printing of a complex smart integrated sensor system
using only five functional inks: the fluoropolymer P(VDF-TrFE) (Poly(vinylidene fluoride
trifluoroethylene) sensor ink, the conductive polymer PEDOT:PSS (poly(3,4ethylenedioxythiophene):poly(styrene sulfonic acid) ink, a conductive carbon paste, a
polymeric electrolyte and SU8 for separation. The result is a touchless human-machine
interface, including piezo- and pyroelectric sensor pixels (sensitive to pressure changes and
impinging infrared light), transistors for impedance matching and signal conditioning, and an
electrochromic display. Applications may not only emerge in human-machine interfaces, but
also in transient temperature or pressure sensing used in safety technology, in artificial skins
and in disposable sensor labels.
Keywords: HMI, P(VDF:TrFE), PEDOT:PSS, touchless, piezo- and pyroelectric sensor, organic transistors,
electrochromic display (ECD)
1. INTRODUCTION
Today, printed electronics promise to radically change our means of fabricating electrical
devices to the direction of low-cost, large-volume, and sustainable production on flexible or
elastic substrates.[1] This increasingly allows applications of electronics “anywhere, not just
everywhere”.[2] Simple printed electronic devices, such as antennas,[3] printed conductive
films,[4] printed memories, [5] batteries,[6] and solar cells,[7] have already been launched on the
market or are close to being launched. Admittedly, a real market penetration of printed
electronic products has not yet taken place mainly due to drawbacks in performance, cost, and
manufacturability.
19
That applies in particular to more complex printed components combining different classes of
devices, such as printed radio frequency identification (RFID) tags,[4] smart labels and
cards,[4] smart packaging,[8] or any other type of smart integrated system. Smart sensors are
very promising with respect to potential application scenarios for detection of physical forces.
Recently, there have been several reports on different types of active matrix sensor networks
based on, for example, an integration of pressure and thermal sensors with organic thin-film
transistors (OTFTs) for an artificial skin system,[9] a combination of a fluoropolymer-based
ultrasonic sensor with OTFTs,[10] large-area image sensors based on organic photodiodes and
OTFTs,[10] and a strain sensor array combining OTFTs and polymer sensors. Although these
devices open up new fields of applications in large-area electronics, the numerous processing
steps, substrates, and materials involved make their fabrication complex, costly, and hard to
control, with expectedly small yield. Hence, developing an active matrix sensor technology
that is fabricated exclusively by printing as few materials as possible constitutes an advance
towards low-cost and large-area sensing films.
P(VDF-TrFE) copolymers have become very attractive as functional materials for high-tech
applications due to a number of excellent inherent physical properties. Apart from the usage
as high-k gate dielectrics in logic gates based on miniaturized organic thin film transistors, a
remnant polarization charge of more than 100 mC/m2 qualifies these copolymers as charge
storage dielectrics in non-volatile memory elements,[12] and high piezo- and pyroelectric
coefficients (up to 40 ȝC/Km2) [13] make them attractive for sensor- and transducer based
organic devices.
The basic Ferroelectric-Active-Matrix-Sensor (FAMS) device is comprised of two
components: (1) a low voltage organic thin film transistor (OTFT) or an electrochemical
transistor (ECT) which is driven by (2) a temperature and/or pressure sensitive pyro- and/or
piezoelectric thin film capacitor based on poly[(vinylidene fluoride-co-trifluoroethylene]
(PVDF-TrFE). By locally changing the temperature or pressure on the active sensor area, an
electrical charge is generated at the sensors electrodes, resulting in a voltage signal that is
used to drive the gate of an OTFT or ECT, respectively. The OTFT/ECT is coupled to the
sensor area by connecting one sensors electrode directly to the gate of the transistor. The
temperature or pressure-dependent gate voltage is then transformed to a temperature or
pressure-modulated drain current signal which is further processed by data read-out
components in central electronic units. Depending on the application, such integrated sensor
elements can be arranged in arrays on different flexible substrates that can easily be attached
to arbitrarily curved surfaces.
2. EXPERIMENTAL
2.1 Fabrication of Lab Scale Devices
The consecutively described shadow-mask based lab scale devices have been the preliminary
stage of the fully printed integrated sensor devices. The design of the lab-scale sensor is very
similar to that of the fully printed ones, with a 3×6 array of circular spot forming the sensor
pixels and organic thin film transistors connected to each column and row. As a substrate a
PET foil with the size of 75×25 mm – thus fitting the sample holder for the shadow mask
process – was used. In the first step an 80 nm film of aluminum was applied by means of
electron beam (e-beam) evaporation via a shadow mask onto the PET substrate, serving as the
bottom electrode of the sensor.
20
Figure 1: Process flow for the fabrication of lab-scale integrated ferroelectric polymer sensor arrays based on
physical vapor-phase deposition (PVD), spin coating and patterning by shadow masks. (a) e-beam evaporation
of aluminum to form the bottom electrodes of the sensor, (b) spin coating of P(VDF:TrFE) film to form the
ferroelectric dielectric of the sensor, (c) e-beam evaporation of aluminum to form the top electrodes of the
sensor and painting of carbon absorption layers. (d) e-beam evaporation of aluminum to form the OTFT gate
electrode, (e) application of the nanocomposite gate dielectric by RF-sputtering of ZrO2 and spin coating of
Poly(vinyl cinnamate) (f) thermal evaporation of the organic semiconductor pentacene, (g) e-beam
evaporation of gold to form OTFT source and drain electrodes, (h) photographic image of a flexible PVDfabricated 3×6 sensor array with integrated OTFTs. The close-up shows the OTFT in detail, (i) AFM height
image of the spin-coated P(VDF:TrFE) layer. The color map corresponds to a height scale ranging from 0116 nm. The rms-roughness is 14.5 nm.
The next step was the deposition of an approximately 2 ȝm thick P(VDF-TrFE) layer
(70:30 mol%) via a spin coating technique. Without any patterning this process formed the
ferroelectric dielectric which was also investigated by AFM. This spin coated ferroelectric
polymer film form a loose network of „spaghetti“. The roughness of the surface (rms) is
relatively high (~14.5 nm) with a maximum peak of ~100 nm. The sensor was finished by
shadow mask evaporation of an 80 nm aluminium top electrode on the ferroelectric material.
For improved IR-absorption a graphite layer has subsequently been applied on the covering
electrode.
The fabrication of the OTFTs started with e-beam evaporation of aluminium (80 nm) through
a shadow mask, serving as the gate electrode being connected to the sensors top electrode.
The next step was the deposition of a nanocomposite gate dielectric by RF-sputtering of ZrO2
(40 nm) and spin coating a thin layer (20 nm) of Poly(vinyl cinnamate) (PVCI) backfilling the
grainy metal oxide film and forming a seed layer for the semiconductor. This gate dielectric
shows an overall capacitance of 100 – 110 nF/cm2 and is described as high-k gate dielectric
on that account.[14] Subsequently, 50 nm Pentacene are evaporated on top of the
semiconductor thus forming the organic semiconductor. The OTFTs have been finalized by ebeam evaporation of Au (80 nm), forming the source- and drain electrodes. The patterning of
the OTFTs was achieved by using shadow masks for all process steps mentioned afore.
21
2.2 Fabrication of Large Area Devices
The process flow of the screen and inkjet-printed active matrix sensor is illustrated in Figure
2. In total, six printing steps are needed to build the sensor array. A flexible 175 ȝm thick PET
(polyethylene terephthalate) sheet is used as a substrate. First, the bottom electrodes of the
pyroelectric and piezoelectric sensor capacitor (sensor pixel) are screen-printed (Figure 2a). A
standard PEDOT:PSS ink was utilized, yielding smooth (root mean square (rms) roughness of
6 nm) and conductive (325 ȍ per square) layers with a thickness in the range of 400–500 nm.
Circular sensor spots, which are interconnected by lines, form arrays of 3x6 (see also: Figure
2g) or 3x4 pixels. In the second step, the ferroelectric polymer PVDF-TrFE is screen-printed
onto the PEDOT:PSS bottom electrodes, followed by a short curing step at 110°C (Figure 2b).
The curing step supports the formation of the crystalline piezo- and pyroelectric ȕ -phase and
accelerates evaporation of the solvent. With an ink containing 18 wt% fraction of the
fluoropolymer, the screen-printed P(VDF-TrFE) layers have a thickness of approximately
5 ȝm. Atomic force microscopy (AFM) analysis of the surface morphology of the P(VDFTrFE) film (Figure 2h) show a grainy structure with an rms roughness of 4.5 nm. The
fabrication of the piezo- and pyroelectric sensor pixels is finalized by screen-printing of either
6 ȝm-thick carbon or 500–600 nm thick PEDOT:PSS top electrodes (Figure 2c). Both
electrode materials exhibit good absorption (> 70%) in the near to mid-IR range. In contrast to
the black carbon electrodes, the PEDOT:PSS electrodes appear light grey in the visible range,
as shown in the photograph of the integrated sensor in Figure 1g. Since P(VDF:TrFE) is
transparent in the visible range, nearly transparent sensor pixels are obtained when
PEDOT:PSS is used for the top and bottom electrodes (Figure 1b). The subsequent printing
steps involve the fabrication of the read-out electronics that is based on ECTs[16]. In order to
minimize the number of printing steps, a lateral ECT architecture was chosen in which the
gate and channel are located in the same layer [15]. Carbon-based contact lines and pads are
screen-printed; this step is combined with the printing of the sensor’s top electrodes (Figure
2c) (unless PEDOT:PSS is used for the top electrodes). The channel and the gate electrode,
both made of PEDOT:PSS, are inkjet-printed (Figure 2d). In order to achieve a sufficient
matching of the input impedance of the ECT with the high impedance of the sensor
capacitance, it was necessary to significantly reduce (i) the dimensions (length and width §
100 ȝm) and (ii) the thickness (400–500 nm for one layer) of the PEDOT:PSS transistor
channel. This was achieved by inkjet printing the channel instead of screen printing thus
reducing the ECTs switching current. The inkjet-printed dielectric SU-8 lacquer layer with a
thickness of 6 ȝm separates the carbon contacts from the electrolyte (Figure 2e). The final
step in the ECT production is the deposition of an electrolyte layer, which is again done by
inkjet printing (Figure 2f). The whole active matrix sensor device is thus fabricated by a
combination of screen and inkjet printing processes.
22
Figure 2: Process flow illustrating the fabrication of printed ferroelectric active matrix sensor arrays. a) Screen
printing with PEDOT:PSS to form the bottom electrodes of the sensor pixel, b) screen printing of the
ferroelectric P(VDF:TrFE) fi lm, c) screen printing of carbon to form the top electrodes of the sensor pixel, d)
inkjet printing with PEDOT:PSS to form the gate and channel of the ECTs, e) inkjet printing of the SU-8
separation layer, f) inkjet printing of the ECT polymeric electrolyte, and g) photograph of an all-printed 3 x 6
sensor array with integrated ECTs. h) Close-up of the channel region of the ECT. i) AFM height image of the
screen-printed P(VDF:TrFE) layer. The color map corresponds to a height scale ranging from 0–55 nm. The
rms roughness of the film is 4.5 nm.[14]
2.3 Poling
For the usage as transducer material, the crystalline domains must be aligned with respect to
their electrical dipoles. Therefore an electrical poling procedure must be applied for aligning
the ferroelectric domains. Two different poling techniques have been used: (i) stepwise poling
as described elsewhere [18][19] and (ii) hysteresis poling.
Hysteresis poling following a Sawyer-Tower circuit also enables monitoring the ferroelectric
properties of the sensor by hysteresis loop measurements (Figure 3). The coercive field
strength required to switch the ferroelectric polarization was between 70-80 Vμm-1 and the
resulting remnant polarization Pr (the displacement at zero electric field) was in the range of
70 mCm-2. These values are close to the best values reported for ferroelectric capacitors based
on spin-coated P(VDF:TrFE)[12]. To this end, hysteresis loops were recorded with electric
fields in the saturation region until stable hysteresis loops were obtained. The poling process
was then stopped at zero electric field. With this process, reproducible piezo- and pyroelectric
characteristics were obtained in the sensor pixels.[15][21][22]
23
Figure 3: Hysteresis loops of a ferroelectric sensor pixel. The lines illustrate the opening of the loop with increasing
electric field amplitude.
2.4 Pyroelectric Characterization
The voltage response of a sensor pixel is induced by thermal excitation with an intensitymodulated light from a laser diode emitting in the near infrared (808 nm) or with intensitymodulated mid-infrared CO2 laser radiation at 10.5 ȝm. Figure 4 displays the voltage
responses for excitation at 808 nm with a maximum intensity of 70 mW measured for sensors
with both carbon and PEDOT:PSS top electrodes. In the frequency range from 5 to 1000 Hz
the voltage response decreases monotonically, as expected for a pyroelectric detector element
[22] [24]
Carbon top electrodes generate a larger output voltage than PEDOT:PSS electrodes
primarily owing to the somewhat higher absorption of carbon in the mid-IR.
Figure 4: Frequency dependence of the pyroelectric voltage response of a sensor pixel excited by intensitymodulated light from a laser diode emitting at 808 nm for carbon and PEDOT:PSS top electrodes. The error
bars illustrate the variation in the pyroelectric response across the pixels of the 3 x 4 printed sensor arrays.
The overall frequency behaviour, the influence of sensor geometry, parasitic capacitance and
impedance, absorption coefficients of the top electrodes, pyroelectric sensitivity of the sensor
layer and influences of fabrication processes have been evaluated and verified using
mathematic modeling of the electric measurements.
3. RESULTS
In Figure 5 the frequency dependent pyroelectric current and voltage outputs of LabScale
sensors with different areas are shown. Similar curves are also obtained for printed PVDFTrFE based sensors. The impinging laser beam always excites a constant area of 3 mm2 being
much smaller than the overall area of the sensor.
24
Figure 5: The influence of parasitic capacities on the pyroelectric sensor output and its mathematically assessment
(solid lines) are shown (left). The overall sensor area is decreased stepwise by successively cutting the
connecting lines between the circular sensor fields at a constant area of excitation (right). Additional
explanations are given in the text.
The overall sensor area is decreased stepwise by successively cutting the connecting lines
between the circular sensor fields (see Figure 5, right). Accordingly the parasitic capacitance
of the sensor is decreased without decreasing the excited area. As is observed in Figure 5
(left), the parasitic capacities are decreasing the voltage output though are not affecting the
current generated by the laser excitation. This is due to the fact that the pyroelectric current
varies as
V
I˜
R
1 Z 2 R 2C 2
(3)
with R and C being impedance and capacitance of the equivalent circuit (thus also including
capacitance and impedance of the measurement circuit). According to (3) the voltage response
varies as I˜R below and I/ȫC above the cut-off frequency. The cut-off frequency is
determined by the RC-time constant of the whole equivalent circuit. The generated
pyroelectric current I can be described as
I
pA Z T
(4)
with p being the pyroelectric coefficient, A being the excited area and T being the average
temperature of the pyroelectric layer. From (3) and (4) it becomes clear that the current
response is independent of the parasitic capacitance whereas the voltage response is
decreased. The resulting simulated response curves are also included in Figure 5 as continues
lines. The excellent correspondence to the experimental data points fully verifies the
theoretical model of the multilayer sensor and the simulation routine. As is expected the
current response is not influenced by the parasitic whereas the voltage response decreases
continuously with increasing parasitic capacitance.
According to the congruent results of the pyroelectric measurements and the mathematic
modeling of the LabScale devices, a new design for large area and fully printed devices was
developed. In this context the new design was used for an integration of the sensor device
with (1) OTFTs and ECTs and (2) in a further step with an electrochromic display (ECD).
3.1 Integration of the Sensor with OTFTs and an ECD
Low leakage currents below 2 nA guarantee a very good impedance matching between the
OTFT and the sensor; resulting in a complete switching of the transistor being biased by the
25
sensor as shown in Figure 6 (left). After the additional combination with the display, the drain
current of the transistor is only marginally reduced (Figure 6 (right)). The small reduction of
the drain current is caused by the current consumption of the display during the reduction /
color change. The dark blue arrows are indicating the regions of the reduction according to
switching the digit on and the light blue arrows are indicating the discharge of the display to
its initial highly conductive “off state”. The frequency of the repeated switching process was
chosen to be 0,01 Hz due to the slow relaxation time of the display that is to be expected in
such a simple configuration without any control electronics.
Figure 6: Sensor output (a) and drain current (b) during the color switching of the printed display. The sensor was
stimulated at 0,01 Hz with a 70 mW laser diode and the drain voltage was -1 Volts. For discharging
(oxidizing) the display, a 500 kŸ resistor was connected between supply voltage and ground. The arrows are
indicating the color changes of the display caused by reduction/oxidation (c) of the electrolyte.
3.2 Integration of a Sensor with ECTs and an ECD
Due to the usage of ink-jet printing for the PEDOT:PSS channel in electrochemical
transistors, the current consumption during the redox process of the electrolyte could be
drastically reduced. Therefore the impedance matching between sensor and ECT could be
adjusted and the complete switching of the transistor biased by the sensor was achieved. The
maximum voltage at the carbon electrodes was about 0.7 V for an input impedance of the
equivalent measurement circuit of 100 MŸ, and 0.3 V at the PEDOT:PSS top electrodes.
Since the sensor is connected in parallel to the gate of the electrochemical transistor, the
voltage response generated by the pyroelectric pixel is sufficient to switch the transistor
current. Figure 7a displays an output characteristic of a printed ECT with a lateral electrode
configuration. Since in an ECT the conductivity of the PEDOT:PSS channel is decreased by
applying a positive gate voltage, these devices are normally on and can be controlled by very
small voltage levels; in this case the printed ECT is switched off by applying a gate voltage of
only 0.75 V (Figure 7b).
26
Figure 7: a) Time curve of the voltage response Vsens of a printed sensor pixel induced by square-wave intensitymodulated laser light (808 nm). The arrows indicate the state of the electrochromic display. b) Time curve of
the ECT drain current induced by Vsens. Since the sensor capacitor and the transistor are connected in parallel,
the ECT gate voltage corresponds to Vsens. The arrows indicate the state of the display.
Figure 8: a) Photograph of a printed active matrix sensor consisting of a 3 x 4 array of sensor capacitors integrated
with a row of electrochemical transistors connected to a printed electrochromic display. b) Activation of
display segments due to infrared radiation from a human finger. c) Activation of display segments with light
from a laser pointer.
REFERENCES
[1] R. Parashkov, Parashkov, R., Becker, E., Riedl, T., Johannes, H.-H., Kowalsky, W., “Large Area
Electronics Using Printing Methods” Proceedings of the IEEE 93(7), 1321-1329 (2005).
[2] S. P. Lacour, in PerAda Magazine 2010 , www.perada-magazine.eu/pdf/003194/003194.pdf
[3] Schreiner, Printing the future, www.schreiner-printronics.com (2011).
[4] PolyIC homepage, PolyIC-“the chip printers”, www.polyic.com (2011).
[5] ThinFilm homepage, World’s first printed ferroelectric memory device, www.thinfilm.se (2011).
[6] a) Enfucell homepage, It’s a paper-thin, it’s a battery and it’s green, www.enfucell.com (2011); b)
Power Paper, Printable batteries, www.powerpaper.com (2011).
[7] Konarka homepage, Converting Light to Energy, www.konarka.com (2011).
[8] Holst Centre homepage, Smart packaging, www.holstcentre.com (2011).
[9] T. Someya, “From the Cover: Conformable, flexible, large-area networks of pressure and thermal
sensors with organic transistor active matrixes,” Proceedings of the National Academy of Sciences 102,
12321–12325 (2005).
[10] T. Someya, A. Dodabalapur, J. Huang, K. C. See, and H. E. Katz, “Chemical and Physical Sensing by
Organic Field-Effect Transistors and Related Devices”, Advanced Materials, 22(34), 3799–3811
(2010).
[11] Someya, T., Kato, Y., Shingo Iba, Noguchi, Y., Sekitani, T., Kawaguchi, H., Sakurai, T., “Integration of
organic FETs with organic photodiodes for a large area, flexible, and lightweight sheet image
scanners,” IEEE Transactions on Electron Devices, 52(11), 2502-2511 (2005).
27
[12] R. C. G. Naber, C. Tanase, P. W. M. Blom, G. H. Gelinck, A. W. Marsman, F. J. Touwslager, S.
Setayesh, and D. M. de Leeuw, “High-performance solution-processed polymer ferroelectric field-effect
transistors,” Nature Materials 4, 243–248 (2005).
[13] N. Neumann, R. Köhler, G. Hoffmann, “Application of P(VDF/TrFE) thin films in pyroelectric
detectros,” Ferroelectrics, vol. 118, pp. 319- 324, (1991).
[14] M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P. A. Ersman, D. Nilsson, D. Platt,
et al., “An All-Printed Ferroelectric Active Matrix Sensor Network Based on Only Five Functional
Materials Forming a Touchless Control Interface,” Advanced Materials 23, 2069–2074 (2011).
[15] M. Zirkl, A. Haase, A. Fian, H. Schön, C. Sommer, G. Jakopic, G. Leising, B. Stadlober, I. Graz, et al.,
“Low-Voltage Organic Thin-Film Transistors with High-k Nanocomposite Gate Dielectrics for Flexible
Electronics and Optothermal Sensors,” Advanced Materials 19, 2241–2245 (2007)
[doi:10.1002/adma.200700831].
[16] D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, and M. Berggren, “Bi-stable and Dynamic
Current Modulation in Electrochemical Organic Transistors,” Advanced Materials 14, 51–54 (2002).
[17] D. Nilsson, N. Robinson, M. Berggren, and R. Forchheimer, “Electrochemical Logic Circuits,”
Advanced Materials 17, 353–358 (2005) [doi:10.1002/adma.200401273].
[18] D. Setiadi, M. Sarro, and L. Regtien, “A 3x1 integrated pyroelectric sensor based on VDF/TrFE
copolymer,” Sensors and Actuators A: Physical 52, 103–109 (1996).
[19] M. Zirkl, B. Stadlober, and G. Leising, “Synthesis of Ferroelectric Poly(Vinylidene Fluoride)
Copolymer Films and their Application in Integrated Full Organic Pyroelectric Sensors,” Ferroelectrics,
vol. 353, no. 1, pp. 173-185, (2007).
[20] S. Bauer, F. Bauer, [Piezoelectricity: Evolution and Future of a Technology], Springer Series in
Materials Science, Berlin, Ch. 6, pp.157 (2008).
[21] J. Groten, M. Zirkl, G. Jakopic, A. Leitner, and B. Stadlober, “Pyroelectric scanning probe microscopy:
A method for local measurement of the pyroelectric effect in ferroelectric thin films,” Physical Review
B 82, 1–11 (2010) [doi:10.1103/PhysRevB.82.054112].
[22] I. Graz, M. Krause, S. Bauer-Gogonea, S. Bauer, S. P. Lacour, B. Ploss, M. Zirkl, B. Stadlober, and S.
Wagner, “Flexible active-matrix cells with selectively poled bifunctional polymer-ceramic
nanocomposite for pressure and temperature sensing skin,” Journal of Applied Physics 106, 034503
(2009) [doi:10.1063/1.3191677].
[23] T. Furukawa, J. X. Wen, K. Suzuki, Y. Takashina, M. Date, “Piezoelectricity and pyroelectricity in
vinylidene fluoride/trifluoroethylene copolymers” J. Appl. Physics, 56, 829, (1984).
[24] P. Muralt, “Micromachined infrared detectors based on pyroelectric thin films” Rep. Prog. Phys.,
64(10), 1339, (2001).
28
BIOGRAPHIC DATA OF DR BARBARA STADLOBER
FullName
Position
Barbara Stadlober
Head of Research Group
Institute of Surface Technologies and Photonics,
Affiliation JOANNEUM RESEARCH, Franz-Pichlerstrasse 30,
A-8160 Weiz
Smart system integration and all-printed active matrix
Titleoftalk
sensors
In organic and large-area electronics smart sensors for detection of physical
forces are very promising with respect to novel user interface applications.
Recently, there have been several reports on different types of active matrix
sensor networks, however in most of these components the large number of
process steps, substrates and materials involved make their fabrication
complex, costly and hard to control, with expectedly small yield. Hence,
developing an active-matrix sensor technology that is fabricated exclusively
by printing and uses as few materials as possible constitutes an advance
towards low-cost and large-area sensing films. Here we demonstrate the
printing of a complex smart integrated sensor system using only five
functional inks: the fluoropolymer P(VDF-TrFE) (Poly(vinylidene fluoride
Abstract
trifluoroethylene) sensor ink, the conductive polymer PEDOT:PSS (poly(3,4ethylenedioxythiophene): poly(styrene sulfonic acid) ink, a conductive carbon
paste, a polymeric electrolyte and SU8 for separation. The result is a touchless
human-machine interface, including piezo- and pyroelectric sensor pixels
(sensitive to pressure changes and impinging infrared light), transistors for
impedance matching and signal conditioning, and an electrochromic display.
Applications may not only emerge in human-machine interfaces, but also in
transient temperature or pressure sensing used in safety technology, in
artificial skins and in disposable sensor labels.
Abrief
biography
Dr. Barbara Stadlober has studied solid state physics at the Karl-FranzensUniversity in Graz. After finishing her PhD at the Technical University in
Garching (Germany) on High Temperature Superconductors she worked six
years as a technology development engineer for power silicon chips at
Infineon Technologies AG in Villach (Austria). Starting from 2002 she
established the Organic Electronics group at the Institute of Nanostructured
Materials and Photonics of JOANNEUM RESEARCH in Weiz (Austria).
Since 2010 she is head of the research unit “Micro- and Nansotructuring” at
the – intermediately enlarged - Institute of Surface Technologies and
Photonics of JOANNEUM RESEARCH. Her main interests are organic
electronics and optoelectronics, process development, novel nanostructuring
methods and organic sensor technologies.
29
O
Organic b
biosensorrs based
d on
biiocompaatible solu
ution-prrocessablle materials
PD Dr.-Ing. Giusepppe Scarpa
Institute for Nanooelectronics, Technische Universität M
München, Geermany
Simple, low-cost porrtable devicess, which can perform reliiable fast diaagnostic proccedures and can
c be used
as disposables in lifee science, coover a wide range of sennsing applicaations, i.e., fo
food and envvironmental
monitoriing or detecction of biological hazaardous materrial. Field-efffect transisttors based oon organic
materialss [organic thhin-film transsistor (OTFT
Ts)] can provvide a promiising answerr in the aforeementioned
field of applications. Besides loow-cost largee-area substrrate-independdent fabricattion capabilitty, organic
materialss offer a unnique opportuunity when used in senssing applicattion, in which the semicconducting
material acts both ass active trannsport layer and sensing component. Here, we rreport on OT
TFTs-based
b
le solution-processable oorganic semiconducting
sensing devices usinng a biofuncctionalized, biocompatibl
polymer,, which can be operated in complex bbiological media
m
in direcct contact with living cellls, opening
up the ppossibility off developing easy to handdle, inexpenssive test strippes for reliabble, and fastt biological
assays.
D Dr.-Ing. G
Giuseppe Sccarpa is Staff
ff Lecturer & Team Leader at the Innstitute for
Bio-Biblliography: PD
Nanoelecctronics, Tecchnische Universität München, Germ
many. He graaduated in ellectrical enggineering at
the Univversity of Rome "Tor Verrgata", Italy, in 1998. In 1999 he joinned the Waltter Schottky Institute of
the Techhnical Univeersity of Munnich (TUM),, Germany, where
w
he recceived his PhD degree working
w
on
design aand fabricatioon of quantuum cascade llasers (2003)). He is currrently staff leecturer at thee electrical
engineerring departm
ment and stafff scientist at
a the Instituute for Nanooelectronics of TUM. Hiis research
focuses on
o the fabriccation of a vaariety of nannostructures ((such organicc devices andd nanomagneets) and on
the development of vvarious nanofabrication teechnologies based on nannoimprint litthography as well as on
biosensoors and biochhips based onn organic matterials.
30
POLYMER SCHOTTKY BARRIER TRANSISTORS
S. Georgakopoulos1, D. Sparrowe2, F. Meyer2, M. Shkunov1
1 Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK
2 Merck Chemicals, Chilworth Technical Centre, University Parkway, Southampton SO16 7QD, UK
Organic Field-Effect Transistors perform well with ohmic source and drain
contacts. However, long term operational stability is typically achieved with high
ionisation potential (IP) semiconductors with IP values more than 5.3eV.[1,2] Due to
high IP ohmic injection it is becoming problematic, and transistors suffer from injection
problems due to the formation of Schottky contacts.
In this work we demonstrate new approach for organic transistors where high IP
indenofluorene copolymers with IP values as high as 5.4eV and 5.8eV are used as the
semiconducting layers. We deliberately utilise Schottky barriers and special transistor
geometry to manipulate charge depletion under the source electrode.[3] As a result we
obtain two significant outcomes:
i)
we can successfully use high IP polymers with excellent environmental
stability,
ii)
we achieve excellent I-V curves with saturated voltage as low as 2V for
transistors with a channel length of 2.5ȝm and thick insulating layer
of ~1 ȝm.
The main feature of these transistor devices is the reverse-biased source Schottky barrier
which depletes the semiconductor of charge in the vicinity of the source electrode. This
depletion extends to the semiconductor/insulator interface and pinches off the channel,
leading to a low saturation voltage, which is preserved without down-scaling the insulator
thickness as required for typical Field-Effect Transistors. Additionally, for sufficiently
strong barriers the output current of the transistor does not scale with channel length. These
properties come at the cost of reduced current due to the highly resistant depletion region,
and other peculiarities.
In summary, we anticipate that organic transistor fabrication technology could benefit from
this new approach by allowing low-cost deposition of thick insulators instead of specialised
ultra-thin insulators for low saturation voltages.
31
Moreover, due to transistor current insensitivity to the channel length,
low-resolution
channel patterning and printing techniques can be utilised for this type of transistors.
References:
1.I.McCullough,M.Heeney,M.L.Chabinyc,D.DeLongchamp,R.J.Kline,M.Colle,W.Duffy,D.Fischer,D.
Gundlach,B.Hamadani,R.Hamilton,L.Richter,A.Salleo,M.Shkunov,D.Sparrowe,S.Tierney,W.Zhang.Adv.
Mater.21,1091(2009)
2. S. Georgakopoulos, D. Sparrowe, F. Meyer, M. Shkunov. Appl. Phys. Lett. 97, 243507 (2010)
3. J.M. Shannon, F. Balon. Solid-State Electronics 52, 449 (2008)
32
CUTOFF FREQUENCY OF ORGANIC FIELD EFFECT
TRANSISTORS: A SIMULATION STUDY
S. SCHEINERT*, I. HÖRSELMANN
Ilmenau Technical University, Institute of Micro- and Nanoelectronics and Center of Microand Nanotechnologies, PF 100565, D-98684 Ilmenau, Germany
Cutoff frequency is a crucial parameter for many applications of the OFET in organic
electronic circuitry. For an ideal transistor this frequency is independent on the insulator
thickness and scales with μ/L2 so that both increasing the mobility μ and reducing the channel
length L result in higher values. However, the behavior of the real OFET is strongly
influenced by high S/D contact barriers, increased contact resistance and traps. We have
carried out 2D simulations of the BOC (S/D bottom contact) and TOC (S/D top contact)
OFETs to describe the influence of these effects on the cutoff frequency. As already described
in the literature for the BOC OFET [1], series resistance and large overlap capacitances
reduce the expected increase of the cutoff frequency reducing the channel length. However
our comparison shows this effect is stronger in the TOC structure. The same tendency is valid
for the influence of the insulator thickness. Whereas the BOC OFET shows the expected
behavior, the cutoff frequency of the TOC one is reduced with reduced thickness. In contrast,
for higher S/D contact barriers the cutoff frequency is stronger reduced in BOC OFETs.
Two dimensional simulations have been carried out with the program Sentaurus [2]. The
program solves the poisson and continuity equations. For the small signal analysis the
complex (small signal) admittance Y matrix is computed giving the current response at a
given node to a small voltage signal at another node j = Y u = A u + i Z C u. The small signal
current gain can be calculated by:
h21
id Adg iZCdg
ig Agg iZCgg
33
(1)
and the cut-off frequency follows from |h21|=1. The simulated device structures are shown in
Fig. 1. The used material parameters for the 50nm thick P3HT layer are the following:
mobility ȝ=0.02cm2V-1s-1, affinity Ȥ =3.0eV, gap Eg=2.0eV, doping level NA=5×1016 cm-3,
density of states in the valence and conduction band NV=NC=1021cm-3. The oxide thickness is
50nm.
Fig.1. Simulated device structure of the BOC (a) and TOC (b) OFET.
In Fig. 2a simulated cut-off frequencies for different overlap length between the S/D- and gate
contacts are shown. Without overlap the simple equation for ft describes well the BOC
transistor behavior. In the TOC OFET the parasitic resistance between the S/D contacts and
the channel at the oxide interface are the reason for the reduced cut-off frequency for shorter
channels. Contact resistance reduce frequency limit for shorter channels. This effect is more
pronounced in the BOC structure as visible from fig.2b. However, also in this case the
analytical equation for ft including series resistance describes well the simulated values.
Fig.2 Cut-off frequency of the TOC and BOC OFET vs. channel length for VGS=-6V,
VDS=-1V and different overlap length (a) and contact resistance (b).
High contact barriers at source and drain reduce the efficient injection of carriers. Consequently, their influence on the frequency was investigated and the results are shown in fig. 3.
34
Fig.3 Cut-off frequency of the TOC and BOC OFET vs. channel length for VGS=-6V,
VDS=-1V and different S/ work functions (a) and the quasi Fermi potential along the
channel near the oxide interface (b).
The cut-off frequency is strongly reduced for higher barriers and again this effect is stronger
pronounced in the BOC OFET than in the TOC one. Furthermore, comparing the results with
that one of the additional series resistance the constant value despite reduced channel length is
visible. The reason for this can be explained with fig. 3b. In the case of contact resistance the
channel length determines strongly the potential drop over the series resistance reducing the
cut-off frequency with reduced channel length. Contrary, for high contact barriers the
potential drop near the contacts does not depend on the channel length resulting in a constant
value of ft.
Traps can also have an influence on the frequency limit. But their effect depends strongly on
the trap parameters. Furthermore, in TOC OFETs bulk traps modify the parasitic resistance
between the S/D contacts and the channel at the oxide interface.
References
[1] A. Hoppe, D. Knipp, B. Gburek, A. Benor, M. Marinkovic, V. Wagner, Organic Electronics 11, 626 (2010).
[2] Sentaurus Device User Guide, Version E-2010.12, December 2010, Synopsys
35
BIOGRAPHIC DATA OF DR SUSANNE SCHEINERT
Degrees
x
Diploma in Electrical Engineering in 1975
x
Dr.-Ing. in 1992 in the field of SOI electronics
x
Dr.-Ing. habil. in 2006 in the field of organic electronics
At present she works in the field of organic electronics with the
main focus on organic field effect devices.
36
THE INFLUENCE OF HOLE INJECTION BARRIERS ON ORGANIC
FIELD-EFFECT TRANSISTORS: CONNECTION WITH
PHOTOEMISSION DATA a
G. PAASCH*1, S. SCHEINERT2, M. GROBOSCH1, I. HÖRSELMANN2, M. KNUPFER1,
and J. BARTSCH2
1
Institute for Solid State and Materials Research IFW Dresden, PF 270116, D-01171 Dresden, Germany
2
Ilmenau Technical University, Institute of Micro- and Nanoelectronics and Center of Micro- and
Nanotechnologies, PF 100565, D-98684 Ilmenau, Germany
The operation of organic devices as organic field-effect transistors (OFET) depends critically
on the contact between the organic layer and the material for source/drain electrodes. Small
barriers for carrier injection are required for efficient operation. In order to support the
understanding of organic devices, photoemission spectroscopy has been used to determine the
properties of metal/organic interfaces. Values for the hole injection barrier determined in the
last decade by different groups are frequently of the order of 0.5...1eV. It is not clear whether
barrier lowering due to the image charge is sufficient to make contacts with such barriers
efficiently for carrier injection. Indeed, no results have been reported where the preparation of
the samples for the photoemission study and for the devices is the same.
Here we discuss results of such an investigation for OFETs with P3HT as active layer and
with gold source/drain contacts [1]. The measured hole barrier at the gold contact of 0.6eV
results from the Au work function of 4.6eV. Taking into account the dependencies of the
mobility on the carrier concentration and on the field for the Gaussian density of states (DOS)
of disordered organics, measured OFET current characteristics cannot be described well with
such contacts but rather for work functions of 4.7eV or larger. Considering the method to
determine the barrier from photoemission data and the Gaussian DOS of the hopping
transport states, we present a quantitative connection [1] between the barrier as determined
from photoemission and the barrier as used in the device simulation. Then we show however,
that the definition of the band edge for a Gaussian DOS is problematical.
*
Corresponding Author: [email protected]
37
In photoemission experiments the hole barrier is extracted by linear extrapolation of the upper
part
of
the
measured
intensity
as
demonstrated in Fig.1. There the Fermi
energy is at binding energy zero and
therefore a barrier of 0.6eV is obtained. A fit
of the upper part of the spectrum by a
Gaussian can be analytically extrapolated
and gives the same value. It is clearly seen in
Fig.1. Measured UPS intensity, linear extrapolation
for barrier determination, Gaussian fit of the upper
part of the spectrum.
Fig.1 that towards the Fermi energy the
spectrum shows a tail. It arises from the
measurement and does not reflect energy states present there. Here a first problem emerges:
From UPS intensities one obtains information in the region down to about 1 to 5 % of the
maximum DOS. On the other hand, OFETs operate in accumulation at concentrations of
about 1018 cm-3, which is slightly more than 10-3 of the total density. Therefore, the tail
measurable with UPS lies far above the DOS values determining the OFET operation.
Moreover, the Gaussian curve approximating the upper part of the spectrum is rather broad
with a variance of 370meV. But in order to
describe the measured current characteristics
one has to assume a Gaussian DOS
D( E )
§ E E 0 2
exp¨¨ 2V 2
2S V
©
1
(Maximum
at
E0
and
·
¸
¸
¹
variance
(1)
V,
determining the mobility) which is much
Fig.2. The tail of the Gaussian fit is replaced by a
steeper tail of the narrower Gaussian DOS.
narrower with a variance much less than
100meV. Such variance is in accordance with values reported for other devices. We suppose
therefore that the narrow Gaussian DOS determining the hopping transport represents the tail
region of the broader Gaussian measured by photoemission. This is demonstrated in Fig.2
showing from the narrow transport DOS only the tail. The continuous transition from the
broad photoemission Gaussian to the narrow Gaussian transport DOS is possible for different
rations m of their maxima. In Fig.2 the values m=10, 20, 30 are chosen. Smaller values are
unrealistic since the transition must occur clearly below the measured UPS intensity.
38
Values larger than m= 30…50 can also be excluded, since transport at densities characteristic
for accumulation must take place in the narrow tail. In this way, the position of the narrow
transport DOS is fixed relative to the photoemission data with an unknown parameter m in the
region indicated in Fig. 2. There remains the question on the position of the band edge relative
to the narrow transport DOS.
For this purpose we show in Fig.3 the hole density as function of the Femi energy (relative to
the maximum E0 of the narrow transport
DOS). The curves with symbols are
calculated numerically. The non-degenerate
approximation (dashed) is given by [2]
p
N0
§ E F ( E0 V 2 / 2kT ) ·
¸¸ .
exp¨¨ kT
¹
©
(2)
In Fig.3 there is shown also the limit for the
transition
Fig.3. Hole density for the system with a Gaussian
DOS: Numerical values with symbols, dashed the
non-degenerate approximation (2). Dash-dotted the
limit (3) for the transition from non-degeneration to
degeneration.
from
low
density
non-
degeneration to degeneration (dash-dotted
line) which occurs at
EF E0 V 2 / kT .
(3)
It is clearly seen in Fig.3 that for smaller variance the non-degenerate approximation works
well for concentrations occurring in OFET accumulation layers. This approximation was used
in the simulations presented in Ref. [1].
Comparing Eq. (2) with the conventional expression for the hole density one can argue that
the valence band edge is situated at EV
E0 V 2 / 2kT . Using this value and the transition
from the photoemission intensities to the Gaussian transport DOS, one can establish a relation
between the barrier as determined by photoemission and that one needed in device simulation.
This was done in Ref. [1]. However, the assumption, that the band edge is situated relative to
the maximum of the Gaussian DOS as EV
E0 V 2 / 2kT is unfortunately not unique. There
are different imaginable possibilities. Thus, one can also assume that the band edge is situated
at the position (3) where the transition from non-degeneration to degeneration takes place.
One can also arbitrarily suppose that the band edge is situated at a position where the density
is decreased to n% (e.g.1%) of its maximum value or that the DOS is decreased to n%
(e.g.1%) of its maximum value.
39
Thus, at present, it is hard to establish a well-defined connection between the barrier as
determined from photoemission and the position of the band edge needed in device
simulation. At first, the position of the transport DOS relative to the measured photoemission
date is not well defined (the factor m given above) and secondly, there is no unique definition
of the position of the band edge relative to the maximum of the transport DOS.
Highly desired are experimental methods for determination of the transport DOS in the
concentration region where the transport takes place.
References
[1] S. Scheinert, M. Grobosch, G. Paasch, I. Hörselmann, M. Knupfer, J. Bartsch: Contact characterization by
photoemission and device performance in P3HT based organic transistors, J. Appl. Phys., in press.
[2] G. Paasch and S. Scheinert: Charge carrier density of organics with Gaussian density of states: Analytical
approximation for the Gauss–Fermi integral, J. Appl. Phys. 107, 104501 (2010)
a
Extended Abstract, 5th International Symposium Technologies for Polymer Electronics TPE12, 22-24 May
2012 Rudolstadt / Germany
40
BIOGRAPHIC DATA OF PROFESSOR GERNOT PAASCH
x
Diplom-Physiker (1967, Technical University Dresden)
x
Dr. rer. nat. (1970,Technical University Dresden)
x
Dr. sc. nat. (1976; converted into Dr. rer. nat. habil.
1991, Technical University Dresden)
x
Technical University Dresden (1967-1977)
x
Lomonossov Moscow State University and Institute of
Physical Problems Moscow (Postdoc with Prof. M.I. Kaganov, Group of I.M.Lifshiz,
1974-1975)
x
Martin- Luther- University Halle- Wittenberg, Dozent (1977-1979)
x
Technical University Ilmenau, full Professor for Theoretical Physics (1979-1987)
x
Institute of Solid State and Materials Research (since 1988) (formerly Zentralinstitut
für Festkörper- und Werkstofforschung), senior scientist, with research Groups
Conducting Polymers, Electrochemistry and Conducting Polymers, Group for
Theoretical Solid State Physics
x
Technical University Ilmenau, Guest Professor for Nanoelectronics 1996-1998
x
Retired 2007, part time employed, guest scientist
41
PRINTED DIODES: PHYSICS AND APPLICATIONS
Donald Lupo, Kaisa Lilja, Petri Heljo, Sampo Tuukkanen, Miao Li
Department of Electronics, Tampere University of Technology,
Korkeakoulunkatu 3, PO Box 692, FI-33101 Tampere
Printed rectifying diodes, while not as heavily studied as printed TFTs, play an important role
in electronic circuitry, e.g. in RF rectification or as an alternative display back plane
structure. Printed organic diodes place less severe demands on charge carrier mobility or
lateral resolution than conventional organic TFTs, but bring their own challenges. In this talk
work done in our group on gravure printed Schottky diodes will be presented, with discussion
of how the printing process can affect interfaces and device physics. The integration of
printed Schottky diodes into circuits such as rectifiers, charge pumps and display backplanes
is then demonstrated.
The development of printed electronic components and circuits offers a novel way to produce
flexible and lightweight electronics using additive roll-to-roll processes on low-cost
substrates. Though there is a wealth of information on organic LEDs (OLEDs), organic
photovoltaics (OPV) and organic thin film transistors (OTFTs), much less has been written
about rectifying diodes. Diodes have a number of applications in electronics, including
rectification of radio frequency signals [1,2] for signal demodulation and for providing DC
power to RFID chips and distributed sensor networks, as well as in display backplanes [3].
We have been working in the Organic Electronics Group at TUT for some years on mass
production compatible, air stable gravure printed organic Schottky diodes, focussing on the
air stable, amorphous organic semiconductor poly(triarylamine) (PTAA).
Interfaces and diode performance
We have fabricated with high (up to 100,000) rectification ratios using very simple, mass
manufacturing compatible processes: screen printing + wet etching to pattern Cu on PET and
gravure printing to deposit PTAA, anode material (silver flake ink) and, in the case of circuits,
dielectrics for crossovers. However, it has been important to study the devices and interfaces
carefully to understand and optimise the diodes, as the printing process can significantly
affect the materials and interfaces and thus device properties. Current-voltage curves
demonstrate high rectification in a device for which Cu is the cathode and Ag is the anode,
which does not fit to a simplistic energy level diagram.
42
In addition, we observed that the diodes behaved differently depending on whether the Cu had
been deposited by evaporation or sputtering; diodes with sputtered Cu cathodes showed a
higher rectification ratio but also a higher onset of space charge limited current (SCLC) [4,5].
Kelvin probe measurements indicated a nominal work function of the Ag flake ink of 5.2 eV,
corresponding to Ag2O, which could explain good hole injection. The measured values for Cu
(4.8 and 5.0 eV) could not explain the fact that Cu acts as a “good” cathode, or the difference
between the two Cu electrodes.
2
Current density (A/cm )
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
10
-10
Cu(s)
Cu(e)
-1
-0.1 0.01
Voltage (V)
0.1
1
10
Figure 1. Log J- log V -performance of diodes with evaporated [Cu(e)] and sputtered [Cu (s)] copper
cathodes. Source: Ref. 4.
The combination of impedance spectroscopy and XPS depth profiles gave more insight into
the phenomenon. The impedance spectra recorded over a range of voltages indicated at least
one additional interlayer, which appeared to be at the semiconductor/cathode interface (Fig.
2). The XPS data show that the interfacial layer in the diodes with sputtered copper cathodes consists
of a thin layer of Cu2O and an organic layer with a combined thickness of 5 - 6 nm. In contrast, the
interfacial layer in diodes with evaporated copper cathodes is thinner, only 4 nm. Based on these
data we can explain the effect based on changes in Fermi level pinning [Lilja2], which
changes the effective barrier between Cu and semiconductor and depends on the interlayers.
Due to the increase in Schottky barrier with increasing interlayer, fewer charge carriers are
able to flow from the cathode to the semiconductor under reverse bias and the reverse current
is lower. However, in forward bias the field is still high enough to assist tunnelling through
the barrier.
43
4
5
6x10
6x10
(a) 5.0 V
4
2x10
0
5
2x10
0
0
4
2x10
6
6x10
4
z'
(c) 0.2 V
4x10
0
4
6x10
5
2x10
z'
7
5
4x10
5
6x10
6x10
(d) 0.1 V
Cu(s)
Cu(e)
6
7
4x10
|-z''|
4x10
|-z''|
Cu(s)
Cu(e)
5
4x10
|-z''|
|-z''|
(b) 0.4 V
Cu(s)
Cu(e)
4
4x10
6
2x10
7
2x10
0
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Cu(e)
0
0
6
2x10
6
z'
4x10
0
6
6x10
7
2x10
z'
7
4x10
7
6x10
Figure 2. Cole-cole plots of the diode impedance with evaporated (open circle) and sputtered (closed
circle) cathode contacts at forward bias voltages of 0.1 V, 0.2 V, 0.4 V and 5 V. The frequency
increases from right to left from 10 Hz to 1 MHz. Source: Ref. 5
Printed Diode Backplanes
As displays become more complex, connecting a lead to each pixel is no longer an option, and
matrix addressing is necessary, in which the display is divided into rows and columns and
addressed a row at a time. Many display media, for example common “electronic paper”
media, require an electronic circuit for each pixel, a so-called active matrix (AM) architecture.
Most commonly, these switching elements are thin film transistors (TFTs), but earlier
displays also used thin film diodes (TFDs) as switching elements. Recently, organic
semiconductors have been used to develop TFT AM display driving circuits [Gel04, Hui02].
However, there are challenges associated with fabricating organic TFT backplanes; fine
patterning is required to define a short channel, and high charge carrier mobility is needed to
get sufficient on current. Diodes, on the other hand, require no fine lateral patterning as
current flows vertically through a thin film, significant currents can be achieved with quite
low mobilities. Therefore it may be useful to return to the TFD concept using organic diodes,
as reported by Nilsson et al. [3], especially for applications where very high resolution and
fast switching are not needed.
We developed a novel backplane architecture which allowed fabrication of a TFD backplane
for a bistable front plane medium with unpatterned front electrode, and implemented this
architecture using the gravure printed diodes described above. Using this architecture, we
were able to demonstrate matrix driving of a simple 4 x 4 prototype based on E Ink
VizplexTM front plane [6], as shown in Figure 3.
44
a)
b)
Figure 3. Diode backplane laminated to Vizplex™; a) an image of a functional display with 7V
driving voltage, b) a schematic presentation of the printed diode backplane. Source: Ref. 6.
RF Rectification
As mentioned above, an important application of rectifying diodes is the rectification of radio
frequency signals. Organic semiconductor rectifier diodes were reported previously [7,8] but
these studies either used evaporated semiconductors and shadow mask evaporation or
photolithographic patterning of electrodes or showed poor high frequency performance
Spin casting has also been used to deposit organic layers but it can not be considered a high
throughput process, and RF performance was poor. High throughput processes, such as
coating and printing technologies, need to be demonstrated if there is to be hope of
manufacturing organic rectifier diodes on a scale that can be of industrial interest.
Figure 4. Frequency dependence of rectification from a printed diode half-wave rectifier for two
different active layer thicknesses. Source: Ref. 9.
45
We found that the Cu/PTAA/Ag architecture described above not only shows high
rectification ratios in dc measurement, but also surprisingly good RF performance. In
reference [2] we reported a half-wave rectifier based on a printed diode that demonstrated
rectification up to 10 MHz. In later work [9] we showed that by optimising the diode
parameters including layer thickness it was possible to rectify significantly higher
frequencies, with a 3 dB point as high as 20 MHz (Figure 4).
For a number of applications, a supply voltage higher than the voltage directly obtainable
from the RF signal is needed, e.g. to drive the logic elements in a chip, especially if organic
transistors are used. Therefore a means of voltage multiplication is needed. For this reason we
have begun looking at printed charge pump circuits based on multiple printed diodes and
capacitors for rectification and amplification of RF input signals. Recently we demonstrated
printed charge pump circuits based on gravure printed Schottky diodes and ink jet printed
capacitors [10]. When the diodes and capacitors were fabricated on the same substrate
Figure 5. Diagram of printed charge pump circuit and output characteristics. Source: Ref. 10.
(monolithic integration) some amplification of an input signal could be seen but performance
was modest due to degradation of the diode contact as a result of the high temperature
sintering of the nano-silver ink used for the capacitors. When the capacitors and diodes were
fabricated on separate substrates to avoid this issue, performance was improved, with 19 V
from 2 stages at 100 kHz for a 10V input sine wave and an output of 10.4 V at 13.56 MHz,
which is over twice the output of a half-wave rectifier. With further optimisation of the diodes
and capacitors as well as additional stages we expect to achieve further voltage amplification,
and work is ongoing in this area. In particular, we are investigating the use of RF energy
harvesting in energy harvesting circuits for distributed sensors.
46
In summary, we have shown that it is possible to use mass production, high throughput
compatible fabrication methods and air stable materials to fabricate diodes with excellent
rectification behaviour, even at radio frequencies, and to use these diodes to fabricate circuits
for applications in RF rectification and display backplanes.
Acknowledgement
The authors thanks T. Joutsenoja, R. Österbacka, H. Majumdar, K. Lahtonen and M. Valden
(co-authors on work cited herein) for their cooperation. This work was supported by the UPM
Kymmene Corporation and by the Academy of Finland.
References
[1] S. Steudel, K. Myny, V. Arkhipov, C. Deibel, S. de Vusser, J. Genoe and P. Heremans. Nat. Mater.
4 (2005) 597
[2] K. E. Lilja, T. G. Bäcklund, D. Lupo, T. Hassinen and T. Joutsenoja, Organic Electronics, 10
(2009) 1011-1014.
[3] B. J. L. Nilsson, U.S. Patent No. 7405775, 29 Jul. 2008
[4] K. E. Lilja, H. S. Majumdar, F. S. Pettersson, R. Österbacka and T. Joutsenoja, ACS Applied
Materials & Interfaces, 3 (2011) 7-10
[5] K. E. Lilja, H. S. Majumdar, K. Lahtonen, P. Heljo, S. Tuukkanen, T. Joutsenoja, M. Valden, R.
Österbacka and D. Lupo, Journal of Physics D: Applied Physics, 44 (2011) 295301
[6] K. E. Lilja, T. G. Bäcklund, D. Lupo, J. Virtanen, E. Hämäläinen and T. Joutsenoja, Thin Solid
Films, 518 (2010) 4385-4389
[7] L. S. Roman, M. Berggren, and O. Inganäs. Appl. Phys. Lett. 75 (1999) 3557
[8] A. Yuming, S. Gowrisanker, H. Jia, I. Trachtenberg, E. Vogel, R. M. Wallace, B. E. Gnade, R.
Barnett, H. Stiegler, and H. Edwards. Appl. Phys. Lett. 90 (2007) 262105
[9] K. Lilja, PRODI workshop, Munich, 2010
[10] P. Heljo, K.E. Lilja, S. Tuukkanen, D. Lupo, Proc. LOPE-C 2011
47
BIOGRAPHIC DATA OF PROFESSOR DONALD LUPO
Donald Lupo studied chemistry at Davidson College in North
Carolina, USA and gained his Ph.D. in physical chemistry at
Indiana University, Bloomington, IN , USA under the
supervision of Prof. George Ewing. Subsequently he worked as
a post-doctoral fellow in the Laboratory for Physical
Chemistrat the ETH in Zürich, Switzerland on IR laser
photochemistry in the group of Prof. Martin Quack. He
reinvented himself as a materials scientist after taking up a
position in central research at Hoechst AG in Frankfurt am
Main, Germany in 1986, where he worked in nonlinear optics
based on Langmuir-Blodgett films, polymer OLEDs and solid
state dye sensitised solar cells (DSSC) based on amorphous
organic semiconductors. At Sony International (Europe) GmbH
he built up the Materials Science Laboratory in Fellbach, Germany and continued his work on
polymer OLEDs and organic solar cells. At NTera Ltd. in Dublin, Ireland he was head of
display R&D for paper-like displays based on electrochromic nanomaterials. He then spent 8
years as a technology consultant working on pass printed electronics, roll to roll printable
displays and dye solar cells with companies such as UPM Kymmene, Merck and G24
Innovations. In 2010 he accepted a call to a professorship in electronic materials in the
Department of Electronics at Tampere University of Technology and joined the faculty in
August 2010. There he is responsible for activities in electronics materials and manufacturing,
with interests in printed diodes, transistors and solar cells and in the effect of printing
processes on materials, interfaces and devices.
48
CURVED AND FLAT AROMATICS: MULTITASK COMPONENTS IN
MOLECULAR MACHINES AND ELECTRONIC MATERIALS
Mateo-Alonso, A.
Dr. Aurelio Mateo-Alonso, 1Freiburg Institute for Advanced Studies, Albertstr. 19 and Institut
für Organische Chemie und Biochemie, Albertstr. 21, 79104 Freiburg, Germany
[email protected]
The design and synthesis of organic molecular and supramolecular materials based on curved
and flat aromatics will be discussed focusing on different applications such as; (i) the design
of fullerene-based molecular machines as models for artificial photosynthetic reaction centers
and property-switchable materials; (ii) the synthesis of oligo- and polyazaacenes for electronic
applications, their self-assembly into nanohybrids (with colloidal particles), and their use as
surfactants for carbon nanotubes.
References:
Molecular Machines and Applications: [1] Chem. Comm. 2010, 46, 9089-9099. [2] J. Am.
Chem. Soc., 2008, 130, 14938-14940. [3] J. Am. Chem. Soc. 2008, 130, 1534-1535. [4]
Angew. Chem. Int. Ed., 2007, 46, 8120-8126. [5] Chem. Comm., 2007, 1945-1947 [6]
Angew. Chem. Int. Ed., 2007, 46(19), 3521-3525. [7] Chem. Comm., 2007, 1412-1414. [8]
Org. Lett., 2006, 8(22), 5173-5176.
Oligo-, Polyazaacenes and Nanocarbons: [1] Chem. Comm. 2011, 47, 514-516. [2] Chem.
Comm. 2010, 46, 9122-9124. [3] Chem. Asian J. 2010, 5, 482-485. [4] Nature Chemistry,
2009, 1, 243-249. [5] J. Mater. Chem., 2009, 19, 4329-4324. [6] Chem. Eur. J., 2008, 14,
8837-8846. [7] J. Am. Chem. Soc., 2008, 130, 8733-8740. [8] J. Phys. Chem. A, 2007, 111,
12669-12673.
49
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57
Part IV:
OLEDs and ECDs
HIGHLY EFFICIENT ORGANIC LIGHT EMITTING DIODES
FOR LIGHTING APPLICATIONS
Björn Lüssem
Starting from a lab-curiosity, organic light emitting diodes have matured into a promising
technology that is beginning to enter commercial markets. OLED displays can be found in a
growing number of appliances such as mobile phones, mp3-players and TV-sets. For lighting
applications several companies are planning to launch first products in the near future.
For both applications, OLED displays and OLED lighting, a high efficiency is essential. The
external quantum efficiency Kq , defined as the ratio of the number of photons emitted from
the OLED to the number of electron/hole pairs injected into the device is given by [1]
Kq
JFKPL[ Kint[
(Eq. 1)
With J the charge carrier balance, F the singlet/triplet ratio, K PL the efficiency of radiative
decay of excitons, and finally [ the outcoupling efficiency. The first three factors are often
combined to the so called internal quantum efficiency Kint , which describes how efficiently
photons are generated inside the device. In state-of- the-art OLEDs, the internal quantum efficiency can be optimized by using highly efficient phosphorescent emitters and charge carrier
blocking layers, and internal quantum efficiencies close to 100% are possible. However, most
photons are trapped inside the OLED cavity, resulting in an outcoupling efficiency [ as low
as 20%. Thus, to increase the efficiency of OLEDs further, improvements in the outcoupling
efficiency are mandatory.
1. Increasing the Outcoupling Efficiency of Monochrome OLEDs
The low outcoupling efficiency is due to the different refractive index of the organic layers,
the glass substrate, and air. Light generated inside the OLED is reflected at the interface between the organic layers (including ITO) and the glass substrate, feeding so called
waveguided modes, and at the interface between glass substrate and air, feeding so called
substrate modes. Furthermore, the molecule can excite plasmons travelling along the metallic
electrode.
The strength of the different optical loss channels can be quantified by advanced optical
simulations [2]. The molecular emitter is modeled as emitting dipole which couples to the
different modes in the OLED cavity. Fig. 1 shows the distribution of loss channels for a red
OLED with varying thickness of the electron transport layer (ETL) [2]. The red OLED consist of BPhen:Cs as ETL with varying thickness, BAlq as hole blocking layer, NPB:Ir(MDQ)2
58
as red emission layer, Spiro-TAD as electron blocking layer, and MeO-TPD as hole transport
layer (Fig. 1a). The simulation results are compared to the experimentally determined external quantum efficiency, shown by black dots (Fig. 1b). The EQE agrees exceptionally well
with the simulation results (outcoupled photons, blue area). It shows an oscillation with the
ETL thickness, which can be explained by the resonance conditions of the OLED cavity – at
70nm the OLED shows a first resonance and at 250nm a second one.
(a)
(b)
(c)
Fig. 1: Different loss channels for a red OLED with varying thickness of the electron
transport layer on low index glass (a) and on high index glass (b) measured with an
outcoupling lense [2]
From Fig. 1b) it can be concluded that at low ETL thickness losses due to coupling to
plasmonic modes are most pronounced. As coupling to plasmons is a near field interaction, it
decreases with increasing distance of the emitter from the cathode, i.e. with increasing ETL
thickness. However, increasing the ETL thickness increases losses due to waveguided modes.
Thus, additional means of avoiding waveguided modes have to be applied to increase the
59
EQE significantly.
In Fig. 1c) one possibility to avoid waveguided modes is shown. The OLED is built on a substrate with an increased refractive index, which is matched to the organic layers. Thus, reflections at the glass substrate are avoided. To further increase the efficiency, an outcoupling
lense is applied to the glass substrate to avoid reflections at the glass/air interface. As can be
seen in Fig. 2c) the waveguided and substrate modes are fully avoided and the device reaches
an efficiency of beyond 50%.
2. Design of Highly Efficient White OLEDs with Increased Outcoupling Efficiency
The concepts shown in Fig. 1 have to be applied to white OLEDs. To obtain a well-balanced,
high quality white emission, the emission of a red, green and blue emitter has to be mixed. To
reach highest efficiency, all emitters should be phosphorescent emitters utilizing the singlet as
well as the triplet state.
In Fig. 2 the design of a highly efficient white OLED utilizing phosphorescent red, green and
blue emitters is shown [3].
(b)
(a)
Fig. 2: Design of a highly efficient white OLED (a). The luminous efficacy of devices
built on low index glass (LI), built on high index glass in the first resonance condition (HI1), and on high index glass in the second resonance condition (HI-2) is shown in (b) [3].
The device consists of an ETL (BPhen:Cs) with two different thickness (corresponding to the
first and second resonance), a hole blocking layer (TPBi), emission layers, an electron blocking layer (NPB) and a hole transport layer (MeO-TPD:NDP2). To increase the efficiency of
the OLED, it is built on low index and on high index glass (Device LI and devices HI). On
60
high index glass, the OLED is built with a small ETL thickness (corresponding to the first
resonance condition, device HI-1) and with a larger ETL thickness (devices HI-2 and HI-3).
The luminous efficacy or power efficiency is measured without any additional outcoupling
structure applied to the substrate (flat device), with a large outcoupling lense applied to the
substrate (half-sphere) and with a high index outcoupling structure consisting of small pyramids cut into the glass attached to the substrate (pattern).
The efficiency of these OLEDs is shown in Fig. 2b. As expected, the highest efficiency is
reached for the OLED on high index glass with thicker ETL. Using a large outcoupling lens,
more than 120 lm/W are obtained. Using a scalable outcoupling structure 90lm/W are
reached.
However, in this so-called all-phosphorescent approach the phosphorescent blue emitter is
prone to degradation and the devices have a lifetime of 1-2h only. An alternative to the allphosphorescent approach is the triplet-harvesting approach, in which the phosphorescent blue
emitter is replaced by a fluorescent emitter without losing efficiency.
3. White OLEDs using Fluorescent Blue Emitters: The Triplet Harvesting Approach
As discussed above, white OLEDs using phosphorescent blue emitters show highest efficiency but exhibit a low lifetime. The lifetime of fluorescent emitters is higher, but fluorescent
emitters only emit from their singlet state and all excitons formed in the triplet state are lost,
which significantly limits device performance.
A solution to this problem is the triplet harvesting approach. A fluorescent blue emitter with a
high lying triplet state is used. The triplet state exceeds the triplet state of the phosphorescent
red emitter, so that triplets formed on the blue emitter are transferred to the phosphorescent
emitter and are harvested there. Thus, in principle all excitons formed in the device are utilized and an internal quantum efficiency of 100% is possible.
61
(a)
(b)
Fig. 3: Stacked white triplet-harvesting OLEDs (TCTA: 4,4’,4’’ tris(N-carbazolyl)triphenylamine,
TPBi:
2,2’2’’-(1,3,5-benzenetriyl)-tris[1-phenyl-1H-benzimidazole],
Ir(dhfpy)2acac: bis(2-(9,9-dihexylfluorenyl)-1-pyridine) (acetylacetonate). Two OLEDs –
a yellow one and a blue/red triplet harvesting OLED – are stacked (a), so that a white
spectrum is reached (b). On high-index glass, 90lm/W are reached with a large
outcoupling lense [4].
The triplet harvesting OLED is shown in Fig. 3a [4]. The OLED is a stacked OLED consisting of a red/blue OLED (4P-NPD and 4P-NPD:Ir(MDQ)2(acac)) and a yellow OLED (having
an emission layer consisting of TCTA:Ir(ppy)3:Ir(dhfpy)2acac). Triplets formed on the blue
emitter 4P-NPD can be transferred to the red emitter Ir(MDQ)2(acac) and are harvested there.
The efficiency of this device is shown in Fig. 3b). For the flat device on low index glass a
luminous efficacy of ~30lm/W is reached. This value can be significantly increased if the
device is built on high-index glass and measured with a large outcoupling lense (90lm/W).
Using the same scalable outcoupling structure as for the all-phosphorescent OLED shown in
Fig 2, a luminous efficacy of 47lm/W is reached.
62
4. Conclusion
Current OLEDs reach internal quantum efficiencies of almost 100% and are mostly limited
by the low outcoupling efficiency. Therefore, to design highly efficient OLEDs, care has to
be taken to avoid plasmonic losses and losses due to waveguided modes.
To realize white OLEDs, the emission of at least 3 emitters has to be combined to obtain a
broad, white spectrum. Combining three phosphorescent emitters yields highest efficiency,
but the phosphorescent blue emitter is prone to degradation. However, in the triplet harvesting
approach the phosphorescent blue emitter can be replaced by a fluorescent one without losing
efficiency. Thus, triplet-harvesting could be a possible route towards highly-efficient and stable white OLEDs.
[1] R. Meerheim, B. Lüssem, and K. Leo, Proceedings of the IEEE 97, 1606 (2009).
[2] R. Meerheim et al., Applied Physics Letters 97, 253305 (2010)
[3] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, Nature 459, 234 (2009).
[4] T. Rosenow, M. Furno, S. Reineke, S. Olthof, B. Lüssem, and K. Leo, Journal of Applied
Physics 108, 113113 (2010)
63
BIOGRAPHIC DATA OF DR BJÖRN LÜSSEM
Björn Lüssem studied electrical engineering at the
RWTH-Aachen and the University of Bath and
obtained his degree as Diplom-Ingenieur in 2003.
He prepared his PhD thesis at the Research Center in Jülich, Germany in the field of molecular
electronics. His thesis concentrates on ScanningTunnelling Microscopy of pure and mixed selfassembled monolayers and has been awarded the
VDE-Promotionspreis and the Günther-LeibfriedPreis. After staying at the Materials Science Laboratory of Sony in Stuttgart from 2006-2008,
he joined Prof. Leo’s group at the TU Dresden, where he is now head of the Organic
Light Emitting Diodes group. His main interests are new semi-conducting
devices based on organic materials and their differences or similarities to
inorganic semiconductors.
64
ROLL-TO-ROLL PROCESSING OF FLEXIBLE OLED
FOR LIGHTING APPLICATIONS
C. MAY, S. MOGCK
Fraunhofer IPMS, Center for Organic Materials and Electronic Devices Dresden COMEDD,
Maria-Reiche-Straße 2, 01109 Dresden, Germany
OLEDs are considered as the second solid-state-lighting technology for new flat, large-area,
and efficient lighting solutions. OLED lighting on glass substrates has been successfully
started the market entry. Flexible OLED will open up new degrees of freedom in design. A
novel approach has been started to develop high efficient small molecule OLED stack
deposition on flexible plastic webs and metal strips in a roll-to-roll vacuum process Within
COMEDD such a roll-to-roll line based on vacuum deposition was successfully installed.
Beside the vacuum evaporation process the paper covers further process approaches like
substrate patterning, inert lamination and inspection issues. Monochrome devices processed
by roll-to-roll on “endless” substrates will be presented.
1. Introduction
With growing maturity and performance of OLED technology, the applications will range
from less demanding such as e.g. signage and decorative lighting up to large area flexible
illumination, automotive applications and general lighting with higher requirements in terms
of efficiency and reliability. At the moment first OLED prototypes for lighting attract a lot of
attention. In case of small molecules devices application of phosphorescent dopants and the
use of the p-i-n design resulted in a white OLED demonstration with efficiency over 90 lm/W
[1]. For significant penetration into the general lighting market, OLED technology will have
to meet or exceed the high standards with respect to energy efficiency, long lifetime and low
production costs that have been set by fluorescent and LED lighting. It is believed that the
price can drop significantly by transferring the batch fabrication into high throughput inline or
roll-to-roll processing (R2R) [2, 3, 4]. The higher throughput and the use of relatively cheap
metal foil and plastic web as substrates can be a major cost reducing step. In the present paper,
the general feasibility of the p-i-n top-emitting OLED design [4] application on flexible
aluminium foil is evaluated in a roll-to-roll process. In addition to the roll-to-roll OLED
vacuum deposition a substrate patterning and the OLED encapsulation process by foil
lamination under protective nitrogen atmosphere will be outlined briefly.
65
2. Roll-to-Roll process line
R2R vacuum deposition and fabrication of small molecule OLEDs on flexible substrates are
carried out in the RC 300-MB roll-to-roll vacuum coater (supplier Von Ardenne
Anlagentechnik GmbH [3], figure. 1). The machine enables processing of metal or plastic
substrates with a width of 300 mm and a thickness of 70 to 500 μm.
14 Linear Organic Evaporators
Lineare Ion Source
Interleaf Winder
Substrate Winder
EM B
EB L
EML
gree
B
EM
HT
L
HB
ET
Substrate with
3-color-white
Anodestack
OLED
layer
cathod
Port
for
Substrate
Inert
Load
2 Metal Evaporators
DC-Magnetron
Lock
Figure 1: Photograph of the RC 300-MB roll-to-roll vacuum coater (left), schematic cross
section (right).
Before the OLED vacuum deposition process can be started a nonconductive passivation layer
must be printed on the aluminium band, as seen in figure 2. The substrate patterning with a
passivation layer is necessary to isolate the anode (aluminium foil) from the metallization
(cathode and metal contact lines) after the OLED stack deposition.
During the organic- and metal evaporation a shadowing with integrated strip masks in the
deposition drum allow a proper patterning of devices. Finally, the processed OLED on
aluminium foil are pre-encapsulated by reactive magnetron sputtering of Al2O3.
After the pre-encapsulation process, the coated coil will be transferred under inert condition to
the encapsulation process performed in the coating- and lamination unit, as seen in figure 3.
66
Figure 2: Photograph of the gravure printing unit for the substrate patterning with a
passivation layer to separate electrically the electrodes of OLED devices.
Figure 3: Photograph of the inert box with integrated substrate patterning- and foil
lamination unit.
The roll-to-roll OLED fabrication process will be supported by an optical inspection system to
discover yield relevant defect issues. High efficient small molecule OLED lighting systems
coated in a vacuum process have a device thickness in sub-ȝm range. Therefore, particles with
a size < 1 ȝm can be critical to the device performance, like efficacy and yield. For this reason
a roll-to-roll optical inspection system is needed which fulfils the defect resolution
requirement in a sub-ȝm range. A winding unit with integrated inspection system of CCD line
scan cameras and a moveable optical microscope has the following inspection modes (see
figure 4) were installed, enabeling:
-
a 100% web inspection with CCD line scan cameras with a pixel resolution down to
14 ȝm.
67
-
a defect review mode for further analysis of defects detected by the 100% inspection.
-
automatic image recording on homogeneous and patterned web to determine the defect
density by image processing of the recorded images to reach a defect resolution in ȝm
range (resolution depends on the used objective).
-
layout recordings to identify patterned defects.
Figure 4: A photograph of the roll-to-roll optical inspection system for metal foil and plastic
films encaged in a clean room carbine of ISO6 to minimize particle contaminations during the
inspection.
The goal of all inspection modes is the quantification and identification of the defect types
which affects the device performance.
3. Results
Working green pin OLED devices with an active OLED area up to 60 x 60 mm² and preencapsulated with Al2O3 were realized on aluminium web as demonstrated in figure 5.
68
Figure 5: Electrical tests of 60 x 60 mm² green pin OLED devices on a aluminium band.
The comparison between a green p-i-n OLED with a 1st p-HTL (black curve, figure 6) and
2nd p-HTL (red and green curve, figure 6) shows one order of magnitude less leakage current.
Figure 6: A diagram of electro-optical characterisation of the green pin OLED devices on
aluminium foil. The 2nd maximum of the hole transport layer (HTL) allows stable OLED
devices. Thicker HTL layers can further planarize rough substrate surfaces to get
reproducible OLED devices (red and green curve).
69
4. Conclusions and Outlook
It was demonstrated that aluminium foils are suitable as substrates for fabrication of high
efficient p-i-n OLEDs in the R2R vacuum processing. Homogenous light emission from
OLEDs with areas on larger scale (60 x 60 mm²) and pre-encapsulated with an thin film Al2O3
layer deposited by reactive magnetron sputtering without significant reduction of the
luminance can be obtained. The R2R patterning process with a passivation layer is suitable to
fabricate 2- dimensional OLED lighting areas. R2R inspection techniques could be
successfully brought into operation which will enable to control the defect levels during the
process. In the future the reproducibility of the OLED R2R process needs to be further
improved using the R2R inspection. The R2R vacuum coater will be equipped with up to 14
organic linear evaporators to allow fabricating high efficient white p-i-n OLEDs. In parallel,
the development of bottom emitting OLEDs on plastic films is planned. The particle
monitoring of the R2R process line will be integrated during the R2R OLED process to setup
efficient defect prevention concept to obtain a stable yield.
5. Acknowledgements
This work was funded by the German Ministry of Education and Science within the project
R2flex (Project ref. 13N11058).
6. References
[1] S. Reineke, et al., White organic light-emitting diodes with fluorescent tube efficiency,
Nature, vol.459, (2009)
[2] C. May, et al., In-line deposition of organic light-emitting devices for large area
applications, Thin Solid Films, vol. 516, p. 4609, (2008)
[3] C. Deus, et. al., Technology and equipment for roll-to-roll processing of small molecule
OLEDs for lighting applications, 8th Int. Conf. on Coatings on Glass and Plastics, ICCG 2010.
Proceedings : June 13-17, 2010, Braunschweig, 2010, ISBN 978-3-00-031387-5, p.117-122
[4] P. Freitag et al: Novel Approaches for OLED Lighting, SID 2011 Digest of Technical
Papers, ISSN 0097-966X/11/4202-1067, p.1067
70
BIOGRAPHIC DATA OF DR CHRISTIAN MAY
Dr. Christian May, born 1967 studied Physical
Metallurgy at Freiberg University of Mining and
Technology. He received his PhD from same
university in 1999. From 1997 he was with Von
Ardenne Anlagentechnik, a supplier of high tech
vacuum equipment, as project manager dealing with
large area thin film deposition. Since 2003 he is with
Fraunhofer Institute of Photonic Microsystems
IPMS in Dresden. First he was responsible for the
development of fabrication technologies for large
area OLED lighting and organic solar cells. Since
beginning 2009 he is acting as Head of the Business
Units “Organic Materials and Systems” first and “Lighting and Photovoltaics” now. Since
2011 he is also deputy of the institute director of COMEDD – Center for Organic Materials
and Electronic Devices Dresden within Fraunhofer IPMS.
71
REALIZING NOVEL AND FUNCTIONAL LIGHT-EMITTING
ELECTROCHEMICAL CELLS
Ludvig Edman*
The Organic Photonics and Electronics Group, Department of Physics, Umeå University, 901
87 Umeå, Sweden.
*
[email protected]
Light-emitting electrochemical cells (LECs) offer a number of important advantages over
competing emissive technologies -- notably the utilization of air-stabile electrodes and very
thick active materials -- but the
critical drawback has been a
short operational lifetime. We
have set out on a quest to
resolve this problem and been
able to identify a number of
lifetime-limiting chemical [1]
and electrochemical [2] side
reactions.
By
following
motivated and straightforward
design principles to minimize
the
extent
of
these
side
reactions, we are now able to repeatedly realize LEC devices that emit with significant
brightness (>100 cd/m2) and good efficiency (>2 lm/W for red emission, >10 lm/W for green
emission) for several months of uninterrupted operation.[3,4] The figure to the right presents
the long-term operation of an optimized red-emitting LEC, and the inset shows a flexible LEC
with a similar promising device performance.
In another development, we have performed a parallel optical probing and scanning Kelvin
probe microscopy study on planar LEC devices during operation, and the acquired light
emission and potential profiles present irrefutable evidence for that electrochemical doping
takes place in-situ in the active material, and that a dynamic p-n junction structure can selfassemble in an LEC during operation.[5] Finally, we have conceptualized and demonstrated a
truly metal-free and “all-plastic” LEC device comprising a graphene cathode and a
conducting-polymer anode.[6] Both electrodes in this device architecture are transparent and
the light emission is accordingly omni-directional. Moreover, all parts of the device can be
processed from solution, which -- in combination with the elimination of expensive and/or
reactive metal materials -- promises to pave the way for a low-cost production of functional
light-emitting devices.
72
References
[1]
Wågberg, T., et al., Advanced Materials, 2008, 20, 1744.
[2]
Fang, J., et al., Journal of the American Chemical Society, 2008, 130, 4562.
[3]
Fang, J., et al. Advanced Functional Materials, 2009, 19, 2671.
[4]
Sandström, et al. Appl. Phys. Lett. 2010, 96, 053303.
[5]
Matyba, P., et al., Nature Materials, 2009, 8, 672.
[6]
Matyba, et al., ACS Nano, 2010, 4, 637.
73
MULTICOLORED ELECTROCHROMIC MODULES FOR ECD
APPLICATIONS
G. Nazmutdinova*1), H. Schache 1), M. Schroedner 1) and D. Raabe 2)
1)
2)
TITK Institute, Department of Physical Materials Research, Breitscheidstraße 97,
07407 Rudolstadt, Germany, e-mail: [email protected]
Neustraße 4, 07774 Dornburg-Camburg
Introduction
Electrochromic materials change their optical properties in response to an electric field.
The interest in these materials has increased in the last few years due to their potential
application on “smart-windows”, automobile mirrors, low-refractive materials in filters and
non-emissive displays. Widely used examples are tungsten oxide, viologens and conjugated
polymers as derivatives of poly(thiophene), poly(pyrrole) and poly(aniline). In fact, organic
electrochromic materials offer several advantages with respect to inorganics, not only in terms
of flexibility, easy of processing and low cost, but also with respect to both efficiency of
coloration and fine-tuning ability of the band gap (and the colour) through chemical structure
modification [1]. The structure of a classical electrochromic device is a typical multilayer
electrochemical cell, consisting of up to seven layers of materials, transparent to visible light.
The electrochromic material is coupled to an ion conductor, solid or liquid electrolyte and an
ion storage layer. These three optically transparent layers are sandwiched between two
conductors, at least one of them must be transparent. The resulting five layers are protected by
two transparent plastic or glass substrates. Alternatively, two electrochromic species can be
present in two symmetrically arranged layers, for such a case, cathodic and anodic coloration
processes are simultaneously driven.
Triphenylamine (TPA)-containing polymers have received considerable interest as
electrochromic [2] and hole-transport materials for use in organic electroluminescence
devices [3], because of their relatively high charge mobility and low ionization potentials [45]. The TPA radical cation is not stable and tends to dimerize rapidly forming
tetraphenylbenzidine (TPB) by tail-to-tail coupling. The last is more easily oxidized than the
starting TPA and undergoes further oxidation at the applied potential [6]. The coupling
reaction could be prevented by incorporating at the para-position of the phenyl group
electron-donating substituents [7-8]. So far, there had been done some attempts to introduce
TPA units into the main or side chain of the polymer backbone and in this way there have
been prepared new high-performance systems with novel optoelectronic functions [9-15].
74
Results
In this paper we report on a device based on an electrochromic non-conjugated copolymer
having alternating TPD (triphenylamine dimer) and diphenylxylylene units in the backbone
with good amorphous film forming properties and high glass transition temperature of Tg ~
240°C. The synthesis of poly[(4-methylphenyl)imino-4,4’-diphenylene-(4-methylphenyl)imino-1,4-phenylene-phenylmethylene-1,4-phenylene-phenylmethylene-1,4-phenylene]
(poly-TPD (4Me)-DPX) polymer is described in [16-17]. The structural formula with the
oxidation scheme is presented in the Fig.1.
CH3
CH3
N
N
CH3
CH3
N
+
N
+
-2e
CH
CH
n
2 steps
CH
CH
n
Fig.1 Electrochemical oxidation of poly-TPD(4Me)-DPX.
The oxidation is carried out in two steps with consistently oxidation of amino groups and
creating radical cations and dications.
The solid-state electrochromic device was constructed using the sandwich structure FTO/
TPD(4Me)-DPX/gel-electrolyte/ion storage layer/FTO. The poly-TPD(4Me)-DPX film was
spin coated from solution to a thickness of 300 nm. The gel-electrolyte, that enabled the
charge transport between the electrochromic and the charge balancing counter electrode
consisted of the salt lithium-bis(trifluoromethylsulfonyl)imide (LiTf2N), the plasticized
polymer poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) and the ionic liquid
1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imid (EMITf2N). The mixture of
cerium and titanium oxides was fabricated by a sol-gel process and used as ion storage layer.
The spin-coated film of the sol with subsequent annealing shows a transmission of about 70
% in the wavelength range from 500 nm up to 1100 nm. All device fabrication was performed
in an inert atmosphere glove box to minimize exposure to water and oxygen.
The optical and electrochemical properties of EC-devices were investigated by UV-vis
spectroscopy and cyclic voltammetry. Fig.2a shows the cyclic voltammogram of the polymer
solution with two oxidation peaks because of successive forming radical cation and dication.
Anodic oxidation was cycled over 50 cycles without noticeable change. In the polymer film
(Fig.2b) the two peaks substantially merge together to form one broad asymmetrical peak.
75
a)
0.00075
6e-5
0,80 V
1,05 V
0.00050
I (Amps)
7e-5
Eox1 =
Eox2 =
5e-5
2
I (Amps/cm )
4e-5
3e-5
0.00025
0.00000
2e-5
1e-5
-0.00025
0
-1e-5
-0.00050
-1.0
-2e-5
-4e-5
0.0
-0.5
0.0
0.5
1.0
1.5
E (Volts)
-3e-5
0.2
0.4
0.6
0.8
1.0
1.2
b)
1.4
E (Volts)
Fig.2. Cyclic voltammograms (a) of poly-TPD(4Me)-DPX in methylene chloride solution, c =
0.005 mol/l, Bu4NPF6, c = 0.1 mol/l, 50 cycles, CE: Pt-disc electrode and (b) of EC-device
from Fig. 3, scan-rate: 15 mV/s, T: 25°C.
The EC-device shows multicolor electrochromic behaviour with color change from neutral
colorless to orange and then through transient green to blue (Fig.3).
Fig. 3. Solid-state multicolor electrochromic device with poly-TPD(4Me)-DPX film, gelelectrolyte PVDF-HFP, LiTf2N in EMITf2N and a CeO2/TiO2 film as ion storage layer.
76
Electrochromism of the device was monitoried by a UV-vis spectrometer at different applied
potentials. The device is ideally colorless in the whole visual optical range with a
transmission of 70 %. When the applied potential increased positively from 0 to 0.45 V, the
film turned into orange to give an absorption band at 475 nm.
80
70
orange form
Transmission (%)
60
50
40
30
20
blue form
10
0
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig.4. Transmission spectra of EC-device from Fig.3 in the neutral (black curve) and two
oxidized states, correspondingly at E=0.45 V (orange curve, Ȝ=475 nm) and at E=1.1 V (blue
curve, Ȝ=770 nm).
Further raising the voltage up to 1.1 V resulted in a colour change of the device into blue
accompanying the increase of absorption around 770 nm. The electrochromic characteristics
of the EC-device from Fig.3 are summarized in Table 1. For a device, with an active
electrochromic area dimension of 3.8 cm x 4.2 cm (15.96 cm² ) the time for switching from a
clear state of 74.5 % of transmission to, for example, a blue state of 22% was measured to be
7.5 s and the reverse switching was completed within 6 s at room temperature (25°C) (Fig. 5).
Colour switch was uniform along the whole surface of the device. Switching time was
obtained by means of applying potential steps between +1.1 V and –0.6 V, and the switching
speed was defined as the time necessary to complete 88 % of the total transmission change
recorded at 770 nm. Optimal devices were reversible during 10000 switching cycles with
negligible change of the coloration efficiency.
77
Table 1. Electrochromic behaviour of the EC-device from Fig.3
Characteristic
colorless/orange
colorless /blue
(Ȝ=475 nm)
(Ȝ=770 nm)
21
52
0.17
0.54
Ș = log (TBleaching/T Colouring) /Q
387
340
Switching time, T (sec)
~10
~10
electrochromic contrast, ǻ%T
optical density
OD = log (TBleaching/T Colouring)
Coloration efficiency (cm2/C)
80
70
Traansmission [%]
60
50
40
30
20
10
0
0
100
200
300
400
500
600
700
800
900
1000
t [s]
0,0020
D, colourless(-0,6 V)/blue (+1,1 V)
2
I(A/cm )
0,0015
0,0010
0,0005
0,0000
-0,0005
-0,0010
0
100
200
300
400
500
600
700
Time (s)
Fig. 5. Changes of optical transmission monitored at 770 nm (above) and electrode current
(below) as a function of time for EC-device from Fig. 3 under application of repetitive pulse
voltage of +1.1 V and –0.6 V.
78
Acknowledgement
Financial support from Federal Ministry for Economy and Technology BMWi (project
IW071048 and MF110097) is gratefully acknowledged.
References
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2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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A. Ito, H. Ino, K. Tanaka, K. Kanemoto und T. Kato, J. Org, Chem., 2002, 67, 491-498.
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2007, 2004-2014
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79
BIOGRAPHIC DATA OF DR GULNARA NAZMUTDINOVA
Gulnara Nazmutdinova was born in Kazan, Russia in 1967. She studied chemistry at the
Kazan State University (at present Kazan (Volga Region) Federal University).
Her Ph.D. work was performed under the supervision of Professor
Alexey V. Zakharov and major research scientist Ph.D. Valery G.
Shtyrlin (Kazan, Russia). From 1995-2000 she worked as scientist
at the Laboratory of Coordination Compounds (Kazan State
University, Russia) researching the complex formation and
chemical exchange in solutions of the transition metals complexes
as well as the biological activity of the metals complexes.
In 2000 she moved to the Thuringian Institute for Textile and Plastics Research (TITK,
Rudolstadt, Germany) and worked as a visiting scientist. Since 2002 she is a staff scientist at
TITK.
Her current research interests cover the polymer electrolytes (for dye-sensitized solar cells,
electrochromic displays and Li-batteries), ion-storage layers as well as the optical and
electrochemical properties of polymers, with special focus on electrochromic systems.
80
Part V:
Materials and
technologies 2
PHOTOGENERATION AND ULTRAFAST DYNAMICS OF EXCITONS
AND CHARGES IN POLYMER/FULLERENE/QUANTUM DOT
BLEND FILMS
Laurens D.A. Siebbeles
Optoelectronic Materials Section, Department of Chemical Engineering,
Delft University of Technology, Delft, The Netherlands
email: [email protected]
Summary
Photogeneration and decay of excitons and charges in the conjugated polymer poly(3hexylthiophene) (P3HT) with PCBM or PbS quantum dots as electron acceptors were studied
with ultrafast laser spectroscopy. Insights in the mechanism of charge carrier formation (via
carrier multiplication) are discussed.
‡
Ǧ
Ϊ
Š
Introduction
Thin blend films of a conjugated polymer and a fullerene derivative are of interest for
application in low cost photovoltaic cells. To date, some of the best efficiencies (near 5%)
have been reported for devices based on a thin blend film of the conjugated polymer
regioregular poly(3-hexylthiophene) (P3HT), and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) as electron acceptor. However, the nature and dynamics of photoexcitations in P3HT
and blend films with PCBM have been subject of debate.
The present work provides insights about the nature of primary photoexcitations in P3HT
and blend films with PCBM and or PbS quantum dots (QDs). QDs are of interest for
application in high efficiency solar cells in which absorption of a single photon leads to
generation of multiple excitons via carrier multiplication.
The photogeneration quantum yield and dynamics of charge carriers and excitons were
studied with ultrafast optical pump-probe spectroscopy and time-resolved terahertz or
microwave conductivity measurements.
Discussion
In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to
0.15 per absorbed photon.[1] The remaining fraction of absorbed photons leads to formation
of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial
value on a timescale of 100 ps followed by decay to a no longer observable yield after 1 ns.
Addition of 50% PCBM by weight, leads to ultrafast (<200 fs) formation of charge pairs with
a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an
order of magnitude faster than in neat P3HT, which is expected to be at least in part due to
interfacial exciton dissociation into charge carriers. The yield of charges in the blend has
decayed to about half its initial value after 100 ps, while no further decay is observed within 1
ns. The small fraction (~1%) of excitons in neat P3HT that is probed by photoluminescence
measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of
81
non-fluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2×10-4 cm2/s, which is an order of
magnitude smaller than reported values for fluorescent excitons. The interaction radius for
second order decay of photoexcitations is as large as 8-17 nm, in agreement with an earlier
result in the literature.[2]
The quantum yield for photogeneration of charge carriers in P3HT:PCBM blends was
found to be virtually temperature independent for timescales up to tens of nanoseconds after
photoexcitation of P3HT.[3] This implies that a description of charge generation on basis of
the Onsager-Braun model with an initial electron-hole distance of the order of nanometers is
inadequate. Factors that can give rise to negligible temperature dependence include coupling
of the initially hot exciton with excess vibrational energy, the high driving force for electron
transfer from the polymer to PCBM, the high dielectric constant along the polymer chain
direction and charge delocalization. The decay of charges due to recombination and/or
trapping on longer times becomes faster at higher temperature as a result of thermally
activated electron and hole mobilities.
Extraction of charges from multi-excitons in PbS quantum dots (QDs) was studied for
blend films with P3HT and/or PCBM. Photoexcitation of the QDs in a blend with P3HT
yielded no observable terahertz (THz) photoconductivity. However, for blends with P3HT
and PCBM a significant THz photoconductivity is observed within 1 ps after photoexcitation
of the QDs. The charges hardly decay on a ns timescale. Multiple-charges could be extracted
from multi-excitons in the QDs. This is of great promise for development of highly efficient
solar cells based on carrier multiplication.
Conclusions
Photoexcitation of P3HT leads to significant generation of free charges in addition to
excitons. Exciton dissociation at an interface with PCBM leads to ultrafast charge generation
with an initial yield that is virtually independent of temperature. Multi-excitons in PbS QDs
can be dissociated into free charges in blend films with P3HT and PCBM.
References
[1] J. Piris, T.E. Dykstra, A.A. Bakulin, P.H.M. van Loosdrecht, W. Knulst,
M.T. Trinh, J.M. Schins, L. D.A. Siebbeles, J. Phys. Chem. C, 113, 14500, 2009.
[2] A. Ferguson, N. Kopidakis, S.E. Shaheen, G.J. Rumbles, J. Phys. Chem. C, 112,
9865, 2008.
[3] W.J. Grzegorczyk, T.J. Savenije, T.E. Dykstra, J. Piris, J.M. Schins and
L.D.A. Siebbeles, J. Phys. Chem. C, 114, 5182, 2010.
82
BIOGRAPHIC DATA OF PROFESSOR LAURENS D. A. SIEBBELES
Laurens Siebbeles (1963) studied chemistry at The
Free University in Amsterdam and obtained his
PhD degree at the FOM-institute for Atomic and
Molecular Physics in Amsterdam. He was a postdoc at the University of Paris Sud in France.
Currently he is head of the opto-electronic
materials section at the Delft University of
Technology in The Netherlands. He studies the
dynamics of charges and excitons in organic materials and semiconductor nanocrystals.
Charges and excitons are produced with high-energy electron or laser pulses and probed by
time-resolved optical and microwave or terahertz measurements. The experiments are
supported by theory of charge and exciton dynamics.
83
POLYMER SOLAR CELLS BLENDED WITH SILICON NANOWIRES
S. Sensfuss1*; H. Schache1, B. Eisenhawer2, G. Andrae2, M. Pietsch2, S.
Shokhovets3, M. Himmerlich4, E. Klemm5, M. Kroll6, T. Pertsch6
1
TITK Rudolstadt, Dept. Functional Polymer Systems and Physical Research, Breitscheidstr.
97, D-07407 Rudolstadt, Germany, *e-mail: [email protected]
2
Institute of Photonic Technology, Dept. Photonic Silicon, Albert-Einstein-Str. 9, D-07745
Jena, Germany
3
Technical University of Ilmenau, Institute for Physics, Weimarer Str.32, D-98684 Ilmenau,
Germany
4
Technical University of Ilmenau, Center for Micro and Nanotechnologies, GustavKirchhoff-Str. 7, D-98693
Ilmenau, Germany
5
Jenpolymer Materials Ltd. & Co. KG, Wildenbruchstr.15, D-07745 Jena, Germany
6
Friedrich Schiller University Jena, Institute of Applied Physics, Max-Wien-Platz 1, D-07743
Jena
Solar cells based on conjugated polymers can be processed from solution or dispersion which
offers a very important technological potential for low-cost fabrication using high-volume
processes like reel to reel technologies. The main limitations of polymer solar cells are
assigned to the absorbance only of a small part of the solar spectrum and the limited charge
carrier mobility, which restricts the possible film thickness of the organic photoactive layer to
below 300 nm. Hybrid inorganic-organic solar cells may be promising candidates to
overcome these limitations and became a subject of increasing interests during the past few
years. The combination of semiconducting polymers with inorganic nanoparticles remain the
advantage of solution processing while benefiting from a broader absorption and/ or a better
charge transport. The application of silicon nanowire (SiNW) structures in solar cells offer the
chance to use their extraordinary light trapping capabilities resulting in an outstanding light
absorption, significantly reduced reflections, superior charge carrier transport properties and a
clearly increased p-n junction area.
Here we report about P3HT:[60]-PCBM polymer solar cells blended with semiconducting or
highly n-doped silicon nanowires in the absorber layer. The silicon nanowires were prepared
by chemical vapour deposition or silver catalyzed electroless etching into silicon wafers
(Fig.1). They were removed from the initial substrate by sonication in chlorobenzene. For first
hybrid solar cells different amounts of the SiNW dispersion were added to the P3HT:[60]PCBM
blend
using
the
usual
device
P3HT:PCBM:SiNW/ Al (Fig.2).
84
architecture
glass/
ITO/
PEDOT:PSS/
Fig.1a Silicon nanowires (SiNW) prepared
by chemical vapour deposition (CVD)
catalyzed by gold colloids
Fig.1b SiNW obtained by silver catalyzed
electroless etching into silicon wafers
Al
P3HT:[60]PCBM
n-doped
Si nanowire
PEDOT:PSS
ITO
glass
h.Q
Fig.2 Device architecture of polymer solar cells blended with SiNW
Presently, the devices with SiNW reached a relative efficiency improvement of about 10 %
compared with the reference cell, which is attributed to a reduced series resistance and an
increase in the short circuit current (Tab.1, Tab.2, Fig.3).
Tab.1 Device properties of glass/ ITO/ PEDOT:PSS/ P3HT:PCBM (1:0.8 w/w):SiNW/ Al
polymer solar cells blended with different amounts of heavily n-doped etched SiNW
(length 3-5 μm)
Isc
Voc
[mA/cm²] [mV]
sample
reference cell (without SiNW)
60wt.-% photoactive solution + 40wt.-% SiNW suspension
50wt.-% photoactive solution + 50wt.-% SiNW suspension
30wt.-% photoactive solution + 70wt.-% SiNW suspension
with spin-coated SiNW (200 rpm)
9.79
9.98
10.45
10.25
9.33
641
639
638
637
635
FF
0.60
0.64
0.62
0.63
0.66
Rs
K AM1.5
[% ] [: cm²]
3.77
4.08
4.13
4.11
3.91
5.78
3.17
3.17
2.81
2.46
The external quantum efficiency is slightly increased with SiNW, but up to now there is no
spectral contribution of the silicon to the photocurrent realizable (Fig.4). The detailed
contribution of the SiNW is not really clarified, we just can exclude an optical effect of light
scattering for which the nanowire density is still too low. Scanning electron microscopy
85
images show the problem of a too low nanowire density with an inhomogeneous distribution
in the P3HT:PCBM film (Fig.5).
reference cell (without SiNW)
60wt.-% photoactive solution + 40wt.-% SiNW suspension
50wt.-% photoactive solution + 50wt.-% SiNW suspension
30wt.-% photoactive solution + 70wt.-% SiNW suspension
with spin-coated SiNW (200 rpm)
6
4
current [mA/cm²]
2
0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
-2
-4
-6
-8
-10
-12
bias [V]
Fig.3 I-V characteristics of glass/ ITO/ PEDOT:PSS/ P3HT:PCBM (1:0.8 w/w):SiNW/ Al
polymer solar cells blended with different amounts of heavily n-doped etched SiNW
(length 3-5 μm)
Tab.2 Device properties of glass/ ITO/ PEDOT:PSS/ P3HT:PCBM (1:0.8 w/w):SiNW/ Al
polymer solar cells blended with different amounts of moderately n-doped etched
SiNW (length 3-5 μm)
sample
Isc
[mA/cm²]
Voc
[mV]
FF
K AM1.5
[% ]
Rs
[: cm²]
reference cell (without SiNW)
50wt.-% photoactive solution + 50wt.-% SiNW suspension
30wt.-% photoactive solution + 70wt.-% SiNW suspension
with spin-coated SiNW (400 rpm)
9.77
10.08
10.13
10.31
626
634
622
621
0.65
0.65
0.63
0.65
3.98
4.15
3.97
4.16
2.96
2.70
2.75
3.12
Fig.4 External quantum efficiency of a P3HT:PCBM solar cell with spincoated SiNW
(moderately n-doped) on the PEDOT:PSS layer in comparison to the reference cell
without SiNW (see Tab.2)
86
Fig.5 SEM images showing a still too low SiNW density in P3HT:PCBM films with an
inhomogeneous distribution and formation of SiNW agglomerates
Further optimization is needed to reach a higher nanowire density in the photoactive layer
with reduced NW agglomeration.
Initial XPS/UPS measurements were carried out to investigate the band alignment at the
polymer/silicon interface. First inverted P3HT:[60]-PCBM cells were prepared on planar Si
wafers using heavily n-doped silicon as bottom and PEDOT:PSS/ Au (7 nm) as transparent
top electrode. The insert of an a-Si layer between the n+-Si and P3HT:[60]-PCBM film seems
to reduce recombination and improves the Voc and fill factor of the cells. Without an a-Si
interlayer both parameters are dropping down (Tab.3).
Tab.3 Device properties of inverted P3HT:[60]-PCBM cells using planar heavily n-doped
silicon wafers as bottom and PEDOT:PSS/ Au (7 nm) as transparent top electrode with
different types of amorphous silicon (a-Si) interlayers
K AM1.5 [% ]
sample
Isc [mA/cm²] Voc [mV]
FF
n+ c-Si
n+ c-Si
n+ c-Si
n+ c-Si
n+ c-Si
n+ c-Si
/
/
/
/
/
+
n a-Si:H (50nm) / i-a-Si:H (5nm)
n+ a-Si:H (50nm)
n+ a-Si:H (25nm)
n+ a-Si:H (30nm) / i-a-Si:H (5nm)
n+ a-Si:H (10nm) / i-a-Si:H (5nm)
3.53
3.05
4.14
4.39
3.85
4.54
352
617
603
604
604
613
0.44
0.69
0.65
0.66
0.60
0.65
0.54
1.30
1.62
1.75
1.40
1.81
The EQE of these non-optimized cells amounts to ~30% in the peak, but does not clearly
show a contribution of the silicon to the photocurrent up to now. Of course, the aim of further
efforts is to be able to use the advantages of silicon (SiNW) and semiconducting polymers.
Acknowledgement
Financial support from the Federal Ministry of Education and Research (BMBF project No.
03SF0333C) is gratefully acknowledged.
87
BIOGRAPHIC DATA OF DR STEFFI SENSFUSS
Steffi Sensfuss received the diploma in Organic Chemistry
in 1983 and a doctorate degree in 1987 in Polymer
Chemistry under the mentorship of Dr. Elisabeth Klemm in
the group of Prof. Hörhold at the University of Jena. During
this time the main research activities had been in the
development of optical adhesives and dental materials based
on thiol/ene polyaddition, epoxy and acrylate polymers.
Since 1998 she is working at the Thuringian Institute for
Textile and Plastics Research in Rudolstadt in the group of
Prof. Roth and Prof. Heinemann, respectively. The main
research activities are actually in the field of applied
research of conducting polymers. Presently she heads the activities of TITK group for
polymer solar cells, hybrid solar cells, polymer gel electrolytes and textile-based dyesensitized solar cells.
88
PRINTED ELECTRONICS BASED ON SOLUTION
PROCESSABLE NANOWIRES
M. SHKUNOV*, C. OPOKU
Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK
Solution-based assembly of thin-film field-effect transistors (FETs) using semiconducting
inks at low temperatures on large areas, and on optically transparent substrates are finding
potential applications in many areas including: sensors, RFID tags[1], memory elements[2], and
flexible
display
applications.[3] Conventional hydrogenated silicon[4] (a-Si:H)
and
polycrystalline silicon (poly-Si) TFTs have limitations due to a trade-off between process
temperatures and performance on transparent/flexible substrates.[5,
6]
Organic field-effect
transistors (OFETs) are gaining popularity as excellent material candidates, but are limited by
their low carrier mobility, sensitive to semiconductor morphology and process conditions.[7]
Single-crystalline semiconducting nanowires (NW) are offering potential breakthrough in the
area of high performance, low cost device assembly due to their proven high charge carrier
mobility and compatibility with solution based assembly techniques.[8]
Small size of
nanowires, typically tens of nanometres in diameter and 10 to 40 micron length allow these
nanomaterials to be dispersed in solvents and processed as inks at room temperature to
‘bridge’ typical device electrodes. Significant advances in NW synthesis can now offer large
quantities of high purity NW.
Most reports of high performance NW FETs have in the past dealt with inorganic
dielectrics that are incompatible with solution assembly techniques.[8] Recently, organic based
dielectrics such as poly(methyl methacrylate) (PMMA)[10], polyimide[11], and self assembly
nanodielectrics (SAND)[12, 13] have been identified as alternative insulators for NW-based
FETs that can be processed at low temperatures. These devices are hybrid where organic and
inorganic components are integrated in a single FET system.[12]
In this report we investigate solution processable silicon nanowire based FETs with both
polymer dielectric and a typical SiO2 dielectric.
Si NWs used in this study were synthesised via the supercritical fluid-liquid solid
method (SFLS)[9]. FETs have been fabricated as follows: several bottom gate (BG) devices
were constructed on n++-Si substrates with a 230nm thermally grown SiO2. Hybrid top gate
(TG) FETs with organic dielectrics were fabricated on glass substrates.
As a first device fabrication step anisole suspended Si nanowires were deposited onto
cleaned substrates by solution coating. Then metal source and drain (s/d) contacts (Au/Cr
89
(100nm/2nm) were patterned on top NWs by the standard lift-off photolithography. After
metallisation, substrates were annealed at ~170oC for 30 minutes. This step completed the BG
FETs
preparation. Extra steps were required to define organic gate insulator and gate
electrode in the hybrid devices, and this was achieved by spin coating ~1μm thick dielectric
followed by shadow mask evaporation of gate electrode (Au, 60nm). Electrical transport
measurements were carried out using Keithley 4200 characterisation system.
Transfer (ID-VG) and output (ID-VD) characteristics for a representative BG Si NW-array FET
with SiO2 dielectrics are shown in Fig. 1a-b. For this device, the on/off ratio is ~107. Notably,
influence of high contacts resistance can be seen in the output scans in Fig. 1b. This can be
explained by the presence of surface layers on Si NWs[9], or energetic barriers resulting from
unoptimised s/d contacts.
The clear increase in Ion with VG is indicative of good gate-channel modulation. The turn-on
voltage (VO), subthreshold swing (s-s), and transconductance (gm) at VD = -6V are extracted
as ~7.7V, 1.4V/dec, and 0.2μS respectively.
10
-6
10
-7
10
-8
10
-9
10
-10
10
-11
10
-12
10
-13
10
VD [V] = -4, -6
7
On/off ratio: 5x10
s-s: 2.1V/dec
gm: 0.2PS
Vo: 7.7V
(2)
'Vo
'Vo: ~27V
(1)
-60
-40
-20
0
20
Gate voltage (V)
b)
8μ
6μ
4μ
2μ
Source current (A)
-5
Source current (A)
Source current (A)
a)
8μ
(-5V step)
6μ
4μ
2μ
0
0
40
VG [V] = 10 to -40V
-10
-8
Figure 1 Transfer (ID-VG) characteristics at VD = -5V and -10V
the same device at VG from 10V to -40V.
-6
-4
-2
Drain voltage (V)
0
b) Output (IS-VD) scans for
A significant shift in VO ('VO or hysteresis) is observed upon forward (denoted as 1) and
reverse sweep (denoted as 2) of the gate voltage. The shift also appears to be independent to
the value of VD. Adsorption of polar species such as water molecules on SiO2 is known to
cause hysteresis effects in carbon nanotubes FET and also in NW FETs.[14] Since our BG
transistors have been measured in a dry N2 atmosphere one would expect the influence of
moisture to be minimal. On the other hand, trap states at the interface between NW and SiO2
cannot be easily eliminated. Wang et al[15] have shown that interface state in ultra high
density Si NWs constructed as FETs can be reduced by forming (5% H2 in N2) gas anneal at
90
470OC. However, such high temperature processes are impractical in low temperature
solution assembly approaches described here. Dielectrics with low trap states are therefore
highly attractive for the realisation of high performance NW-based FETs that can be
processed at relatively low temperatures.
Due to the cylindrical nature of NWs, extraction of the gate capacitance requires special
treatment to account for the electrostatic fringing between individual NWs that constitute the
active channel. The capacitance is obtained using Eq. 1 which describes the capacitance for
individual NWs according to cylinder on infinite plate model[16]
‫ܥ‬ேௐ ൌ ଶSఌ೚ ఌ೔ ௅
௖௢௦௛షభ ቀ
(1)
ೝశ೏
ቁ
ೝ
where Ho, H represent the permittivity of free space and the dielectric constant (~3.9) of the
gate dielectric (SiO2 in this case); r (= 5nm) is the average radius of individual NWs, d is the
dielectric thickness (230nm) and L (= 10μm) is the channel length. Accordingly, the total
capacitance (CT = N x CNW) is then obtained by multiplying the number of NWs (N) in the
FET channel. Using Eq. 1 and the parameters of the BG device structure, CNW is
approximated as 0.62fF, and the total channel capacitance (CT) for the 5NWs in the
representative BG is estimated to be ~3.1fF.
With the estimated channel capacitance CT, the field effect mobility can be calculated for the
representative BG device using the Eq. 2 in the linear regime
݃௠ ൌ
ஜ஼೅
௅మ
ܸ஽
(2)
where gm (~0.2μS at VD = -6V) is the transconductance, defined as ˜IS/˜VG, μ is carrier
mobility (holes in the present case) and VD of the drain voltage bias. Using Eq. 2 and the
measured parameters for the device we estimate the field effect hole mobility to be
~11cm2/V-s which is noticeably higher than that of typical organic FETs.
The transfer and output scans for a representative hybrid TG Si NW FET with organic
gate insulator are shown in Fig. 2. Despite much thicker dielectric layer devices demonstrate
excellent transistor characteristics and also offer significantly reduced hysteresis. From the
transfer scans (Fig. 2a) for this device, we extract the current on/off ratio, s-s, and gm as ~106,
1.6V/dec, 0.2μS respectively. High on/off ratio exhibited by the device suggests that the gate
91
can effectively modulate the NW conductance even with a 1μm thick organic dielectric. The
peak currents at VD = -10Vand -15V are 1.9μA and 2.3μA.
-5
10
-6
10
-7
10
-8
10
-9
10
-10
10
-11
10
-12
Hybrid TG Si NW-array FET
VD [V]: -5, -10, -15
(2)
106
On/off :
VO: -0.8V
s-s: 1.6V/dec 2μ
gm: 0.2PS
(@ -10V)
1μ
(1)
-40
-20
0
Gate voltage (V)
20
b)
3μ
Source current (A)
10
Source current )A)
Source current )A)
a)
VG [V] = 5V-40V
(in -5V steps)
1.0μ
0.0
0
Hybrid TG Si NW-array TFT
2.0μ
-10
-8
-6
-4
Drain voltage (V)
-2
0
Figure 2 Transistor characteristics for a representative hybrid TG Si NW-array FET a)
Transfer characteristics for VD = -10V and -15V. b) Output scans for the same device
measured from VG = 5V to -40V in -5V steps.
Hole mobility is extracted as follows: the TG channel length and NW density (N) are 10μm
and 3NWs respectively. Thus CNW based on the cylinder on infinite plate model for a 1μm
thick organic film with low dielectric constant is estimated to be ~0.24fF. So for the channel
capacitance of the representative hybrid TG device containing just 3 NWs is ~0.72fF. Using
the extracted gm at VD = -10V, μ is calculated to be ~28cm2/V-s, which is substantially higher
than that attained in the BG FET with SiO2 dielectric.
From Fig. 2a the subthreshold slope is estimated as ~1.6V/dec, which suggests low density of
interface traps at nanowire/organic dielectric interface during consecutive gate sweeps. Of
particular relevance is the hysteresis loop ('VO ~9V) in the transfer scans which is
considerably smaller than that of the BG device in Fig. 1a. The relatively small hysteresis may
be attributed to the low density of localised states at the semiconductor-dielectric interface.
The output scans (Fig. 2b) also show excellent gate modulation. The device exhibits some
contact resistance (at VD < 2V) which can be explained by the non ohmic contact formation.
In summary we have demonstrated hybrid top-gate nanowire field-effect transistors
where the active channel is composed of Si nanowires. The device deposition methods are
compatible with solution based fabrication techniques envisioned for printed plastic
92
electronics. The devices with organic dielectric show excellent transistor characteristics with
on/off ratio for106, low hysteresis, field effect mobility ~28cm2/V-s.
The high mobility, and the small hysteresis exhibited by the hybrid TG Si NW-array FETs
make solution processable nanomaterials very attractive candidates for printed electronics.
Acknowledgements
The authors thank EPSRC UK for the provided support CASE/CNA/07/79 and grant
EP/I017569/1.
References:
93
BIOGRAPHIC DATA OF DR MAXIM SHKUNOV
Maxim
Shkunov
studied
physics
and
applied
mathematics at Moscow Institute of Physics and
Technology and trained at Russian Academy of Sciences
(Moscow) prior to receiving his PhD in condensed matter
physics from the University of Utah, USA, where he
conducted research in ultrafast spectroscopy and laser
action in conjugated polymers, microcavities and
photonic crystals. He has since developed and evolved his
research to encompass charge transport phenomena in
organic semiconductors, plastic field-effect transistors,
self-assembly
at
organic/inorganic
interfaces,
self-
organisation with pi-conjugated liquid crystals and polymers, semiconducting nanowires,
solution processable nanowire electronics and organic-inorganic hybrid devices. He has 20
years of experience in the field and worked at Cavendish Laboratory (U. Cambridge) and
Merck Chemicals (UK). Maxim (co)-authored more than 90 publications, including articles
in peer reviewed journals, book chapter and patents. He is now a Lecturer in Nanoelectronics
at the Advanced Technology Institute, University of Surrey (UK) with a range of research,
teaching, and administrative duties.
94
ORGANIC AND HYBRID ORGANIC HETEROJUNCTIONS IN
ORGANIC ELECTRONICS AND SPINTRONICS APPLICATIONS
S. Braun,1 L.M. Andersson,1 P. Sehati,1 Y.Q. Zhan,1,2, M.P. de Jong,3 M. Fahlman1*
1
Department of Physics, Chemistry and Biology, Linköping University, Sweden.
2
Microelectronics Department, IT School, Fudan University Shanghai, China
3
MESA+ Institute for Nanotechnology, University of Twente, Enschede, the Netherlands
E-mail: [email protected]
Organic electronic and spintronic devices such as solar cells and spin valves are multi-layered
devices where their ultimate performance is to a large extent dominated by the electronic
processes at interfaces. The relative position of energy levels across a stack of thin organic
layers is important for charge/spin injection and exciton separation, and hence for device
engineering and optimization. Here we will present some recent results on P3HT and PCBM
where we explore the effects of inter- and intra-molecular order at the interface on the EICT+,and how these parameters affect exciton dissociation and charge transport [1]. In the latter case
P3HT, both in a well ordered state and with temperature induced disorder, is characterized and
analyzed in terms of the ICT model, the Gaussian disorder model and a simple polaronic
approach consistent with Marcus theory. In a separate set of experiments we explore so-called
spinterfaces, where the energy level alignment and spin-polarization of the molecular orbitals
of S-conjugated molecules are studied for the Alq3/Fe and C60/Fe systems. We demonstrate
hybridization and exchange coupling between a S-conjugated orbital in Alq3 molecules
adsorbed on the Fe surface. The hybridization results in an Ohmic-like contact and efficient
charge injection. Furthermore, the exchange coupling induces spin-polarization of a
S-conjugated orbital in Alq3, which enable the efficient spin injection [2]. For the C60/Fe
system, hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a
significant magnetic polarization of C60 ʌ*-derived orbitals [3].
[1] Harri Aarnio, et al, Adv. Energy Mater, 1, 792 (2011).
[2] Yiqiang Zhan, et al, Adv. Mater., 22, 1626 (2010).
[3] T.L.A. Tran, et al, Appl. Phys. Lett., 98, 222505 (2011).
95
BIOGRAPHIC DATA OF PROFESSOR MATS FAHLMANN
Personal data:
Date of birth - 22 of June, 1967
Academic preparation:
Undergraduate degree- 1991, Master of Applied Physics
and Electrical Engineering (Civ. Ing. Y-linjen)
PhD degree- 1995, Surface Physics and Chemistry
Post-Doc- 1996 - 1998, Department of Physics, The Ohio
State University, USA.
Docent- Linköping University, 2000.
Current position:
Professor in Surface Physics and Chemistry at the Department of Physics, Chemistry and
Biology (IFM), 2008 - present.
Former positions:
x Professor in Experimental Physics at the Department of Science and Technology, Campus
Norrköping, Linköping University, 2005 – 2008.
x Associated professor at the Department of Science and Technology, Campus Norrköping
Linköping University, 1999 – 2005.
x Assistant professor at the Department of Physics, Chemistry and Biology, Linköping
University 1998-1999.
Current and previous appointments:
x Director of the Division of Surface Physics and Chemistry at IFM, Linköping University,
2008 – present.
x Principal investigator in SUNFLOWER (EU FP7), 2011-2015.
x Principal investigator in HINTS (EU FP7), 2011-2014.
x Principal investigator in MOLESOL (EU FP7), 2010 -2013.
x Principal investigator in ONE-P (EU FP7), 2009-2011.
x Principal investigator in MINOTOR (EU FP7), 2009-2012.
x Director of the division of Physics and Electronics, Department of Science and
Technology, Campus Norrköping, Linköping University, 2001- 2005. 2006 (Dec.) – 2007
(July).
x Principal investigator in OFSPIN, (EU FP6), 2007 – 2009.
x Deputy director of the Electronics Engineering, Physics and Mathematical programs at
Linköping University, 2007 – 2008.
x Director of the Master of Engineering Programs at the Norrköping Campus, Linköping
University, 2003-2006.
x Principal Investigator in LAMINATE (EU FP5), 2003-2005.
x Principal Investigator in COE (Organic Electronics center funded by Swedish Foundation
for Strategic Research), 2003-2008.
x Dean of Physics and Electronics Education at Campus Norrköping, Linköping University,
2001-2002.
x Coordinator of Industrial Research in the Center for Advanced Molecular Materials (center
funded by Swedish Foundation for Strategic Research), 2000 – 2008.
96
Commissions of trusts:
x Chairman of the FASM (the Association for Synchrotron Radiation Users at MAX-lab)
board, 2004-2006.
x Member of the Board of Directors, National Synchrotron Radiation Laboratory MAX-Lab,
Lund, Sweden, 2001 – 2010
x Opponent and committee member on Ph.D. defences; referee in numerous journals,
organizer of symposia, spring/summer schools, etc.
Bibliometrics
Field-normalized crown indicator (2003-2010): 1.98
Bibliography data (Career, ISI Web of Knowledge, 2012-03-26):
x 128 items
x h-index: 31
x average citation: 26.2
x Total citations: 3354
See http://www.researcherid.com/rid/A-1524-2009 for complete list.
97
Part VI:
Solar cells / OPV 1
NARROW BANDGAP COPOLYMER DERIVATIVES BASED ON 4HCYCLOPENTA(2,1-B;3,4-B´)DITHIOPHENE UNITS: SYNTHESIS AND
PHOTOVOLTAIC PERFORMANCE
D. J. M. VANDERZANDE1,2, S. VAN MIERLO1, L. MARIN1, P. VERSTAPPEN1, A.
HADIPOUR5, M.J. SPIJKMAN4, N. VAN DEN BRANDE3, B. RUTTENS1, J. KESTERS1,
J. D’HAEN1, G. VAN ASSCHE3, D. M. DE LEEUW4, T. AERNOUTS5, J. MANCA1, L.
LUTSEN2, W. MAES1
1
University of Hasselt, Institute for Materials Research (IMO), Agoralaan Bld D, B-3590 Diepenbeek, Belgium
2
IMEC/ IMOMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium
3
Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussel, Belgium.
4
Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands
5
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
Organic or polymer solar cells (OSC’s) offer a real potential toward their application as a new
renewable energy source, as they combine unique features such as potential low cost by largearea fabrication (printing), light weight, solution processability, (semi)transparency and be
produced on a flexible substrate.1 Bulk heterojunction (BHJ) OSC’s based on regioregular
poly(3-hexyl-thiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) as
active layer donor and acceptor materials, respectively, have achieved reproducible power
conversion efficiencies (PCE’s) of 4–5%.1,2 However, one of the main problems associated
with the P3HT:PC61BM combination is the mismatch between the OSC absorption window
and the terrestrial solar emission spectrum due to the relatively large band gap of the
polythiophene donor polymer and the limited absorption width of the material blend. The
most popular approach to obtain low bandgap structures is based on alternating
copolymerization of (heteroaromatic) donor and acceptor moieties.1 Incorporating electron
rich and electron deficient subunits within one polymer structure produces in this way a
significant decrease in the bandgap due to intramolecular charge transfer.
4H-cyclopenta>2,1-b:3,4-b’@dithiophene (CPDT) has emerged as an attractive heterocyclic
system for organic photovoltaics (OPV) with good electron-donating properties, a rigid
coplanar structure favoring ʌ-ʌ intermolecular interactions, and the possibility of side-chain
manipulation to influence solubility (and processability).3 On the other hand, electron
deficient systems such as 2,1,3-benzothiadiazoles (BT),1,3 thiazolo[5,4-d]thiazoles (TzTz),4
and quinoxalines (Qx)5 have been introduced as interesting building blocks for integration in
OPV devices due to their strong electron-withdrawing properties, high oxidative stability and
straightforward synthesis. In our research group, CPDT units have been combined with these
electron poor systems in donor-acceptor alternating copolymers. Furthermore blends of the
98
resulting donor polymers with PC71BM as acceptor have been investigated as active layers in
bulk heterojunction OSC’s.
High-performance CPDT-based low bandgap copolymers are typically synthesized from
symmetrically dialkylated (either branched or linear) CPDT building blocks.3 Nowadays it is
generally accepted that the shape and size of the solubilizing alkyl side chains have a crucial
effect on the photovoltaic performances of blends of polymers with fullerenes - affecting
stacking properties, the phase separation process and as such the nano-morphology of the
blend. To enable a larger variability of solution characteristics of CPDT derivatives a novel
synthetic protocol was developed allowing a versatile introduction of different types of side
chains (Scheme 1).
Scheme 1. Three-step synthetic protocol toward asymmetrically substituted 4Hcyclopenta>2,1-b:3,4-b’@dithiophenes.
A convenient and efficient three-step route toward both symmetrically and non-symmetrically
functionalized CPDT’s has been developed within our group.6 Using this method a broad
collection of functionalized bridged bithiophenes can smoothly be accessed. Starting from 3bromo-2,2’-bithiophene, prepared by Kumada coupling of 2-thienylmagnesium bromide with
2,3-dibromothiophene under Pd(dppf)Cl2 catalysis, lithiation and subsequent reaction with
dialkyl ketones afforded (non-)symmetrically dialkylated tertiary alcohol derivatives. By
means of a Friedel-Crafts type dehydration and cyclization in sulfuric acid medium diluted
with octane, these derivatives were converted to the 4,4-dialkyl-CPDT’s in satisfactory yield.
A first series of materials were synthesized using one particular CPDT derivative with a
branched and a linear alkyl side chain, in casu the 4-(2-ethylhexyl)-4-octyl-CPDT. As the
most performant CPDT-based copolymer to date is composed of alternating 4,4-bis(2ethylhexyl)-CPDT and 2,1,3-benzothiadiazole units, affording 5.5% PCE,3b we first
synthesized the analogous PCPDT-BT material. The dibrominated CPDT precursor was
combined in a Suzuki polymerization reaction with the BT-bis(boronate) (Pd(PPh3)4, K2CO3,
Aliquat, toluene, 80 °C for 3 days), affording a PCPDT-BT polymer in 80% yield after
successive Soxhlet extractions and precipitation from MeOH (Figure 1).
99
Figure 1. Overview of the synthesized polymers: PCPDT-BT, PCPDT-DTTzTz and
PCPDT-Qx.
Toward the synthesis of the second polymer, the CPDT-bis(boronate) was prepared and
combined
with
a
dibrominated
dithienyl-TzTz
precursor7
under
similar
Suzuki
polycondensation conditions, affording a PCPDT-DTTzTz material in 64% yield after a
similar work-up (Figure 1). For the electron deficient quinoxaline building block, a monomer
with an extended absorption window was prepared by expanding the conjugated system in the
vertical direction. Toward the desired PCPDT-Qx copolymer (Figure 1), the dibrominated
quinoxaline was combined with the bis(trimethylstannyl)-CPDT analogue in a Stille
polycondensation reaction. The donor polymers so obtained were blended with PC71BM as an
electron acceptor, BHJ OSC’s were fabricated and the photovoltaic properties of the devices
were investigated. Preliminary non-optimized device results are presented in Table 1.
Table 1. PCE’s for the PCPDT-X Copolymers.
Polymer
PCPDT-BT
PCPDT-DTTzTza
PCPDT-DTTzTzb
a
Mn
1.3 x 104
1.7 x 104
1.4 x 104
PDI
3.3
2.9
2.4
Voc
0.64
0.58
0.67
Jsc
7.44
9.0
11.13
FF
0.38
0.47
0.54
PCE (%)
1.81
2.43
4.03
Unpurified polymer. b Polymer purified by recycling preparative SEC.
When the PCPDT-BT polymer, as obtained after Soxhlet extraction and precipitation (Mn 1.3
x 104, PDI 3.3), was blended with PC71BM in a 1:3 w/w ratio and the active layer was spin
coated from o-dichlorobenzene (with the addition of 1,8-octanedithiol), the resulting solar cell
device (ITO/PEDOT:PSS/active layer/Yb/Ag) showed a rather moderate performance with an
open circuit voltage (Voc) of 0.64 V, a fill factor (FF) of 0.38, a short-circuit current density
(Jsc) of 7.44 mA/cm2, and a resulting PCE of 1.81% under air mass 1.5 global illumination
conditions (AM 1.5G; 100 mW/cm2) (Table 1). Compared to the top PCPDT-BT
100
performance,3b the Jsc and FF are not at the required level. The PCPDT-DTTzTz copolymer,
as obtained after Soxhlet extraction and precipitation (Mn 1.7 x 104, PDI 2.9), was also
blended with PC71BM in a 1:3 w/w ratio. The active layer was spin coated from
chlorobenzene (without any additive) and the resulting solar cell (ITO:PEDOT/PSS:active
layer:Ca-Ag) showed a slightly better performance with a Voc of 0.58 V, a FF of 0.47, a Jsc of
9.0 mA/cm2, and a resulting PCE of 2.43% (Table 1). As polymer molecular weight and
purity are essential parameters influencing the opto-electronic properties and the final solar
cell outcome, purification and fractionation of the PCPDT-DTTzTz copolymer were pursued
by recycling preparative SEC. The solar cell device, prepared in an identical way, obtained
from a purified batch of the polymer (Mn 1.4 x 104, PDI 2.4) showed a significant increase in
PCE till 4.03% by noticeable improvement of all three parameters (Voc 0.67 V, FF 0.54, Jsc
11.13 mA/cm2; Table 1). The thin-film transistor field-effect mobility, calculated in the linear
regime, was found to be 1.0 x 10-3 cm2/Vs, one order of magnitude higher than reported
values in literature for different TzTz-based copolymers,4a and hence in good agreement with
the photovoltaic properties. The PCPDT-DTTzTz polymer showed semi-crystalline
behavior, as evidenced by DSC and XRD experiments8.
In conclusion a series of novel low bandgap copolymers with asymmetrical alkyl substitution
on the cyclopentadithiophene building block PCPDT-X (X = 2,1,3-benzothiadiazole,
thiazolo[5,4-d]thiazole or quinoxaline), have been successfully synthesized, by Suzuki and
Stille polycondensation protocols. Preliminary investigations of these materials in organic
solar cells have afforded a power conversion efficiency of 4.03% for the PCPDTDTTzTz:PC71BM combination, without extensive optimization work. A noticeable increase
in efficiency was obtained upon purification of the polymer by recycling SEC.
Acknowledgement. The authors gratefully acknowledge the IWT for their financial support
via the SBO-project 060843 ”PolySpec”. We also acknowledge the ONE-P project for the
financial support from the European grant agreement n° 212311 related to the HasseltEindhoven collaboration.
(1) (a) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954. (b)
Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47, 58. (c) Boudreault, P.
T.; Najari, A.; Leclerc, M. Chem. Mater. 2011, 23, 456.
(2) Dang, M. T.; Hirsch, L.; Wantz, G. Adv. Mater. 2011, 23, 3597.
(3) (a) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec,
C. Adv. Mater. 2006, 18, 2884. (b) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses,
D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (c) Bijleveld, J. C.; Shahid, M.;
Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2009, 19, 3262. (d) Coffin, R.
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C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009, 1, 657. (e) Tsao, H. N.; Cho, D. M.;
Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spies, H. W.;
Müllen, K. J. Am. Chem. Soc. 2011, 133, 2605. (f) Manceau, M.; Bundgaard, E.; Carlé, J.
E.; Hagemann, O.; Helgesen, M.; Søndergaard, R.; Jørgensen, M.; Krebs, F. C. J. Mater.
Chem. 2011, 21, 4132.
(4) (a) Jung, I. H.; Yu, J.; Jeong, E.; Kim, J.; Kwon, S.; Kong, H.; Lee, K.; Woo, H. Y.; Shim,
H.-K. Chem. Eur. J. 2010, 16, 3743. (b) Lee, S. K.; Cho, J. M.; Goo, Y.; Shin, W. S.; Lee,
J.-C.; Lee, W.-H.; Kang, I.-N.; Shim, H.-K.; Moon, S.-J. Chem. Commun. 2011, 1791. (c)
Subramaniyan, S.; Xin, H.; Sunjoo Kim, F.; Shoaee, S.; Durrant, J. R.; Jenekhe, S. A. Adv.
Energy Mater. 2011, 1, 854. (d) Zhang, M.; Sun, Y.; Guo, X.; Cui, C.; He, Y.; Li, Y.
Macromolecules 2011, 44, 7625. (e) Helgesen, M.; Madsen, M. V.; Andreasen, B.;
Tromholt, T.; Andreasen, J. W.; Krebs, F. C. Polym. Chem. 2011, 2, 2536.
(5) (a) Wang, E.; Hou, L.; Wang, Z.; Hellström, S.; Zhang, F.; Inganäs, O.; Andersson, M. R.
Adv. Mater. 2010, 22, 5240. (b) Lee, S. K.; Lee, W.-H.; Cho, J. M.; Park, S. J.; Park, J.-U.;
Shin, W. S.; Lee, J.-C.; Kang, I.-N.; Moon, S.-J. Macromolecules 2011, 44, 5994. (c) He,
Z.; Zhang, C.; Xu, X.; Zhang, L.; Huang, L.; Chen, J.; Wu, H.; Cao, Y. Adv. Mater. 2011,
23, 3086. (d) Zhang, Y.; Zou, J.; Yip, H.-L.; Chen, K.-S.; Zeigler, D. F.; Sun, Y.; Jen, A.
K.-Y. Chem. Mater. 2011, 23, 2289.
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102
BIOGRAPHIC DATA OF PROFESSOR DIRK VANDERZANDE
Name:
Address:
Dirk Vanderzande
University of Hasselt, Institute of Material
Research (IMO), Agoralaan, Building D,
B-3590 Diepenbeek, Belgium
Tel.:+32 1126.83.21, Fax: +321126.83.01,
E-mail: [email protected]
Born: 12-6-1957, Genk, Belgium.
Master in Chemistry at KULeuven in 1979;
PhD at KULeuven in 1986 in Organic synthesis and studies of
mechanisms of thermal induced rearrangement reactions of
ortho-quinodimethane systems.
Permanent position as senior post-doc researcher since January 1987 at the University of
Hasselt (Belgium) joining the research group “Organic and Polymeric Chemistry”. Expert
domains: Organic Synthesis, Polymer Synthesis, Advanced Organic and Polymeric Materials
for Optical and Electronic applications.
1988: Start up of his research in the field of conjugated polymers. Two topics were started.
The first topic relates to low band gap conjugated polymers, their synthesis, structural
characterization and the underlying polymerization mechanisms. The second topic relates to
the polymerization behavior of p-quinodimethane systems towards the synthesis of precursors
for conjugated polymers.
1992: Appointed as assistant professor at U Hasselt.
1992: A new route was developed and optimized at the level of synthetic routes towards the
monomer, the polymerization conditions and conditions of conversion of the precursor
polymer.
1995: appointed as associated professor at U Hasselt.
1997 Concerning latter topic important breakthroughs were achieved and the so developed
methodologies extended towards other systems. Start of material development specific
towards polymer LEDs in collaboration with Philips and Covion.
1999: Appointed as “hoogleraar” at U Hasselt.
2000: Extending the efforts of material development towards applications of conjugated
polymers in thin film FET transistors and organic solar cells. Start of studies related to defect
structures in conjugated polymers.
2001: Start of association of the group as a division IMOMEC in IMEC vzw.
2003: Appointed as full professor (gewoon hoogleraar) at U Hasselt.
2003: New route was discovered towards Poly(Thienylene Vinylene) via the so called
dithiocarbamate route which is under further investigation;
Since 1th of October 2010 Director of the institute imo-imomec at UHasselt.
Full partner in many EC research projects in collaboration with industry over the last 10
years.
More than 226 refereed papers in international journals.
16 original patents in the field of synthetic routes towards conjugated polymers.
103
The abstract of the lecture Dr. Hoppe
entitled
“Imaging methods for quality control and degradation
analysis of organic solar cells”
was not available to the editorial deadline.
104
The abstract of the lecture Dr. Troshin
entitled
“Novel methods for controlling the quality and
evaluation of the degradation profiles of conjugated
polymers designed for photovoltaic applications”
was not released for publication.
105
HIGHLY EFFICIENT FLEXIBLE PLASTIC SOLAR CELLS MADE BY A REEL-TO-REEL
COATING PROCESS
M. Schrödner, L. Blankenburg, K. Schultheis, H. Schache, S. Sensfuss
TITK Rudolstadt, Dept. Functional Polymer Systems and Physical Research
Breitscheidstr. 97, 07407 Rudolstadt, Germany
e-mail: [email protected]
Summary
We could demonstrate the feasibility of flexible polymer thin film bulk-heterojunction solar
cells with photo-conversion efficiencies (PCE) of more than 3 % using a reel-to-reel slot-die
coating process for the preparation of the hole transport and the photoactive layer. The
photoactive material was a blend of poly(3-hexylthiophene) (P3HT) as donor and indene-C60
bisadduct (ICBA) as acceptor, which enables more than 6 % PCE on glass. Using flexible
PET foils with ITO as substrate we achieved a maximum PCE of 4.55 % by spin coating and
3.2 % by reel-to-reel coating, which are with respect to substrate and coating technique
among the highest reported PCE values in the literature so far.
1. Introduction
Organic thin film solar cells have made much progress during the last years. Since the
discovery of the ultrafast charge transfer in p-conducting polymer/fullerene composites by
Sariciftci and Heeger in 1992 [1] the power conversion efficiency (PCE) of bulk heterojunction solar cells was increased up to 10 % [2]. These efficiencies together with the
possibility to use low cost materials and low cost reel-to-reel coating or printing techniques
enable costs of production of 0.63 €/Wp and below [3] making polymer solar cells
competitive to other organic or inorganic solar cell devices. But there is still a lack in
efficiency between best laboratory cells and flexible solar cells which had been made in a
reel-to-reel (R2R) process. Best flexible organic solar cells on foils made by spin coating have
an efficiency of 3.66 % [4]. Using R2R-processing techniques the maximum efficiency
reported so far is 2.33 % only [5]. So, it is a challenge to improve R2R-processed flexible
solar cells markedly to close the gap to the best laboratory cells on glass.
Several continuously working printing and coating techniques like gravure and flexographic
printing [6, 7], slot-die and knife coating [8-11] were examined with respect to polymer solar
cell production.
106
In this work we demonstrate flexible polymer solar cells with a PCE of more than 3 % made
by two reel-to-reel coating steps. The photoactive system is poly(3-hexylthiophene) (P3HT)
as donor and bisindene fullerene (ICBA) as acceptor, which was shown to enable more than
6% PCE on glass [12-14].
2.Materials
Ready to use P3HT/ICBA solution PV2000 and regioregular P3HT (Plexcore OS 2100) were
obtained from Plextronics. ICBA was also synthesized by ourselves via a [4+2]-cycloaddition
reaction
of
fullerene
and
bisindene.
PEDOT:PSS
(poly(3,4-ethylenedioxy-
thiophene):polystyrene-sulfonate) was obtained from H. C. Starck (Clevios PH and Clevios
4083). Alternately to PEDOT:PSS also an hole-transport material from Plextronic (Plexcore
PV 2000 HTL ink) was tested. All solutions were filtered before use.
ITO coated PET foil (125 μm thick) was purchased from Southwall. The ITO thickness was
about 100 nm and the surface resistance 45 :sq. ITO glasses were obtained from Merck (13
:sq).
I-V curves were recorded with a Keithley SMU 2400 Source Meter by illuminating the cells
from the ITO side with 100 mW/cm² white light from a Steuernagel solar simulator to realise
AM 1.5 conditions. The illuminating light was calibrated using a silicon reference cell from
ISE (Freiburg). All cells were measured in a glove box.
3. Results
3.1 Solar cells made by spin coating
For optimisation of the layer setup solar cells were first processed on ITO coated glass. The
photoactive layer was made from the Plexcore PV 2000 ink containing regioregular poly(3hexylthiophene) (P3HT) and the fullerene derivative ICBA as electron acceptor. Compared
with the mostly used fullerene acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
the ICBA has a 0.17 eV higher LUMO level resulting in a higher open circuit voltage VOC >
0.8 V. The optimum thickness of the photoactive layer was about 200 nm. Annealing up to
170 °C leads to an improved PCE.
The hole transport layer (HTL) between ITO anode and photoactive layer was made from
PEDOT:PSS (Clevios 4083) or from the Plexcore PV 2000 HTL ink, which were annealed
after spin-coating at 120 °C and 150 °C respectively. The cathode was made by thermal
evaporation of Ca (15 nm) and Al (50 nm). The complete layer sequence is
ITO/HTL/P3HT:ICBA/Ca/Al.
107
The resulting solar cells reach PCE values up to 5.8 % (ISC = 10.3 mA/cm², VOC = 0.823 V;
FF = 0.68), Fig. 1. Besides the high open circuit voltage also high fill factors up to 0.71 were
observed.
P3HT
indene C60 bisadduct (ICBA)
Flexible solar cells on ITO/PET foils were processed analogous to glass cells with the
exception that Clevios PH was used instead of Clevios 4083 as HTL. As expected the PCE is
lower compared to cells on glass, which is mainly attributed to a lower current. The maximum
PCE was 4.55 % (ISC = 8.56 mA/cm², VOC = 0.804 V; FF = 0.66), Fig. 1.
The choice of the HTL material (PEDOT:PSS vs. Plexcore PV2000 HTL ink) had nearly no
effect on the device performance.
10,0
current density [mA/cm2]
8,0
6,0
4,0
R2R on foil
JSC: 7.31 mA/cm2
VOC: 800 mV
FF: 0.55
KAM1.5: 3.20 %
spin-coating on foil
JSC: 8.56 mA/cm2
VOC: 804 mV
FF: 0.66
KAM1.5: 4.55 %
spin-coating on glass
JSC: 10.3 mA/cm2
VOC: 823 mV
FF: 0.68
KAM1.5: 5.80 %
2,0
0,0
-2,0 0
0,2
0,4
0,6
0,8
-4,0
-6,0
-8,0
-10,0
-12,0
voltage [V]
Fig. 1: J-V-characteristics of P3HT:ICBA solar cells made on glass and foil by spin coating
(circles) and reel-to-reel coating (squares)
108
3.2
Solar cells made by reel to reel coating
Taking advance of the results from the spin coating experiments R2R-coating of the
photoactive layer and the hole-transport layer was investigated. The reel to reel coater LBA
200 used for these experiments has a maximum web width of 200 mm and can be equipped
with several slot-die casters for application of the coating fluid (Fig. 2). By this coating
technique the fluid is forced through a slot which allows an accurate adjustment of the wet
film thickness by flow rate and web speed. The lips of the die are placed some ten
micrometers above the web surface so that a stable meniscus is formed. The stability of the
meniscus is determined by the coating window which depends on ink properties (viscosity,
surface tension) and process parameters (web speed, lip design).
PEDOT:PSS was coated from a Clevios PH dispersion diluted 1:1 by isopropyl alcohol at a
web speed of 2 m/min and fluid feed rates of 1.2 ml/min for 40 mm coating width and 2.1
ml/min for 7 cm coating width. Drying was done by infrared radiation followed by a hot air
dryer (90°C). The thickness of the resulting layer was 98 nm.
Fig. 2: Application of the photoactive ink with the slot-die coater and cutting samples from
the coated foil for preparation of solar cells
The photoactive layer was coated from a 3 wt.% solution of P3HT (Plexcore OS 2100) and
ICBA (ratio 1:0.8). The solvent was a mixture of o-dichlorobenzene and chloroform which
allows a fast drying before the web is deflected from the horizontal to a perpendicular
direction for the first time. The layers were only dried by mild heating at 30°C. The web
speed (1 m/min) together with feed rates of 0.3 ml/min at 40 mm width and 0.55 ml/min at 70
mm width gives resulting layer thicknesses of between 220 nm and 250 nm. After drying
109
pieces (5 x 5 cm²) were cut from the coated foil (Fig. 2) and annealed at temperatures up to
170°C.
Thereafter Ca and Al were evaporated as cathode and the cells were measured as described
above. The maximum PCE obtained for these reel-to-reel processed cells was 3.2 % (Fig. 1)
which is the highest reported yet.
4. Discussion
As can be seen in Fig. 1 there is a loss of efficiency of P3HT/ICBA solar cells when the
substrate is changed from ITO/glass to ITO/foil and again when the spin coating technique is
replaced by reel-to-reel coating. The first drop is known from former experiments and from
the literature [11, 15] and can be ascribed to the lower surface conductivity of ITO on foil
compared to ITO on glass resulting in a higher series resistance.
Changing layer processing from spin to reel-to-reel coating the losses are linked equally with
a decrease of the short circuit current and the fill factor whereas the values of the open circuit
voltage remain nearly unaffected. This is an indication that the morphology of the photoactive
layer is quite different from those made by spin coating. The reason for this is not clear yet,
but may be connected with different properties of the photoactive materials used (commercial
Plexcore PV 2000 for spin coating vs. self-made P3HT/ICBA solutions for reel-to-reel
coating). Moreover, addition of chloroform to the coating solution could also be
counterproductive for the development of an optimised morphology because of a faster drying
kinetics.
So there is still some space and need for further improvements to overcome the 4 % hurdle
also with reel-to-reel coated solar cells on plastic foils. In combination with alternative
solution processable materials for the electrodes together with new cell concepts (inverse, ITO
free) a complete reel-to-reel processing with high through-puts up to some thousand m²/h
should enable low production costs and make polymer solar cells competitive.
5. Conclusions
We could demonstrate the feasibility of flexible polymer thin film bulk-heterojunction solar
cells with efficiencies of more than 3 % using a reel-to-reel die-coating process for the
preparation of the HTL and the photoactive layer. The maximum PCE achieved so far is more
than half the value obtained with the same materials on glass by spin coating and it is about
70 % the value obtained by spin coating on the same plastic substrate. Nonetheless both the
PCE values of polymer solar cells on plastic foil made by spin-coating (4.55 %) and reel-toreel coating (3.2 %) are the highest reported in the literature up to now.
110
Further improvement of the PCE of large area flexible polymer solar cells is a challenging
task for future technology development as well as increasing the through-put up to some
thousands of square meters per hour and replacing all steps which are not compatible to reelto-reel processing.
Acknowledgement
Financial support from the Thuringian Ministry TMWAI (2004 WI0282), from the German
Federal Ministries BMBF (project “SonnTex”, 03X3518A) and BMWi (IW061016; 1136/03,
VF071005, IW082026, VF090063 and KA0406302DA7) is gratefully acknowledged.
References
[1]
N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258 (1992) 1474
[2]
M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt. Res. Appl. 2012, 20, 12
[3]
N. Espinosa, M. Hösel, D. Angmo and F. C. Krebs, Energy Environ. Sci. 2012, 5, 5117
[4]
K.-H. Tsai, J.-S. Huang, M.Y. Liu, C.-H. Chao, C.-Y. Lee, S.-C. Hung, C.-F. Lin, J. Electrochem. Soc. 156(10) (2009)
B1188-B1191
[5]
F.C. Krebs, S. A. Gevorgyan, J. Alstrup, J. Mater Chem. 2009, 19, 5442-5451
[6]
M.M. Voigt, R.C.I. Mackenzie, C.P. Yau, P. Atienzar, J. Dane, P.E. Keivanidis, D.D.C. Bradley, J. Nelson, Sol.
Energy Mater. Sol. Cells 95 (2011) 731-734.
[7]
P. Kopola, T. Aernouts, S. Guillerez, H. Jin, M. Tuomikoski, A. Maaninen, J. Hast, Sol. Energy Mater. Sol. Cells 94
(2010) 1673-1680
[8]
L. Blankenburg, K. Schultheis, H. Schache, S. Sensfuss, M. Schrödner, Sol. Energy Mater. Sol. Cells 93 (2009) 476483.
[9]
F.C. Krebs, Sol Energy Mater Sol Cells (2009) 93, 465.
[10] Wengeler, L., et al., Chem Eng Process (2011) 50, 478.
[11] Y. Galagan, I.G. de Vries, A.P. Langen, R. Andriessen, W.J.H. Verhees, S.C. Veenstra, J.M. Kroon, Chem. Eng.
Process.: Process Intensification 50(5-6) (2011) 454-461
[12] Y. He, H.-Y. Chen, J. Hou, Y. Li, J. Am. Chem. Soc. 132 (2010) 1377-1382
[13] G. Zhao, Y. He and Y. Li, Adv. Mater. 2010, 22, 4355-58
[14] Y. Sun , C. Cui, H. Wang and Y. Li, Adv. Energy Mater. 2011, 1, 1058
[15] M. Al-Ibrahim, H.-K. Roth, U. Zhokhavets, G. Gobsch, S. Sensfuss, Solar Energy Mater. & Solar Cells 85 (2005) 1320
111
BIOGRAPHIC DATA OF DR MARIO SCHRÖDNER
born:
1958 in Burgstaedt (Saxonia, Germany)
studies:
Physics at the Leipzig University, 1979-1984, diploma degree
PhD
1989 at Leipzig University
ESR spectroscopy on radiation induced radicals
1988-1995
1995- now
scientific assistant at the Technical University of Leipzig
-
intrinsically conducting polymers;
-
laser processing of polymers for applications in micro system technologies
Thuringian Institute of Textile and Plastics Research (TITK) in
Rudolstadt
-
polymer electronics, photovoltaics and actuators
-
laser micro machining of polymers
112
Part VII: Materials and
technologies 3
INTERFACE ENGINEERING AND ELECTRONIC FUNCTIONALITY
VIA MOLECULAR SELF-ASSEMBLY
M. HALIK
University Erlangen-Nürnberg, Organic Materials & Devices – OMD, Department of
Materials Science, Martensstrasse 07, D-91058 Erlangen, Germany
Functionalized molecules that organize to self-assembled monolayers (SAMs) are gaining
importance in organic electronic devices. They are fully compatible with flexible substrates,
are amenable to low-cost processing, and show reliable film-forming behavior [1]. Starting
from auxiliary layers, which improve and modify surfaces and interfaces in traditional thinfilm devices, the applications of SAMs develop towards molecular scale electronics. Hybrid
dielectrics composed of tiny oxide layers and self-assembled molecules provide low-voltage
device operation and control the molecular orientation of subsequently deposited organic
semiconductors [2,3]. By adjusting the molecular dipole of such SAM molecules, the
threshold voltage in organic transistors can be tuned [4]. With multifunctional molecules, in
which several layer functions of a device are implemented, self-assembled monolayer field
effect transistors (SAMFETs) can be realized with p- and n-type transport [5]. Detailed
studies on the impact of SAM morphology to the device performance indicate that a balance
between dense surface packaging, optimized SS-interaction and reduced leakage current can
be realized by mixed SAMs and the stoichiometric control of the composition of different
SAM-forming molecules [6]. A theoretical approach to describe the transport in SAMFET
devices will be presented. Finally, an example of SAMs in organic solar cells will document
the general potential of the self-assembly approach [7].
[1]
M. Halik and A. Hirsch, Adv. Mater. 23 (2011), 2689-2695.
[2]
H. Klauk, et al., Nature 445 (2007) 745 – 748.
[3]
M. Novak, et al., Appl- Phys. Lett. 98 (2011), 093302.
[4]
A. Y. Amin, et al., Langmuir 27 (2011), 15340-15344.
[5]
M. Novak, et al., Nano Letters 11 (2011), 156-159.
[6]
A. Rumpel, et al., Langmuir 27 (2011), 15016-15023.
[7]
T. Stubhan, et al., Advanced Energy Materials (2012), DOI: 10.1002/aenm.201100668
113
BIOGRAPHIC DATA OF PROFESSOR MARCUS HALIK
Marcus Halik
Prof. Dr. rer. nat., Dipl.-Chem.
geboren am: 13.01.1971 in Heiligenstadt
Email: [email protected]
Web: http://www.omd.uni-erlangen.de
Education
09/1990 – 07/1995
Study of Chemistry at the Technischen Hochschule LeunaMerseburg and Martin Luther University Halle-Wittenberg
09/1995 – 10/1998
Dissertation: “2,2-Difluor-1,3,2-(2H)-dioxaborine als Bausteine zur
Darstellung von langwellig absorbierenden Methin-farbstoffen” at the
Martin-Luther-University Halle-Wittenberg
Scientific Career
03/1999 – 03/2000
Post-Doc with Prof. Dr. S.R. Marder, University of Arizona (USA),
04/2000 – 08/2005
Engineer - Infineon Technologies AG
09/2005 -
W2-Professur für Polymerwerkstoffe (Organic Materials & Devices –
OMD) at the Institute of Polymer Materials at the Friedrich-Alexander
University Erlangen-Nürnberg
114
FUNCTIONAL OLIGOTHIENYLENEVINYLENE-BASED MATERIALS
FOR OPTOELECTRONICS
F. LANGA*; R. CABALLERO; A. ALJARILLA, L. LOPEZ-ARROYO; M. URBANI;
B. PELADO; P. DE LA CRUZ
Instituto de Nanociencia, Nanotecnología y Materiales Moleculares. Universidad de Castilla-La Mancha.
Campus de la Fábrica de Armas, 45071-Toledo (Spain)
Linear S-conjugated oligomers with well-defined chemical structures have been used as
molecular wires in molecular electronics or nanoscopic systems; among the different Sconjugated oligomers, oligothienylenevinylene oligomers (nTVs) are excellent wires besides
good electron donors. Here, we will report on the synthesis and properties of some new
derivatives (Figure 1) were nTVs behave as wires or electron donors in donor-bridge-acceptor
linking porphyrins and fullerenes, were the phophysical events were studied, or used as dyes
in dye sensitized solar cells, acceptor-bridge-acceptor or donor-bridge-donor systems,
studying the mixed valence systems.
n= 1,3,5,7
Figure 1
References:
E. Mª Barea, R. Caballero, F. Fabregat-Santiago, P. de la Cruz, F. Langa, J. Bisquert.
ChemPhysChem 2010, 11, 245.
M. Urbani, B. Pelado, P. de la Cruz, K. Yamanaka, O. Ito, F. Langa, Chem. Eur. J. 2011, 17,
5432.
S. Rodríguez, R. González, M. C. Ruiz Delgado, R. Caballero, P. De la Cruz, F. Langa, J. T.
López-Navarrete, J. Casado. J. Am. Chem. Soc. 2012 (in press).
Maxence Urbani, Kei Ohkubo, Shafiqul D. M. Islam, Shunichi Fukuzumi, Fernando Langa.
Chem. Eur. J. 2012 (in press).
115
BIOGRAPHIC DATA OF PROFESSOR FERNANDO LANGA
Prof. Fernando Langa is Professor or Organic Chemistry at
Univ. de Castilla-La Mancha (Spain) since 2002. Graduate of
the University Complutense of Madrid (UCM), he worked as a
postdoctoral fellow at University of Dundee (Scotland). After
joining
(1990) the Organic Chemistry Department at the
University of Castilla-La Mancha (Campus of Toledo) as
Assistant Professor, he has been Visiting Professor at the
University of Paris-Sud (France) and later, in 1997, at the
Institute for Polymers and Organic Solids (IPOS) at the
University of California, Santa Barbara (UCSB). He is actually
Director of the Institute for Nanoscience, Nanotechnology and Molecular Materials of UCLM
at Toledo.
His research interests span a range of targets with emphasis on the chemistry of carbon
nanostructures such as fullerenes, carbon nanotubes, graphene, ʌ-conjugated systems as
molecular wires, and electroactive molecules, in the context of electron-transfer processes,
photovoltaic applications, and nanoscience.
He is editor of the book “Fullerenes: Principles and Applications” (Royal Society of
Chemistry) as well as author of several book chapters and more than 130 publications in
international journals.
116
INORGANIC AND ORGANOMETALLIC LOW COST DOPANTS FOR
TRANSPORT LAYERS IN ORGANIC ELECTRONIC DEVICES
GUENTER SCHMID1, JAN HAUKE WEMKEN1, ANNA MALTENBERGER1, MARINA
A. PETRUKHINA2, THOMAS DOBBERTIN3, ARNDT JAEGER3
1
Siemens AG, Guenther-Scharowsky-Strasse 1, 91058 Erlangen
2
Department of Chemistry, State University of New York University, Albany, NY 12222
3
Osram Opto Semiconductor, Wernerwerks-Strasse 2, 93049 Regensburg
Highly conductive transport layers are one of the technical prerequisites to manufacture
organic light emitting diodes (OLEDs) with high luminous efficacies above 50 lm/W. The
conductivity is usually enhanced by doping of the organic hole or electron conductor host
material. In the class of inorganic and organometallic compounds, the electron withdrawing or
donating properties can be varied in a very large range.
In the presentation, we show a structure-property relationship within a series of p-dopants
based on organometallic Lewis-acids. The conductivity enhancement of the hole conductor
was balanced to other important OLED requirements, such as cost, transparency and off-stateappearance. The fundamental properties of the doped layers were characterized by currentvoltage measurements, impedance, UV/Vis and photoluminescence spectroscopy. The
maximum conductivity was found to be in the range of 10-4 to 10-3 Scm-1 without a significant
additional absorption owing to a charge transfer band or the radical cation of the host
material.
The selected dopant was incorporated into a white OLED stack. Luminous efficacy and
lifetime data will be presented.
The synthetic depth of the preparation of the dopants determines its cost and thus, its
contribution to an OLED product. The compounds were prepared in large quantities from
readily available basic chemicals in only two steps. The compounds were characterized by
standard chemical analytics and X-ray crystallography. Molecular modeling supported the
course of Lewis acidity within the series of compounds.
117
BIOGRAPHIC DATA OF DR GÜNTER SCHMID
Principal Research Scientist
Global Technology Field Organic Electronics
Dr. Günter Schmid is a Principal Research Scientist at Corporate
Technology of Siemens AG in the global technology field “Organic
Electronics”. His current focus is the material development of
organic light emitting diodes for lighting applications. He earned his
PhD degree from the University of Ulm (Germany) in 1993 and
joined 1994 the Laboratory for Molecular Structure and Bonding at the Texas A&M
University for a postdoctoral position. From 1996 to 1999 he developed organic dielectrics for
the application in silicon based semiconductors. Since 1999 organic electronics became his
main field of interest, first as project manager for ultra low cost electronics at Infineon
Technologies AG until 2005 and second at his current position.
118
Hydrogen-bonded Indigoids and Acridones: Highly Ordered
Semiconductors for High Performance Organic Electronics
Mihai Irimia-Vladu1,*, Eric Glowacki1, Gundula Voss1, Lucia Leonat2, Uwe
Monkowius1, Günther Schwabegger1, Zeynep Bozkurt3, Helmut Sitter1, Siegfried Bauer1
and Niyazi Serdar Sariciftci1
1
Johannes Kepler University Linz, Austria
2
Polytechnica Bucharest, Romania
3
Sabanci University, Turkey
*
Corresponding author: [email protected]
The burgeoning field of organic electronics is driven by the notion of mass-producing
cheap and sustainable electronic devices. Future large-scale application of organic electronics
should also focus on sustainability in order to address the current problem of electronic waste. In
our recent activity we considered investigating natural and nature inspired materials for organic
electronics applications1-4. We have demonstrated that nature is an immense reservoir of
inexpensive materials that are suitable for the fabrication of organic electronic devices operating
at state-of-the-art level1-4.
Our investigation of naturally occurring pigments revealed interesting properties of those
materials, such as strong dipolar interaction, i.e. hydrogen bonding. This type of chemical bond
is a stronger dipolar force than typical van der Waals interaction featured by synthetic S–
conjugated chromophores currently employed in organic electronics. H-bonding is ubiquitous in
natural systems, being responsible for the unique properties of water and the forces holding
together DNA and RNA strands, three of the systems that allows the existence of life on Earth.
The consequence of H-bonding is the presence of highly ordered organic films that have a high
dielectric constant. This long range intermolecular interaction tremendously influences the
dynamics of photogenerated excited states in such molecules. Indigo and its synthetic derivatives
represent an interesting class of organic semiconducting materials. Indigo for example is among
the very few known blue-colored chromophores of truly natural-origin. Indigo (extracted from
plants) and 6,6’-dibromoindigo (Tyrian purple, extracted from sea shells) have been exploited for
thousands of years as valuable dyestuffs. We recently found that vacuum-evaporated indigo and
Tyrian purple films show high ordering with a crystalline texture featuring a single preferential
orientation and have high dielectric constants (in the range of 4–6). These properties are
responsible for those materials displaying high carrier mobilities. We measured for example,
high and well-balanced field effect mobility of ~ 0.4 cm2/Vs for both electrons and holes for
119
OFET devices with Tyrian purple channel. Tyrian purple was also remarkable for its air-stable
transport of both electrons and holes, a consequence of its deep LUMO level of ~ -4.0 eV.
With the knowledge that the exciton binding energy is inversely proportional to H2, we
sought to create nonexcitonic single-material solar cells. We achieved interesting results with the
dye quinacridone, also known as pigment violet 19, used extensively in paints and cosmetics.
Quinacridone features intermolecular hydrogen bonding similar to indigo, and can be perceived
as an H-bonded pentacene analogue. Different than indigo, it does not have characteristic a fast
internal conversion. In effect quinacridone is highly fluorescent and has a long excited state
lifetime, two facts that make quinacridone an interesting candidate for the development of a
“single layer” organic solar cell. Single-layer metal-quinacridone-metal structures produced
short-circuit photocurrents in the milliamp per square centimeter range and external quantum
efficiencies of 10% at the absorption peak of the dye. Moreover, temperature dependence of
photocurrent experiment showed excitation binding energies below 100meV. As a comparison to
the above values, single-layer polymer cells featuring poly(thiophene) or poly(phenylene
vinylene), showed short circuit currents in the microamperes range and quantum efficiencies of
less than 1%. Our finding demonstrates that highly ordered organic materials that have strong
intermolecular interactions could be efficiently used to produce high efficiency nonexcitonic
organic solar cells.
Our investigation of natural and nature-inspired materials proved that those materials can
be employed in the fabrication of organic devices operating at state-of-the-art performance.
Better understanding of the strong intermolecular interactions in these materials and making use
of those interactions is the focus of our present and future research activity. We are aiming to
synthesize better absorbers to make viable single-layer solar cells as well as to control and
optimize film growth conditions in order to maximize mobilities and semiconductor performance
in transistor-based devices.
References
1. E.D. Gáowacki, L. Leonat, G. Voss, M. Bodea, Z. Bozkurt, M. Irimia-Vladu, S. Bauer, N.S.
Sariciftci, Natural and nature-inspired semiconductors for organic electronics, Proc. SPIE 8118,
81180M (2011).
2. M. Irimia-Vladu, E.D. Gáowacki, P. Troshin, G. Schwabegger, L. Leonat, D. Susarova, O.
Krystal, M. Ullah, Y. Kanbur, M.A. Bodea, V.F. Razumov, H. Sitter, S. Bauer, N.S. Sariciftci,
Indigo—a natural pigment for high-performance ambipolar organic field effect transistors and
circuits, Adv. Mater. 24(3), 375-380 (2012).
3. E.D. Gáowacki, L. Leonat, G. Voss, M.A. Bodea, Z. Bozkurt, A. Montaigne Ramil, M. IrimiaVladu, S. Bauer, N.S. Sariciftci, Ambipolar organic field effect transistors and inverters with the
natural material Tyrian Purple, AIP Adv. 1, 042132–042137 (2011).
4. M. Irimia-Vladu, P.A. Troshin, M. Reisinger, L. Shmygleva, Y. Kanbur, G. Schwabegger, M.
Bodea, R. Schwödiauer, A. Mumyatov, J. Fergus, V.F. Razumov, H. Sitter, N.S. Sariciftci, S.
Bauer, Biocompatible and biodegradable materials for organic field-effect transistors, Adv.
Funct. Mater. 20(23), 4069–4076 (2010).
120
The abstract of the lecture Mr. Kolbusch
entitled
“Production technologies for large area printed
flexible electronics”
was not available to the editorial deadline.
121
Part VIII: Solar cells / OPV 2
STRUCTURAL CORRELATIONS IN THE GENERATION OF POLARON PAIRS IN
COPOLYMERS FOR PHOTOVOLTAICS
Enrico Da Como1, Raphael Tautz1, Jochen Feldmann1, Ullrich Scherf2 and Elizabeth
von Hauff3
1PhotonicsandOptoelectronicsGroup,LudwigǦMaximiliansǦUniversitätMünchen,80799,Munich,Germany
[email protected]
2
DepartmentofChemistryandInstituteofPolymerChemistry,WuppertalUniversity,42097Wuppertal,Germany
3FraunhoferInstituteforSolarEnergySystems,Freiburg,Germany
The most recent advances in the power conversion efficiency of organic solar cells considered
the design of low-bangap copolymers with an extended absorption in the near infrared. These
materials, based on the donor-acceptor concept have attracted a considerable interest. The low
bandgap offers optimal light-harvesting characteristics and has inspired much work towards
the achievement of record power-conversion efficiencies in solar cells. It remains an open
question what are the fundamental photoexcitations in these macromolecular semiconductors.
In conjugated homopolymers an exciton is formed upon light absorption. This can later
dissociate forming a polaron pair because of the energetic disorder in the polymer solid
system. Donor-acceptor copolymers show an intrinsic alternation of units with different
electron affinities, which may favour the formation of polaron pairs.
Here, we report on how the chemical structure of donor and acceptor moieties controls one of
the primary steps in light absorption and photocarrier generation, i.e. the yield of polaron
pairs. We analyze several copolymers in which the acceptor part is modified according to
different electron affinities and the distance between the center of mass between the donor
and acceptor moiety is varied. The experimental technique for such measurements is based on
a femtosecond pump-probe setup with a near infrared pump resonant with the first absorption
peak of the polymers and a probe in the middle infrared (MIR) monitoring the polaron
absorption bands. This allows us for a reliable and accurate estimation of the polaron
generation yield upon photoexcitation. We focussed our attention on four different
copolymers having the same 4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b`]-dithiophene
donor moiety indicated as CPDT and different acceptors; benzo-[1,2-c;4,5c']bis[1,2,5]thiadiazole]
(BDT),
(2,1,3-benzothiadiazole)
(BT)
di(thien-2-yl)-2,3diphenylthieno[3,4-b]pyrazine (2TTP) and di(thien-2-yl)- (2,1,3-benzothiadiazole) (2TBT).
The chemical structures for such systematic series of compounds are reported as insets in
Figure 1 together with the absorption spectra in the main figure frame.
a)
Absorption of polymer films (arb. units)
PCPDT-BDT
800 nm
b)
PCPDT-BT
800 nm
c)
800 nm
PCPDT-2TTP
d)
PCPDT-2TBT
660 nm
500
1000
1500
Wavelength (nm)
2000
Figure 1 Absorption spectra of the polymers: a) PCPDT-BDT, b) PCPDTBT, c) PCPDT-2TBT, d) PCPDT-2TTP.
We make the important observation that copolymers with
donors and acceptors spatially adjacent along the
polymer backbone (PCPDT-BDT and PCPDT-BT),
show a more efficient formation of polaron pairs (> 24%
of the initial photoexcitations). On the other hand, we do
not observe any clear correlation between the acceptor
strength, i.e. the electron affinity of the acceptor moiety,
and the yield. Interestingly, also the dynamics of the
recombination process differ and can be correlated to the
structure. The results provide useful input into the
understanding of how structure-property relationships
determine the initial ultrafast branching of
photoexcitations in low-bandgap polymers for
photovoltaics.
122
BIOGRAPHIC DATA OF DR ENRICO DA COMO
E-Mail: [email protected]
Website: http://www.phog.physik.uni-muenchen.de/
Educational Background
-
01/03 to 03/06 Ph.D. in Chemical
Physics, University of Bologna (Italy)
-
09/97 to 10/02 Laurea in Chemical
Physics (equivalent to 5 years M.Sc.),
110/110 cum laude. University of Modena
(Italy)
Appointments
-
Since 05/12: Reader in Physics at the Department of Physics,
University of Bath (United Kingdom)
-
01/08 to 04/12: University Assistant (W1 level), in Experimental
Physics at the Photonics and Optoelectronics group, Department of
Physics, Ludwig-Maximilians-University Munich (Germany)
-
08/08 and 08/09 (1 month per visit): visiting scientist at the
Department of Physics, University of Utah, UT (U.S.A.) (Group
Prof. J. Lupton)
-
04/06 to 12/07: Postdoc at the Photonics and Optoelectronics group,
Department of Physics, Ludwig-Maximilians-University Munich
(Germany) (Chair of Prof. J. Feldmann)
-
01/06 to 03/06: Research Assistant at the at the Istituto per lo
Studio dei Materiali Nanostrutturati, National Research Council
(C.N.R.), Bologna (Italy)
Current Fields of Interest
-
Optical spectroscopy of excitons and polarons in organic
semiconductors and conductors.
Single molecule and ultrafast spectroscopy of semiconductor
nanocrystals, graphene and plasmonic nanoparticles.
Morphological studies on nanostructured materials by optical,
scanning-probe and electron microscopy (both transmission and
scanning).
123
ORIGIN OF SUB-BANDGAP ABSORPTION IN P3HT:
PCBM SOLAR CELLS
Martin Presselt, Felix Herrmann, Erich Runge, Sviatoslav Shokhovets, Harald Hoppe,
Gerhard Gobsch
To explain the origin of sub-bandgap (SBG) absorption contributing to the photocurrent in
bulk-heterojunctions (BHJ) made of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]phenylC61-butyric acid methyl ester (PCBM) at least four different models are discussed in
the literature. In an earlier work we have shown that an exponential in addition to a Gaussian
function is needed to reproduce SBG external quantum efficiency (EQE) spectra. There, the
exponential function was assigned to a disorder related absorption tail, while the SBG EQE
Gaussian was not assigned unambiguously. In the present work, the SBG EQE Gaussian is
assigned to a hole-polaron transition at P3HT rather than to a direct charge transfer transition
from the P3HT HOMO to the PCBM LUMO or absorption of molecularly dispersed PCBM
as concluded from temperature dependent EQE measurements.
124
POLYMER SOLAR CELLS – VISUALIZING VERTICAL PHASE
SEPARATION IN SOLUTION-PROCESSED FILMS OF POLYMER
FULLERENE BLENDS
Ana Sofia Anselmo,1 Andrzej Dzwilewski,1 Jakub Rysz,2 Andrzej Budkowski2, Krister
Svensson1, Jan van Stam3, Ellen Moons1
1 Department of Physics and Electrical Engineering, Karlstad University, SE-65188 Karlstad,
Sweden
2 Institute of Physics, Jagiellonian University, Kraków, Poland
3 Department of Chemistry and Biomedical Sciences, Karlstad University, SE-65188
Karlstad, Sweden
[email protected]
Recently power conversion efficiencies for polymer solar cells of over 8% have been
reported.1 Morphology control has been one major key to the improvements in energy
conversion efficiency of polymer-based photovoltaic devices. Polymer bulk heterojunction
solar cells consist of solution-cast thin films of electron donor and electron acceptor
molecules mixed with one another, a so-called bulk heterojunction. This molecular
distribution has a strong effect on the charge generation processes in the solar cell, such as the
separation of excitons into mobile charges at the donor/acceptor interface and the transport of
these mobile charges to the electrodes.
When a thin film is prepared by spincoating a blend of a conjugated polymer and the
fullerene-based acceptor material, PCBM, demixing determines the nanostructure in the film,
which is influenced by the polymer-fullerene-solvent interactions, the molecules’ tendency to
self-organise, and the kinetics of the film formation. During spincoating, characterized by
rapid solvent evaporation, the kinetics of crystallization and of liquid-liquid phase separation
compete.2 The formation of lamellar phases and vertical concentration gradients has been
reported for several blend systems, among which APFO3:PCBM3 and P3HT:PCBM.4-5
Characterization of the composition at these interfaces requires techniques that exhibit both
the chemical contrast and the lateral or depth resolution required to unveil the nanostructure
of these bulk heterojunctions. A combination of surface techniques with depth profiling
methods is necessary to obtain a full picture of the film.
125
Depth profiling methods such as dynamic secondary ion mass spectrometry (dSIMS),3,6,7
electron
tomography,8-10
neutron
reflectivity11-12
and
variable-angle
spectroscopic
ellipsometry (VASE)13-14 have been used successfully to demonstrate the spontaneous vertical
phase separation in thin films of most polymer:fullerene systems studied for OPVs.
Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has proven to be a
powerful technique to study compositions at the surface and near the substrate of films of
blends of P3HT (poly(3-hexylthiophene)) with PCBM.15-17
Here we have used a combination of dynamic Secondary Ion Mass Spectrometry (d-SIMS),
and Near-Edge Absorption Fine Structure (NEXAFS) spectroscopy to probe the surface and
bulk composition of blend films of APFO3 (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'di-2-thienyl-2',1',3'-benzothiadiazole]) and PCBM ([6,6]-phenyl-C61-butyric acid methyl
ester) with 1:1 and 1:4 (w/w) blend ratios.
The different depth ranges of two NEXAFS detection modes, partial (PEY) and total electron
yield (TEY), provide a tool to access compositional gradients in the surface region of
APFO3:PCBM blend films. The PEY-NEXAFS spectra of APFO3, PCBM and a 1:4 blend of
APFO3 and PCBM are shown in Figure 1. Because the characteristic X-ray absorption
resonances of APFO3 and PCBM were easily resolved, the composition ratio of the blend
films could quantified by fitting the blend NEXAFS spectrum with a linear combination of
the pure component spectra. Results, shown in Table 1, show strong polymer enrichment of
the top surface, both in blend films spin-coated from chlorobenzene (CB) and from
chloroform (CF) solutions. Differences in composition between surface, sub-surface, and bulk
are observed and form clear evidence for vertical phase separation.
126
PEY-NEXAFS
Signal Intensity (arb. units)
PCBM
APFO3
blend 1:4 w/w (20% polymer)
fit 1:1 w/w (50% polymer)
residual plot
280
290
300
310
Photon Energy (eV)
320
Figure1.ͳ•Ǧ‡†‰‡•’‡…–”ƒ‡ƒ•—”‡†ƒ––Š‡ƒ‰‹…ƒ‰Ž‡‹’ƒ”–‹ƒŽ‡Ž‡…–”‘›‹‡Ž†
ȋȌ‘ˆȋ–‘’Ȍ–Š‡„Ž‡†…‘’‘‡–•ƒ†ȋ„‘––‘Ȍ–Š‡ͳǣͶ™Ȁ™„Ž‡†‘ˆ͵ƒ†
’”‡’ƒ”‡†ˆ”‘…ŠŽ‘”‘ˆ‘”Ǥ–Š‡„‘––‘‰”ƒ’Š–Š‡„‡•–Ž‹‡ƒ”…‘„‹ƒ–‹‘ˆ‹–‹•
ƒŽ•‘•Š‘™Ǥ
Table1.‡‹‰Š–„Ž‡†”ƒ–‹‘•…ƒŽ…—Žƒ–‡†ˆ”‘–Š‡„‡•–Ž‹‡ƒ”…‘„‹ƒ–‹‘ˆ‹–‘ˆ–Š‡
‡š’‡”‹‡–ƒŽ„Ž‡†•’‡…–”ƒǤ
50:50
20:80
PEYǦNEXAFS
͸͵ǣ͵͹
Ͷͺǣͷʹ
TEYǦNEXAFS
ͶͶǣͷ͸
ʹ͵ǣ͹͹
PEYǦNEXAFS
͸͹ǣ͵͵
ͷͶǣͶ͸
TEYǦNEXAFS
Ͷͷǣͷͷ
ʹͺǣ͹ʹ
CB
CF
APFO3:PCBM
127
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Progress in photovoltaics: research
and applications 2011, 19, 84-92.
Nilsson, S. et al, Macromolecules 2007, 40, 8291.
C.M. Björström, A. Bernasik, J. Rysz, A. Budkowski, S. Nilsson, M. Svensson,
M. R. Andersson, K. O. Magnusson, E. Moons, J. Phys.: Condens. Matter 2005, 17,
L529.
D. S. Germack, C. K. Chan, B. H. Hamadani, L. J. Richter, D. A. Fischer, D. J. Gundlach,
D. M. DeLongchamp Appl. Phys. Lett. 2009, 94, 233303.
B. Xue, B. Vaughan, C.-H. Poh, K. B. Burke, L. Thomsen, A. Stapleton, X. Zhou, G.W.
Bryant, W. Belcher, P. C. Dastoor J. Phys. Chem. 2010, 114, 1.
Björström, C. M.; Nilsson, S.; Bernasik, A.; Budkowski, A.; Andersson, M.; Magnusson,
K.; Moons, E. Applied Surface Science 2007, 253, 3906-3912.
A. S. Anselmo, L. Lindgren, J. Rysz, A. Bernasik, A. Budkowski, M. R. Andersson, K.
Svensson, J. van Stam, E. Moons Chem. Mater. 2011, 23, 2295.
Yang, X.; Loos, J. Macromolecules 2007, 40, 1353-1362.
Bavel, S. van; Sourty, E.; With, G. de; Loos, J. Nano letters 2009, 9, 507-513.
Andersson, B. V.; Herland, A.; Masich, S.; Inganäs, O. Nano letters 2009, 9, 853-855.
Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Soft Matter
2010, 6, 641-646.
Parnell, A. J.; Dunbar, A. D. F.; Pearson, A. J.; Staniec, P. A.; Dennison, A. J. C.;
Hamamatsu, H.; Skoda, M. W. A.; Lidzey, D. G.; Jones, R. A. L. Advanced materials
2010, 22, 2444-2447.
Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.;
Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Nature materials
2008, 7, 158-164.
Müller, C.; Bergqvist, J.; Vandewal, K.; Tvingstedt, K.; Anselmo, A. S.; Magnusson, R.;
Alonso, M. I.; Moons, E.; Arwin, H.; Campoy-quiles, M.; Inganäs, O. Journal of
Materials Chemistry 2011, advance article (DOI: 10.1039/c1jm11239b).
Germack, D. S.; Chan, C. K.; Hamadani, B. H.; Richter, L. J.; Fischer, D. A.; Gundlach,
D. J.; DeLongchamp, D. M. Applied Physics Letters 2009, 94, 233303.
Germack, D. S.; Chan, C. K.; Kline, R. J.; Fischer, D. a; Gundlach, D. J.; Toney, M. F.;
Richter, L. J.; DeLongchamp, D. M. Macromolecules 2010, 43, 3828-3836.
Xue, B.; Vaughan, B.; Poh, C.-how; Burke, K. B.; Thomsen, L.; Stapleton, A.; Zhou, X.;
Bryant, G. W.; Belcher, W.; Dastoor, P. C. Solar Cells 2010, 15797-15805.
128
BIOGRAPHIC DATA OF PROFESSOR ELLEN MOONS
Ellen Moons is professor in materials physics at
Karlstad University in Sweden. Her present research
focus is the morphology control in solution-deposited
films of conjugated polymers and fullerenes for the
optimization of the performance of polymer solar cells.
By synchrotron-based electron spectroscopy and depth
profiling techniques she visualizes the development of
vertical phase separation in molecular films.
photo: Andreas Reichenberg
Ellen Moons received her Ph.D. from the Weizmann Institute (Israel) in 1995. After
postdoctoral research on dye-sensitized solar cells at the University of Technology in Delft,
The Netherlands and at EPFL in Lausanne, Switzerland, she moved in 1998 to the Cavendish
Laboratory in Cambridge, UK, and to the spin-off company Cambridge Display Technology,
where she studied the effect of morphology in thin films of conjugated polymer blends on the
performance of polymer light-emitting diodes. In 2000 she moved on to Karlstad University
in Sweden. In 2011 she received the Göran Gustafsson prize in physics of the Royal Swedish
Academy of Sciences.
129
Part IX:
Materials and
technologies 4
LOW TEMPERATURE SINTERING OF INKJET PRINTED SILVER
TRACKS
J. Perelaer,1,* Ulrich S. Schubert1,*
1
Laboratory of Organic and Macromolecular Chemistry (IOMC) and Jena Center for Soft
Matter (JCSM), Friedrich-Schiller-University Jena, Humboldtstraße 10, 07743 Jena,
Germany.
E-mail: [email protected] and [email protected]
Inkjet printing is an interesting technique for the production of microelectronic structures. The
main advantage of printing lies in its flexibility, low cost, ease of processing and its potential
for mass production. During the last years, there has been a growing interest in printing silver
inks.[1-4]
After printing the silver nanoparticles are usually sintered in an unselective manner by heating
them to 220 °C for 60 minutes, but we have developed a novel method to sinter nanoparticles
in a fast and selective way by using microwave radiation.[2] Hereby, not only the sintering
times are reduced to 3 minutes only, but also low Tg polymer foils can be used, like PET. The
resistivity of the conductive features was similar to those sintered conventionally at 220 °C
for 60 minutes. Moreover, when applying conductive antennae structures that absorb the
electromagnetic waves in a more efficient manner, the sintering times were even improved
down to a single second – a process now referred to as flash sintering.[6,7]
These conductive features may be used in many plastic electronic applications, such as radio
frequency identification (RFID) tags, lighting & signage, electrodes in sensors, etc.
References
[1]
J. Perelaer, P. J. Smith, D. Mager, D. Soltman, S. K. Volkman, V. Subramanian, J. G.
Korvink, U. S. Schubert, J. Mater. Chem. 2010, 20, 8446.
[2]
J. Perelaer, B.-J. de Gans, U. S. Schubert, Adv. Mater. 2006, 18, 2101.
[3]
T. H. J. van Osch, J. Perelaer, A. W. M. de Laat, U. S. Schubert, Adv. Mater. 2008, 20, 343.
[4]
C. E. Hendriks, P. J. Smith, J. Perelaer, A. M. J. van den Berg, U. S. Schubert, Adv. Funct.
Mater. 2008, 18, 1031.
[5]
I. Reinhold, C. E. Hendriks, R. Eckardt, J. M. Kranenburg, J. Perelaer, R. R. Baumann,
U. S. Schubert, J. Mater. Chem. 2009, 19, 3384 (including Back Cover).
[6]
J. Perelaer, M. Klokkenburg, C.E. Hendriks, U.S. Schubert, Adv. Mater. 2009, 21, 4830.
[7]
J. Perelaer, R. Abbel, S. Wünscher, R. Jani, T. van Lammeren, U. S. Schubert, Adv. Mater.
2012, DOI: 10.1002/adma.201104417.
130
BIOGRAPHIC DATA OF DR JOLKE PERELAER
Jolke Perelaer obtained his masters in chemistry at the
University of Utrecht in 2004. In 2009 he finished his PhD
within the group of Prof. Schubert (Eindhoven University of
Technology,
the
Netherlands)
with
the
research
title
"microstructures prepared via inkjet printing and embossing
techniques." He continued his work with Prof. Schubert as
project manager of the inkjet group at the Friedrich-SchillerUniversity in Jena (Germany). The topics include printed
electronics
(photovoltaic,
OLED,
RFID),
materials screening and printed bio-materials.
131
combinatorial
IMPROVING THE PHOTOVOLTAIC PERFORMANCE OF POLYMER
BASED SOLAR CELLS WITH MOLECULAR DOPING
Antonietta De Sio1, Ali Veysel Tunc1, Enrico Da Como2, Jürgen Parisi1, Elizabeth von Hauff3
1
2
3
Energy and Semiconductor Laboratory, Institute of Physics,
Carl von Ossietzky University of Oldenburg
Photonics and Optoelectronics Group, Department of Physics and CeNS LudwigMaximilians-University, Munich, Germany
Department of Physics, University of Freiburg and Fraunhofer ISE, Freiburg (Germany)
Abstract
The power conversion efficiency of polymer:fullerene solar cells is limited by the low hole
mobility in the polymer phase, and recombination effects at the donor-acceptor interface 1. In
this contribution, we demonstrate a novel strategy to improve the power conversion efficiency
of polymer based solar cells. The electronic properties of the low bandgap polymer poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3benzothiadiazole)] (PCPDTBT) are modified by doping with 2,3,5,6-TetraÀuoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ) molecules via co-solution. We show that at low doping
concentrations the hole conductivity and mobility increases. Transient and steady state
photophysical investigations demonstrate an increase in the efficiency of charge separation 2
leading to higher polaron densities in the blend. We discuss the increase in performance in
terms of trap filling with increased carrier density, and reduced recombination correlated to
the improvement in mobility. The results demonstrate a new approach to tune the charge
separation and transport efficiency to achieve optimised solar cell performance 3.
References:
1
M. Hallermann, E. Da Como, J. Feldmann, M. Izquierdo, S. Filippone, N. Martin, S. Juchter E. von Hauff,
Appl. Phys. Lett. 97, 023301 (2010).
2
F. Deschler, E. Da Como, R. Tautz, E. Von Hauff, U. Scherf J. Feldmann, Phys. Rev. Lett. 107 127402 (2011)
3
A. De Sio, A. V. Tunc, E. Da Como, J. Parisi, E. von Hauff, submitted (2011)
132
BIOGRAPHIC DATA OF PROFESSORIN ELIZABETH VON HAUFF
Elizabeth von Hauff studied Physics at the University of
Alberta, in Edmonton, Canada. From 2002-2005 she did
her PhD work under the supervision of Prof. Dr. V.
Dyakonov at the University of Oldenburg in Germany.
From 2006-2011 she was an assistant professor in the
group of Prof. J. Parisi in Physics, at the University of
Oldenbug. Since July 2011 she is an associate professor
in Physics at the University of Freiburg, Germany, and
works in close cooperation with the Fraunhofer Institute for Solar Energy Systems (ISE). Her
research focuses fundamental processes in organic semiconductors and the development of
novel devices based on organic materials.
133
SILICON/ORGANIC HYBRID HETERO–JUNCTION INFRARED
PHOTODETECTOR OPERATING IN THE TELECOM REGIME
Gebhard. J . Matt‫ כ‬, Mateusz Bednorz‫ ככ‬, Eric Daniel Glowacki‫ כככ‬, Christoph
J. Brabec‫ כ‬, Markus Scharber‫ ככככ‬, Thomas Fromherz‫ככ‬
‫כ‬Lehrstuhl für Werkstoơe der Elektronik- und Energietechnik, FriedrichAlexander-Universität Erlangen-Nürnberg, Martensstraße 7, 91058 Erlangen
(Germany); E-mail: [email protected]
‫ ככ‬Institute for Semiconductor and Solid State Physics, Johannes Kepler University,
Altenbergerstrae 69, 4040 Linz (Austria)
‫ כככ‬Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University,
Altenbergerstrasse 69, 4040 Linz (Austria)
‫ ככככ‬Konarka Austria, Altenbergerstrasse 69, 4040 Linz (Austria)
The authors report on the fabrication of a Silicon/organic heterojunction based IR
photodetector. It is demonstrated that an Al/p-Si/perylene-derivative/Al heterostructure
exhibits a photovoltaic effect up to 2.3 μm. Although the devices are not optimized, at room
temperature a rise time of 300 ns, a responsivity of ‫׽‬0.2 mA/W with a speci¿c detectivity of
D‫ ׽ כ‬7 · 107 Jones at 1.55μm is found [1]. The achieved responsivity is two orders of
magnitude higher compared to our previous efforts [2]. It will be outlined that the
photocurrent originates from an absorption mechanism at the organic/inorganic interface. The
non-invasive deposition of the organic interlayer onto the Si results in compatibility with the
CMOS process, making the presented approach a potential alternative to all inorganic
device concepts.
[1] Mateusz Bednorz, Gebhard J. Matt, Eric Daniel Glowacki, Christoph J.
Brabec, Markus C. Scharber, Helmut Sitter, and Thomas Fromherz. Silicon/organic
hybrid hetero–junction infrared photodetector operating in the telecom regime. submitted,
2012.
[2] Gebhard J. Matt, Thomas Fromherz, Mateusz Bednorz, Saeid Zamiri, Guillaume
Goncalves, Christoph Lungenschmied, Dieter Meissner, Helmut Sitter,
Niyazi Serdar Sariciftci, Christoph
J Brabec, and Günther Bauer. Fullerene sensitized silicon for near- to mid-infrared light
detection. Advanced materials (Deer¿eld Beach, Fla.), 22(5):647–50, February 2010.
Available
from: http://onlinelibrary.wiley.com/doi/10.1002/adma.200901383/abstract,
134
IMPACT OF TRAP-ASSISTED RECOMBINATION ON THE
PERFORMANCE OF POLYMER-FULLERENE BULK
HETEROJUNCTION SOLAR CELLS
C. DEIBEL, A. FÖRTIG, J. LORRMANN, J. GORENFLOT,
A. WAGENPFAHL, D. RAUH, J. RAUH, AND V. DYAKONOV
1
Experimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilians University
of Würzburg, Am Hubland, 97074 Würzburg, Germany
Recombination of photogenerated charges in organic photovoltaic devices limits the power
conversion efficiency. Understanding its detailed nature is therefore an important prerequisite
for a directed performance optimisation. We investigated devices made from P3HT and
PTB7, as well as their blends with fullerene derivatives by temperature dependent transient
absorption and charge extraction measurements. These experimental methods allow us to
explore the important role the trapping of charge carriers plays in the charge transport and
nongeminate recombination. Whereas the recombination in neat P3HT polymer is mainly
determined by a recombination order of two, as expected for the recombination of free
electrons and holes, we observe in the P3HT:PCBM blends a trap-limited process with
recombination orders clearly exceeding two. For the latter, the current-voltage characteristics
Figure 1: Slow nongeminate recombination of a free and a formerly trapped charge
carrier at at donor-acceptor heterointerface within a polymer-fullerene solar cell. In this
case, the emission rate from the trap determines the recombination rate.
135
can be completely reconstructed by considering nongeminate recombination with the
experimentally determined, carrier concentration dependent recombination rate and the carrier
concentration at a range of different voltage bias values. However, in order to explain these
results in view of the nature of nongeminate recombination, we present a multiple-trappingand-release model, optimised for describing charge recombination in organic solar cells based
on coupled continuity equations with continuous trap distributions. With this approach, we
can describe the experimentally determined charge carrier dynamics quantitatively. We find
that a trap-induced delay of the bimolecular recombination leads to the higher apparent
recombination orders. This effect is amplified if the donor-acceptor phase separation is
considered: charge carriers trapped within one of the material phases can only recombine with
a mobile charge (before extraction from the device) if and when they are re-emitted from the
trap. We discuss our findings in terms of the impact of trapping on the recombination rate and
the solar cell performance.
[1] D. Rauh, C. Deibel, V. Dyakonov, Charge density dependent nongeminate recombination
in organic bulk heterojunction solar cells, accepted for publication by Adv. Funct. Mater.
(2012) [arXiv:1203.6105]
136
BIOGRAPHIC DATA OF DR CARSTEN DEIBEL
Dr. Carsten Deibel is junior researcher and group leader at Prof.
Vladimir Dyakonovâ™s Chair of Experimental Physics VI,
University of Würzburg, Germany. He obtained his Master of
Philosophy (Physics) in 2000 from University of Sussex,
Brighton. His PhD research was done ain the group of Prof.
Jürgen Parisi at the University of Oldenburg, Germany; it
concerned the characterisation of defects in inorganic thin film
solar cells made of Cu(In,Ga)(S,Se)2 in cooperation with Shell
Solar (now Avancis). He obtained his PhD in natural sciences
(Physics) in 2002. From 2003-2005, he had a postdoctoral
position at the Interuniversity Microelectronics Center, Leuven,
Belgium, in the Polymer and Molecular Electronics Group of
Prof. Paul Heremans, working on polymer solar cells and discotic liquid crystalline devices.
Since 2005, he holds the position in Würzburg, where he is responsible for the fundamental
research of charge generation and transport in organic semiconductors and optoelectronic
devices, which bears direct relevance to organic and hybrid photovoltaics. In 2010, he was
selected to join the Förderkolleg of the Bavarian Academy of Sciences and Humanities.
137
Part X:
Posters
CHARGE SEPARATION DYNAMICS AT ZNPC: C60 BULK HETEROJUNCTIONS USING TIME-RESOLVED TERAHERTZ
SPECTROSCOPY
Andreas F. Bartelt*, Christian Strothkämper, Rainer Eichberger
Helmholtz Center Berlin for Materials and Energy, Hahn-Meitner-Platz 1, D-14109 Berlin,
Germany
[email protected]
In organic donor-acceptor material compositions, photoexcitation results in the formation of
excitons which can be separated at the donor-acceptor interface. However, for the generation
of free charges the Coulomb attraction of the electron and hole across the interface needs to
be overcome. In many cases these bound polaron pairs, or charge-transfer states, can result in
enhanced geminate recombination of charges, which is detrimental for the photovoltaic
performance. In this contribution, the influence of the donor-acceptor morphology on the
ability to separate interfacially bound charges is analyzed for the model system ZnPc:C60. The
morphologies of ZnPc:C60 blend film can be optimized by controlling the growth temperature
during deposition, inducing phase segregation and crystallinity. Here, the influence of growth
temperature induced structural changes on the generation of free charges in ZnPc:C60 blend
films is investigated using optical-pump terahertz-probe spectroscopy (OPTP)1. The growth
temperature was varied between 25°C and 120°C. The all-optical OPTP method allows
determining the photoconductivity of thin films with sub-picosecond time resolution and
without contacts.
Excitation at 800 nm results in the creation of ZnPc-excitons and charge-transfer states. No
C60 excitation occurs at this wavelength. The photo-generated ZnPc-excitons lead to the
formation of vibrationally hot charge-transfer states at the interface with C60. Taking
advantage of the excess energy, some free charges are generated within 1 ps. The generation
of a photoconductivity is detected within the time resolution of the experiment. In competition
to the charge separation is the relaxation to the charge transfer ground state of Coulombically
attracted charges, which usually occurs within picoseconds. An increased time-dependent
photoconductivity is observed for improved C60 crystallinity and domain size in films with
higher growth temperature. While for amorphous blends the interfacial charge-transfer state
hampers the generation of free charges, growing crystalline C60 nanodomains help reaching
the charge-separated state by separation of the relaxed interfacial charge transfer state. In this
case, a sequential charge separation process is observed, which entails both vibrationally hot
and relaxed charge transfer states, on time scales between <100 fs and ~100 ps. The
separation from relaxed charge transfer states is in competition with geminate recombination,
which can take place within hundreds of picoseconds to nanoseconds. The increase of
photoconductivity with film growth temperature correlates with formerly observed solar cell
photocurrent improvements.
Fig.1: Photoconductivity transients following optical excitation of ZnPc:C60 blend layers grown on
substrates with temperatures TS=25°C, 90°C and 120°C. With increasing TS, the instantaneous
photoconductivity increases while a second and slower rise appears.
High local mobilities of minimal μ~0.3 cm2/Vs are obtained from the measured
photoconductivities, which increase with higher growth temperatures. According to the
Onsager-Braun theory, higher mobilities foster the probability for charge separation.
Furthermore, when crystalline C60 domains are formed, the electrons become gradually more
mobile and can relax to more delocalized states. The increased geminate charge distance
lowers their binding energy and favors charge separation. Additionally, the dissociation yield
improves as the average C60 domain size approaches the Onsager capture radius rc ~ 15 nm
which corresponds to the average C60 domain sizes in our TS=120°C samples. We will also
discuss the possibilities of an unfavorable interfacial band bending which might be reduced
with increasing growth temperatures.
Fig 2: The separation of vibrationally hot charge–transfer states is in competition with thermal
relaxation of charge-transfer states. Separation from thermalized charge-transfer states can result in
the formation of the charge-separated state, and is in competition with geminate recombination.
1
Andreas F. Bartelt, Christian Strothkämper, Wolfram Schindler, Konstantinos Fostiropoulos,
und Rainer Eichberger, Appl. Phys. Lett. 99, 143304 (2011).
Curriculum vitae
Dr. Andreas F. Bartelt
Dr. Andreas F. Bartelt
Helmholtz-Zentrum Berlin für Materialien und Energie
Institut für Solare Brennstoffe und Speichermaterialien, E-I6
Hahn-Meitner-Platz 1
14109 Berlin
Career
Since 09/2007
Helmholtz-Center Berlin for Materials and Energy
2004 - 2007
Lawrence Berkeley National Laboratory (USA)
2002 - 2004
Princeton University (USA)
1997 - 2002
Free University Berlin (Germany)
1996 - 1997
Max-Planck-Institut für Strömungsforschung Göttingen
(Germany)
University
1997 – 2002
Dissertation at the Free University Berlin, physics department
Degree: Dr. rer. nat. (2002)
1993 – 1997
Graduate school in physics (Hauptstudium) at the GeorgAugust-University Göttingen (Germany)
Degree: Diploma in physics (1997)
1992 – 1993
ERASMUS-scholarship for a one-year university exchange
programm with the University of Lisbon (Portugal),
department of physics
1989 – 1992
Undergraduate school in physics at the Georg-AugustUniversity Göttingen (Germany)
IN SITU REFLECTANCE IMAGING OF ORGANIC THIN FILM
FORMATION FROM SOLUTION
J. BERGQVIST1*; H. ARWIN2; O. INGANÄS1
1
Biomolecular and Organic Electronics, IFM, and Center of Organic Electronics, Linköping
University, SE-581 83 Linköping, Sweden
E-mail: [email protected]
2
Laboratory of Applied Optics, IFM, Linköping University, SE-581 83 Linköping, Sweden
The rapid progress of organic photovoltaic devices during the last decade, with power
conversion efficiencies now exceeding 8%1, has brought the technology close to an industrial
breakthrough. Organic photovoltaic devices (OPV) based on polymer:fullerene blends has the
benefit of being solution processable, thereby roll to roll printing is desired to gain the
production advantage. The formation of the photoactive material from solutions needs to be
controlled and optimized. Therefore a suitable method to monitor the deposition process is
needed as deviations of drying times1 and drying rates2 during the coating process have
proven to generate morphology variations causing variations in photocurrent generation.
In this work we demonstrate how reflectance imaging can be used as a non-invasive tool for
areal surveillance of the drying process, both for spin coating and blade coating deposition. A
blue LED with a narrow bandwidth is used as light source to generate specular reflections
imaged by a CMOS camera. The thinning of the wet film can then be observed by thin film
interference. Each pixel display a sine shaped reflectance with increasing amplitude versus
time due to the alternating fulfillment of the conditions for constructive and destructive
interference, as the thickness of the wet film decrease. This enables an estimation of the
solvent evaporation rate for each pixel during the later stage of coating, as the evaporation
rate is proportional to the frequency of the reflectance curve. By averaging the frequency for
each pixel and subsequently map this over the substrate, variations in evaporation rate can be
illustrated. For spin coating the evaporation rate is shown to increase with the distance from
the rotation center, whereas the air flow is the determining parameter during blade coating. By
mapping the times when interference ceases, lateral variations in drying time are visualized.
For blade coating a mixed solvent of toluene and orthodichlorobenzene (oDCB) is used, as
OPV efficiency is superior from oDCB but toluene is preferred as being less hazardous to the
environment. An initial fast evaporation is observed from a high frequency of the reflectance
oscillations as the toluene evaporate, while the final stage of the drying is dominated by a
slower evaporating oDCB phase. Moreover lateral thickness variations of the dry film can be
visualized by scanning ellipsometry. The possibility to monitor the thin film formation as well
as lateral variations in thickness in-situ by a non-invasive method, is an important step for
future large scale applications where stable high performing generating morphologies have to
be formed over large areas.
References:
1
Service, R.F. Outlook brightens for plastic solar cells (2011) Science, 332 (6027), 293.
2
Schmidt-Hansberg, B.; Sanyal, M.; Klein, M.F.G.; Pfaff, M.; Schnabel, N.; Jaiser, S.; Vorobiev, A.; Müller, E.;
Colsmann, A.; Scharfer, P.; Gerthsen, D.; Lemmer, U.; Barrena, E.; and Schabel, W., ACS Nano 5 , 2011, 85798590
3
Hou, L.; Wang, E.; Bergqvist, J.; Andersson, V.B.; Wang, Z.; Müller, C.; Campoy-Quiles, M.; Andersson,
M.R.; Zhang, F.; Inganäs, O., Adv. Func. Mat. 21 , 2011, 3169–3175
BIOGRAPHIC DATA OF JONAS BERGQVIST
Jonas Bergqvist
[email protected]
2010
Master of Science in Applied Physics, Linköping University, Sweden
2011
Started Ph.D. on the topic of Optical methods to study material and components
for polymer solar cells, Biomolecular and Organic electronics, IFM, Linköping
University
MULTI-FREQUENCY TRANSCONDUCTANCE TECHNIQUE ON OFET’S
I. Hörselmann1; S. Scheinert1
1
Ilmenau University of Technology, Department of solid state electronics, P.O.
Box 100565, D-98684, Ilmenau, Germany
The characterization of the interface properties is import to improve the performance of
organic field effect transistors (OFET’s). In [1] the application of the multi-frequency
transconductance (MFT) technique, developed for silicon based MOSFET’s [2,3], on OFET’s
was described to estimate the surface trap state density. Motivation of our contribution is to
check the applicability of the MFT-technique on OFET’s with two dimensional numerical
simulation methods.
The simulated bottom contact thin film transistor is similar to paper [1], with 100nm gate
oxide (C"ox=34.5nFcm-2). Only the channel length was reduced from 50 to 2μm and the
mobility of the semiconductor increased, to ensure the channel transit time is small against
trap time constant. Fig. 1 shows the cross section of simulated transistor. The semiconductor
material is a 35nm thin P3HT layer with following
properties:
mobility
ȝ=1cm2V-1s-1,
affinity
Ȥ
=3.0eV, gap Eg=2.0eV, doping level NA=5×1016
Fig.1: Simulated device structure.
cm-3, density of states in the valence and
conduction band NV=NC=1021cm-3 and thermal
velocity vth=10cms-1. Source and drain contacts are ohmic, the gate work function was chosen to 4.17eV. The drain-source voltage for all following simulation results is VDS=-0.2V. The
interface trap density was set to N”it=1.5×1012cm-2. Fig. 2 illustrates the influence of different
trap levels on the transfer characteristic compared with the trap free case. The diagram 3
shows the difference between high- (gmHF) and low- (gmLF) frequency transconductance (gm)
with donor-like interface traps. At high frequencies the trap states are not able to follow the
ac-signal, consequently gm increases at frequencies higher then trap time constant. The publication [1] suggest to calculate the trap density per unit energy from the difference of gmHF
and gmLF with the following equation:
Dit''
I D Cox"
k BT
ª
º
1
1
«
LF
HF »
¬ Re{g m } Re{g m } ¼
(1).
-7
3 E -E (eV)
t
V
-8
10
-9
10
gm (nS/μm)
ID (A/μm)
10
Et-EV (eV)
trap free
0.1
0.2
0.3
0.4
-40
1
NF
gm f=1Hz
NF
5
gm f=10 Hz
-10
10
2
trap free
0.1
0.2
0.3
0.4
-30
-20
-10
0
-40
0
-30
VGS (V)
-20
-10
0
VGS (V)
Fig. 2: Transfer characteristic for donor-like
interface trap states in comparison to trap free
case.
Fig. 3: Transconductance in case of donor like
interface trap states simulated for 1 and
1×105 Hz.
The results are shown in Fig. 4. However, this approximation is valid for weak inversion [3], but not
for the accumulation or depletion, were the OFET operates. As a result the calculated trap
densities are too high and the MFT-method is not applicable to OFET’s without a
modification. We modified the MFT-technique starting with eq. 2 and 3, for VDS<VGS:
ID
gm
w
μQ "p
L
wI D
wVGS
dM Fp
w
μV DS (Qsc" Qit" )
dx
L
wM S (Qsc" Qit" )
w
μV DS
wVS
wM S
L
|
( 2)
(3).
At high frequencies there is no contribution from Q”it to gm because the trap states can’t
follow the ac signal, together with the voltage balance on the MOS-structure, and the
approximation CscNF=CscHF we obtain eq. 4:
"
it
D
Cox"
e
ª
º
g mLF
g mHF
«
» with
LF
KVDS g mHF ¼
¬ KVDS g m
K
w
PCox"
L
(4) .
Fig. 5 contains the D”it calculated by the modified MFT-method (eq. 4). As expected, using eq. 4
the calculated trap densities per unit energy multiplied by the energetic distribution of 2*kBT
are not higher than the implemented interface density N”it.
60
ET-EV (eV)
6
0.1
0.2
0.3
0.4
4
ET-EV (eV)
0.1
0.2
0.3
0.4
12
-1
-2
Dit (10 eV cm )
-2
30
20
2
"
"
12
40
-1
Dit (10 eV cm )
50
10
0
-40
-30
-20
-10
0
-40
0
-30
-20
VGS (V)
-10
0
VGS (V)
Fig. 4: D”it calculated by MFT-method eq. 1
Fig. 5: D”it calculated by modified MFT-method
eq. 4
The value of interface trap density N”it itself can easily be determined by integration the
difference of gmHF and gmLF over the gate voltage with eq. 5. The results of eq. 5 are listed in
tab. 1.
N it"
L
ewPVDS
³
VGSstop
VGSstart
g
HF
m
g mLF dVGS
(5)
The calculated interface trap densities are lower than 1.5×1012cm-2, because trap states close
to the valence band are able to follow the ac-signal even in HF-case and deeper trap states
don’t change the occupation during the gate sweep.
Tab.1: Calculated trap densities, using eq. 5
ET-EV (eV)
0.1
0.2
0.3
0.4
N”it (cm-2)
0.45×1012
1.38×1012
1.26×1012
1.01×1012
With the modified MFT-technique it is possible to calculate the trap density per unit energy
D”it and the interface trap density N”it on OFET’s.
References
[1] P. Srinivas et al. A simple and direct method for interface characterization of OFETs. In Proceedings of 14th
IPFA 2007, Bangalore, India, pages 306–309, 2007.
[2] Sheng S. Li P.C. Yang, H.S. Chen. Measurements of interface state density in partially- and fully-depleted
silicon-on-insulator MOSFETs by a high-low-frequency transconductance method. Solid-State Electronics,
35(8):1031–1035, 1992
[3] Hisram Haddara and Gérard Ghibaudo. Analytical modeling of transfer admittance in small mosfets and
application to interface state characterization. solid state electronics, 31(6):1077–1082, 1987.
BIOGRAPHIC DATA OF INGO HÖRSELMANN
Name:
Telephone:
E-mail:
Ingo Hörselmann
+49 3677 693406
[email protected]
Organization:
TU Ilmenau, Institute of micro- and
nanoelectronics
Gustav-Kirchhoff-Straße 7
98693 Ilmenau
Germany
Adress:
Ingo Hörselmann has received his diploma in electrical engineering at the University Ilmenau
in 2003. In this year he joined the working group polymer electronics at the department of
solid state electronics, under the leadership of PD Dr. S. Scheinert. Main research is numeric
simulation and dynamic behavior of organic field effect transistors.
DIFFERENT APPROACHES FOR IMPROVING
ORGANIC SOLAR CELLS
M. SCHIEK1,2,*, N. TRAUTWEIN3, A. OSADNIK2, J. JENSEN1,
L. BEVERINA4, A. LÜTZEN2, H. BORCHERT3, J. PARISI3, F. BALZER1
1
NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, DK-6400
Sønderborg, Denmark
2
Kekulé-Institute of Organic Chemistry and Biochemistry, Rheinische Friedrich-WhilhelmsUniversity of Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany
3
Energy and Semiconductor Research Laboratory, Department of Physics, University of
Oldenburg, D-26111 Oldenburg, Germany
4
Department of Materials Science, University of Milano-Bicocca, Via Cozzi 53, I-20125
Milano, Italy
Organic solar cells currently struggle to compete with their inorganic counterparts made from
silicon, but there are a lot of possibilities to improve their performance. Four different
approaches are presented here:
1) Lifetime and efficiency can be enhanced by employing new active materials such as
squaraine dyes. These small molecule donor materials are readily available with a large
variety of structural motifs, they are environmentally stable and show broad absorption
within the visible-infrared in the solid state.
2) Controlling the device architecture and nanoscale morphology allows better chargecarrier separation and collection and hence increases efficiency. Thin films of the
polymeric donor material P3HT are imprinted with a nanostructured silicon mold and
subsequently covered with the PCBM acceptor to pave the way to an ideal, interdigitating
heterojunction.
3) Light trapping additives such as silver nanoparticles help to exploit the incident sunlight
more efficiently. Forward scattering of light lengthens the optical path in the active layer
resulting in a higher degree of light absorption. Additionally, the silver nanoparticles’
surface plasmon resonance leads to a local field enhancement increasing the number of
excitons generated.
4) Novel transparent electrodes such as silver nanowire mesh electrodes avoid the rare
material indium and allow the device fabrication on more flexible substrates. They are
fully solution processed with high throughput, and the random mesh of interconnected,
highly conductive nanowires is bendable and transparent over the visible-infrared range.
In all cases the key to improvement is to understand the complex structure-propertyperformance interplay. Due to the nature of charge generation and transport in solar cells
occurring on the nanoscale it is crucial to correlate nanoscopic performance with bulk
measurements. This can be realized by advanced AFM investigations, namely Kelvin Probe
Force Microscopy (KPFM) and photoconductive AFM (pc-AFM).
PHOTOVOLTAIC PERFORMANCE OF PPV-PPE COPOLYMERS:
EFFECT OF THE FULLERENE DERIVATIVE
Olga A. Mukhacheva1, Pavel A. Troshin1, Andrey E. Goryachev1, N. Serdar Sariciftci2,
Daniel A. M. Egbe2, and Vladimir F. Razumov1,
1
Institute for Problems of Chemical Physics of Russian Academy of Sciences, Semenov
Prospect 1, Chernogolovka, Moscow region, 142432, Russia, Email: [email protected]
2
Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz,
Altenbergerstrasse 69, A-4040 Linz, Austria
Several conjugated PPV-PPE copolymers were studied as electron donor materials in bulk
heterojunction organic solar cells in combination with a library of electron acceptor fullerene
derivatives. It was shown that molecular structure and solubility of the fullerene counterpart
affect significantly photovoltaic performance of both polymers. Use of [60]PCBM as electron
acceptor material yielded quite moderate power conversion efficiencies. The best results were
achieved when some fullerene derivatives with better suiting molecular structures and
solubilities were applied. In some cases the photovoltaic performance of the
polymer/fullerene blends shows direct correlation with the molecular structures of the
materials.
The obtained results suggest that every newly designed conjugated polymer should be
evaluated in solar cells using a library of fullerene derivatives instead of just conventional
PCBMs. We believe that only this combinatorial approach might bring the best performing
donor/acceptor combinations for future generations of efficient organic solar cells.
ADVANCED CALORIMETRY FOR ANNEALING STUDIES OF
ORGANIC PHOTOVOLTAICS
Niko Van den Brandea, Fatma Demira, Sabine Berthob, Dirk Vanderzandeb, Bruno Van Melea,
Guy Van Asschea
a
Physical Chemistry and Polymer Science (FYSC), Vrije Universiteit Brussel (VUB),
Pleinlaan 2, B-1050 Brussels, Belgium.
b
Institute for Materials Research (IMO-IMOMEC), Hasselt University, Wetenschapspark 1,
3590 Diepenbeek.
Introduction
Bulk-heterojunction (BHJ) solar cells are composed of a nanoscale co-continuous composite
of donor and acceptor phases, facilitating exciton dissociation. Conjugated, light-excitable
polymers are most often used as an electron donor, while fullerene derivatives are the most
widespread type of electron acceptor due to their high electron affinity and ability to transport
charge [1]. An important advantage of such a system is that it can be cast from solvent,
facilitating
processing.
Post-production
annealing
of
such
polymer:fullerene
bulk
heterojunction solar cells is vitally important, not only for fine-tuning the morphology and
thus increasing the efficiency, but also for retaining the desired morphology during long-term
operation [2-3]. However, knowing the optimal conditions for annealing temperatures and
times requires knowledge about thermal transition temperatures and annealing kinetics of the
blend systems. Using advanced fast-scanning thermal analysis techniques, the formation of
nuclei and growth of crystals during heating or cooling can be reduced or avoided, allowing
for the study of the crystallization processed during annealing. In this study, non-isothermal
and isothermal crystallization kinetics of the P3HT:PCBM (poly(3-hexyl thiophene: [6,6] –
phenyl C61 – butyric acid methyl ester) were studied by Rapid Heating Cooling Calorimetry
(RHC) [4] and Fast Scanning Differential Chip Calorimetry (FSDCC) [5].
Methodology
P3HT (Merck, Mw=35 000 g mol-1, Mw/Mn = 1.8; regioregularity greater than 98.5%) is mixed
with PCBM (Solenne) in a 1:1 ratio and dissolved in chlorobenzene (CB) at a concentration of
about 1-2.5 wt %, stirring overnight at 50 °C. The solutions were deposited by drop-casting
on large glass plates in a glove box under a nitrogen atmosphere to form films with a
thickness of 1 μm. After drying in nitrogen atmosphere at room temperature for 50 hours to
remove the residual solvent, the remaining solid films were scratched off the glass substrates
and collected as a powder for RHC and FSDCC measurements.
RHC experiments were performed on a prototype instrument made available by TA
Instruments equipped with a liquid nitrogen cooling and specifically designed for operation at
high scanning rates. Sample masses in the order of 250 - 300 μg were used in aluminium
RHC crucibles. This allows for heating and cooling rates in the order of 2000 K.min-1.
FSDCC was performed on a prototype Fast-Scanning Differential Chip Calorimeter (Rostock
Univeristy, Germany) using Xensor Integration XI39399 chips, with an active area of 100 μm
by 100 μm, having two on-chip heaters and 6 thermopiles for accurate temperature
measurements. The whole chip calorimeter is submerged in a liquid nitrogen vessel, providing
effective fast cooling. The highest rates achieved so far with this setup are in the order of 106
K.s-1, for sensor chips with a heated area of about 10 μm by 10 μm. The rates used in this
work are limited to 5000 K.s-1. For both devices thermal annealing from the melt and from the
glass were simulated by a temperature program as illustrated in figure 1, were the thermal
transitions seen during the heating runs after isothermal treatment were used for analysis.
Temperature
Annealing from
the glass
Annealing from
the melt
ISO
ISO
time
Figure 1: Temperature program used to simulate thermal annealing, both from the glass and from the melt. The
heating runs after the isothermal segments were used for data analysis.
Results & Discussion
Controlled fast heating and cooling are imperative for an isothermal crystallization study of
rapidly crystallising systems such as P3HT:PCBM.. The fast heating and cooling capacity of
advanced fast-scanning thermal analysis techniques enables the reduction of nuclei formation
and crystal growth during heating or cooling, by restricting the time available for the blend to
crystallise by quickly passing through the crystallisation temperature window that extends
from the melting temperature down to the glass transition temperature.
The thermal annealing process in P3HT:PCBM blends was studied first by performing RHC
isothermal annealing experiments at 110 °C for a wide range of isothermal times. All
measurements were performed with a heating rate of 500 K.min-1 and ballistic cooling.
Ballistic cooling reaches more than 1500 K.min-1 at the beginning of the cooling, which drops
down to 750 K.min-1 around 60 °C.
Figure 2 shows the subsequent heating curves of the system for increasing annealing times. It
is clear that the cold crystallization enthalpies are decreasing and melting enthalpies are
increasing by longer annealing times. Besides, by longer annealing at the isothermal
annealing temperature, the step in Tg is getting smaller since less amorphous fraction is
remaining in the material, and the Tg shifts to higher temperatures. As discussed in previous
work, the crystallization of P3HT (having a lower Tg) from the blend is enriching the fraction
of PCBM (having a higher Tg) in the remaining amorphous phase, leading to a higher Tg in the
subsequent heating. Of course, the Tg is further increased by the close presence of crystalline
domains, as is the case for semi-crystalline homopolymers. Another important remark is the
clear difference between isothermal crystallisation from the melt and from the glass (without
or with cooling before annealing). The crystallization rate is clearly higher when annealing
from the glass [6].
-1
-1
1 W.g
1 W.g
6 min
Heat Flow (W.g-1)
Heat Flow (W.g-1)
6 min
Exo
0 min
-50
50
150
Exo
0 min
250
-50
50
150
250
Temperature (°C)
Temperature (°C)
Figure 2: RHC heating cycles at 500 K.min-1 for a P3HT:PCBM 1:1 blend after isothermal annealing both from
the melt (left) and from the glass (right)..
This difference between the two possible paths of thermal annealing can be explained by a
significant difference between the two in the amount of crystal nuclei. This suggests that the
rates available in RHC are not sufficient to avoid most of the nucleation.
When similar tests are conducted using FSDCC, using heating and cooling rates of 5000 K.s-1,
cold crystallization disappears (see figure 3). There is also no longer a significant difference
between isothermal crystallization from the melt and from the glass. Based on these results, it
can be concluded that nucleation is mostly avoided by using heating and cooling rates
available in FSDCC, and the crystallinity seen results only from the isothermal treatment.
This allows for a reliable investigation of the crystallization processes during isothermal
annealing, both from the glass and from the melt.
10 min
Exo
0 min
20
70
120
10 min
0.1 mW
Heat flow (mW)
Heat flow (mW)
0.1 mW
170
Exo
0 min
220
20
Temperature (°C)
70
120
170
220
Temperature (°C)
Figure 3: FSDCC heating cycles at 5000 K.s-1 for a P3HT:PCBM 1:1 blend after isothermal annealing, both from
the melt (left) and from the glass (right).
Conclusion
The isothermal crystallisation at the annealing temperature of 110 °C was investigated for 1:1
P3HT:PCBM using the advanced fast-scanning thermal analysis techniques RHC and FSDCC.
RHC allowed for an in depth study of the isothermal crystallisation of these systems, but does
not avoid most of the nucleation, leading to a difference in isothermal crystallisation rates for
annealing treatments from either the glass or the melt. This difference, as well as cold
crystallisation, disappears when FSDCC is used, owing to the much higher rates of heating
and cooling. Because of this it is now possible to obtain only information about the isothermal
crystallisation, without unwanted effect of the preceding thermal treatment.
References
1.
Thompson B.C. and Frechet J.M.J., Angewandte Chemie-International Edition, 2008.
47(1): p. 58.
2.
Erb T., Zhokhavets U., Gobsch G., Raleva S., Stuhn B., Schilinsky P., Waldauf C.,
and Brabec C.J., Advanced Functional Materials, 2005. 15(7): p. 1193.
3.
Hoppe H. and Sariciftci N.S., Journal of Materials Chemistry, 2006. 16(1): p. 45.
4.
Danley R.L., Caulfield P.A., and Aubuchon S.R., American Laboratory, 2008. 40(1):
p. 9.
5.
Minakov A.A., van Herwaarden A.W., Wien W., Wurm A., and Schick C.,
Thermochimica Acta, 2007. 461(1-2): p. 96.
6.
Demir F., Van den Brande N., Van Mele B., Bertho S., Vanderzande D., Manca J., and
Van Assche G., Journal of Thermal Analysis and Calorimetry, 2011. 105(3): p. 845.
A COMPARATIVE STUDY OF BISFUNCTIONALIZED
[60]FULLERENE DERIVATIVES AS ELECTRON ACCEPTOR
MATERIALS FOR ORGANIC SOLAR CELLS
D. K. Susarova, A. E. Goryachev, P. A. Troshin, and V. F. Razumov
Institute for Problems of Chemical Physics of Russian Academy of Sciences, Semenov
Prospect 1, Chernogolovka, Moscow region, 142432, Russia. E-mail: [email protected], [email protected]
One of the main trends in the ongoing research in the filed of organic photovoltaics is
the development of fullerene-based electron acceptor materials with decreased electron
affinity. Such compounds have higher lying LUMO energy levels compared to conventional
PCBM (C60 or C70 versions) and therefore they yield higher open circuit voltages in organic
solar cells.
We report the synthesis and investigation of eight different fullerene bis-adducts. All the
compounds were characterized using several physicochemical methods that enabled them to
establish the molecular and isomeric composition. All prepared fullerene derivatives were
evaluated as electron acceptor materials in fullerene/polymer solar cells under identical
conditions with the reference material bis-[60]PCBM. It is shown that the molecular
compositions of the [60]fullerene bis-adducts affect strongly their photovoltaic performance.
In particular, some of the designed fullerene-based materials outperform mono[60]PCBM and
bis[60]PCBM in solar cells due to their optimized molecular structures.
INFLUENCE OF VARIOUS STRESS TYPES ON THE DEGRADATION
OF POLYMER/FULLERENE FILMS
Vida Turkovic, Sebastian Engmann, Gerhard Gobsch, and Harald Hoppe
Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau,
Germany
In search of ecoǦfriendly, sustainable energy sources that could reduce and eventually make
the fossil fuel and nuclear power redundant, the attention within the recent years has been
directed towards polymer solar cells. Being lowǦcost, highly scalable, flexible, lightǦweight,
and having a short energy payback time, makes this type of solar cells an interesting
commercially applicable product.
Recently, record efficiencies over 10% PCE have been reported by Mitsubishi. Still,
improving their long term stability remains a serious issue that has to be resolved in order to
make them commercially widespread. Their organic nature makes them especially sensitive to
chemical decay in contact with light, oxygen, heat, and water. As all of the mentioned factors
are present in the normal working environment of the solar cells, it is important to investigate
the degradation mechanisms and possibly find solutions how to overcome them.
An important shortcoming is their low charge carrier mobilities and short lifetimes, which
gives special importance to obtaining and maintaining the optimal morphology within the
device. To balance the charge carrier separation on one side, and charge transport on the
other, a continuous path in form of interpenetrating network of polymer and fullerene is
needed1. However, as the optimal morphology is not thermodynamically stable, the fullerene
molecules in blends with polymeric donors tend to diffuse and recrystallize after deposition,
which is especially pronounced with increase in temperature2. This process accelerates under
thermal exposure, and eventually leads to a morphological destruction of the organic solar
cell3.
In order to investigate the chemical4 and morphological5 stability of the photoactive layer in
thin film devices, we developed a simple set of experiments performed within ambient and
inert atmosphere using various stress conditions with respect to photo-, thermal and oxidative
1
HoopeH.,SariciftciN.S.,“Morphologyofpolymer/fullerenesolarcells”,J.Mater.Chem.16,p45(2006)
2
ZhaoJ.,SwinnenA.,VanAsscheG.,MancaJ.,VanderzandeD.,VanMeleB.,„PhasediagramofP3HT/PCBM
blendsanditsimplicationforthestabilityofmorphology“,J.Phys.Chem.B113,p1587(2009)
3
BerthoS.,JanssenG.,CleijT.J.,ConingsB.,MoonsW.,GadisaA.,D’HaenJ.,GoovaertsE.,LutsenL.,ManceaJ.,
VanderzandeD.,“Effectoftemperatureonthemorphologicalandphotovoltaicstabilityofbulkheterojunction
polymer:fullerenesolarcells”,Sol.EnergyMater.Sol.92,p753(2008)
4
TurkovicV.,EngmannS.,GobschG.,HoppeH.,„Theinfluenceoftypicaldegradativestressesonthe
photoactivelayerofpolymer:fullerenesolarcells“,inpreparation
5
TurkovicV.,EngmannS.,GobschG.,HoppeH.,„Methodsindeterminationofmorphologicaldegradationof
polymer:fullerenesolarcells“,Synt.Met.161,p2532(2012)
stability. The thin films were characterized with standard spectroscopic measurements,
whereas devices – completed on degraded layer stacks – were probed by electrical
measurements.
Figure 1. Optical microscopy images of the P3HT:[60]PCBM films exposed to elevated
temperature (120°C) in absence of air (N2 glovebox)
Dark GB 120°C
Ref
5 min
15 min
30 min
1h
2h
4h
8h
17 h
21 h
Abs (O.D.)
0.8
0.6
0.4
0.2
0.0
300
Dark GB 120°C
Ref
5 min
15 min
30 min
1h
2h
4h
8h
17 h
21 h
2.0
norm-PL (a.u.)
1.0
1.5
1.0
0.5
350
400
450
500
550
600
650
0.0
500
700
Wavelength (nm)
600
700
800
900
Wavelength (nm)
Figure 2. Absorbance and photoluminescence spectra of ITO/PEDOT:PSS/P3HT:[60]PCBM
layer stacks exposed to elevated temperature (120°C) in absence of air
REVEALING BULK HETEROJUNCTION BLEND MORPHOLOGY BY
SPECTROSCOPIC ELLIPSOMETRY
Authors:
S. Engmann, V. Turkovic, H. Hoppe, and G. Gobsch
Affiliation:
Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau,
Germany
Abstract:
The nanoscale morphology of bulk heterojunction solar cells is very crucial for their
performance[01]. Hence, a lot of effort has been invested to reveal and control it. In order to
elucidate the nanoscale morphology, many different complex techniques have been applied.
Recently, TEM-tomography has been used to portray the structure of the polymer-fullerene
active layer in all three dimensions [02]. In this work we used another non-destructive method,
in order to gain information about the predominant morphology and the donor-acceptor
distribution within P3HT:PCBM blend films. We demonstrate that spectroscopic ellipsometry
can be used to investigate the composition profile over the film thickness[03] and provides
detailed information about the shape and extension of phase separation. Furthermore, the
shape of nano-inclusions of the fullerene phase within the polymer matrix was determined[04].
The non-invasive character of the method allowed studying morphological aging and changes
in the concentration profile, initiated by surface-directed spinodal decomposition[05] of the
film, could be traced[06].
1
P3HT [ vol.-%]
60
2/3
Substrate
0
E
Spherical
Screening factor q
E
1/3
10
20
30
40
References:
[01]
[03]
[04]
[05]
[06]
20
0
50
60
70
80
90
100
Film Surface
0
PCBM content in blend [ wt.-%]
[02]
40
Substrate
Columnar Structure
0
Storage at RT
1d
14d
21d
43d
80
Laminar Structure
20
40
60
80
100
Film thickness [ %]
Hoppe, H. & Sariciftci, N. S.
Journal Of Materials Chemistry, 2006, 16, 45-61
van Bavel, S.; Sourty, E.; de With, G.; Frolic, K. & Loos, J.
Macromolecules,, 2009, 42, 7396-7403
Germack, D. S.; Chan, C. K.; Kline, R. J.; Fischer, D. A.; Gundlach, D. J.;
Toney, M. F.; Richter, L. J. & DeLongchamp, D. M.
Macromolecules, 2010, 43, 3828-3836
Engmann, S.; Turkovic, V.; Gobsch, G. & Hoppe, H.
Advanced Energy Materials, 2011, 1, 684-689
Vaynzof, Y.; Kabra, D.; Zhao, L. H.; Chua, L. L.; Steiner, U. & Friend, R. H.
Acs Nano, 2011, 5, 329-336
Engmann, S.; Turkovic, V.; Hoppe, H. & Gobsch, G.
Synthetic Metals, 2012, 161, 2540-2543
POLYMER SOLAR CELL LIFETIME: DEPENDENCE ON METAL
BACK ELECTRODE AND ENCAPSULATION
Authors:
Roland Rösch, Kai-Rudi Eberhardt, Gerhard Gobsch and Harald Hoppe
Affiliation:
Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau,
Germany
Presentation Type:
Poster
Abstract:
We investigated the degradation of sealed and unsealed polymer solar cells based on
PCDTBT:PCBM photoactive layer with different metal back electrodes in classical device
architecture [1]. Lifetimes were determined via automated IV-characterisation under
illumination. A titaniumoxide interlayer was used to increase the lifetime compared to bare
metal electrodes (compare with the figure below). Furthermore and to learn about the acting
degradation mechanisms, lateral inhomogeneities like shunts or electrode oxidation, were
studied by luminescence imaging and lock-in thermography.
unsealed
unsealed, stabilized
normalized PCE
1.0
0.8
0.6
0.4
0.2
0.0
0.1
1
10
100
Time in h
A lifetime increase for unsealed devices by a factor of ~100 was observed.
[1]
Roland Rösch et al., in preparation (2012)
1000
QUALITATIVE AND QUANTITATIVE CHARACTERIZATION OF
POLYMER SOLAR CELLS BY LATERALLY RESOLVED
DETECTION OF LUMINESCENCE
Marco Seeland, Roland Rösch, Gerhard Gobsch and Harald Hoppe
Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau,
Germany
Imaging of luminescence patterns by laterally resolved detection of luminescence using
appropriate cameras is a versatile characterization tool for polymer solar cells and modules
[1]. The non-invasive nature of this characterization method allows applications ranging from
quality control after processing of polymer solar modules [2] to advanced investigation of
local device degradation [3]. We present results where the combined application of electroand photoluminescence imaging was used to localize the degradation effects both laterally
and in depth with respect to the affected layer [4]. Furthermore the application of
electroluminescence imaging yields direct information concerning the local current flow
trough the active layer, where a lateral gradient in the electroluminescence and thus the
current distribution is observed. A quantitative interpretation of electroluminescence profiles
is shown and the effect on the lateral photovoltaic response is discussed [5].
References:
[1]
[2]
[3]
[4]
[5]
M. Seeland, R. Rösch and H. Hoppe, Imaging Techniques for Studying OPV Stability
and Degradation, in Stability and Degradation of Organic and Polymer Solar Cells,
edited by F. C. Krebs (John Wiley & Sons, 2012).
M. Seeland, R. Rösch, B. Muhsin, et al., Energy Procedia (Proceedings of Organic
Photovoltaics: Science and Technology (E-MRS 2011, Symposium S)), acc. (2012).
R. Rösch, D. M. Tanenbaum, M. Jorgensen, et al., Energy Environ. Sci. (2012).
M. Seeland, R. Rösch and H. Hoppe, J. Appl. Phys. 109 (6), 064513 (2011).
M. Seeland, R. Rösch and H. Hoppe, J. Appl. Phys. 111 (2), 024505 (2012).
INFLUENCE OF ORGANIC ACIDS ON DEVICE PERFORMANCE OF
P3HT : PCBM SOLAR CELLS
Authors:
B. Muhsin, F. Herrmann, C.-R. Singh, G. Gobsch, M. Presselt, and H. Hoppe
Affiliation:
Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau,
Germany
Abstract:
We have investigated the effects of addition of organic acids to polymer solar cells based on
poly(3-hexylthiophen):phenyl-C61-butyric-acid-methyl-ether (P3HT:PCBM) [1]. We have
observed a significant increase of the device performance based on improved fill factors [2].
We correlate these results with dark and photogenerated concentrations of charge carriers
evaluated from CELIV measurements as well as sub-bandgap absorption performed by photothermal-deflection (PDS) spectroscopy. The material class is rated in terms of processing
agents.
800
0.5
0.4
Absorption
900
0.3
0.2
10
0.1
0.0
300
1000
30
0,5%
0,25%
25
0,1%
0,05%
0,025%
20
0,01%
0,005%
0,0025%
15
0% CB
0% CB:CF
5
400
500
600
700
800
900
0
1000
70
0.0001
0.001
0.01
0.1
1
68
66
64
62
O in nm
0.0001
0.01
0.1
1
CB without Additive
700
CB:CF without Additive
600
Fill factor (%)
500
3
400
PL Normalized x 10
300
0.001
Additive (%)
Absorption and photoluminescence of P3HT:PCBM films processed by different amounts of
additives (left). Fill factors of solar cells in dependence of additive concentration (right).
[1]
[2]
J. A. Renz, T. Keller, M. Schneider, S. Shokhovets, K. Jandt, G. Gobsch, and H.
Hoppe, Sol. Energy Mater. Sol. Cells 93, 508 (2009).
F. Herrmann, B. Muhsin, C.-R. Singh, G. Gobsch, M. Presselt, and H. Hoppe, in
preparation (2012)
OPTIMIZATION OF ORGANIC SOLAR CELLS BASED ON
BTD/DPP COPOLYMERS
Olesia Synooka(a), Florian Kretschmer (b, c), Martin D. Hager (b, c, d) , Ulrich S. Schubert (b, c, d),
Gerhard Gobsch (a) and Harald Hoppe(a)
(a) Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693
Ilmenau, Germany
(b) Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-SchillerUniversity Jena, Humboldtstr. 10, 07743 Jena, Germany.
(c) Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University Jena, Humboldtstr.
10, 07743 Jena, Germany.
(d) Dutch Polymer Institute (DPI), John F. Kennedylaan 2, 5612 AB Eindhoven, The
Netherlands
Conjugated polymers for organic solar cells offer a lot of advantages. To influence their light
harvesting properties it is essential to tune the band gap and absorption of the polymers.
Synthesis of a ʌ-conjugated donor-acceptor polymer series based on thiophene,
diketopyrrolopyrrole, benzothiadiazole and fluorene was accomplished by means of a Suzuki
cross coupling reaction. The variation of the monomer contents strongly influences the light
absorption of the polymers along with the HOMO and LUMO levels. In this case it is
interesting to systematically vary the properties of the resulting materials for organic
photovoltaics by combinatorial chemistry. Increasing amounts of diketopyrrolopyrrole lead to
a lower band gap and enhanced absorption of red light caused by the stronger accepting
properties of this moiety (compared to benzothiadiazole). We have optimized solar cell
devices based on these materials and reveal difference between copolymer blends and these
terpolymers [1]. Also different types of solvent additives were investigated to improve solar
cells performance.
BTD/DPP(%)
90/10
80/20
70/30
60/40
50/50
40/60
30/70
20/80
10/90
100/0
0/100
0.9
Absorbance (O.D.)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
250000
BTD/DPP(%)
90/10
80/20
70/30
60/40
50/50
40/60
30/70
20/80
10/90
100/0
0/100
200000
Normalized PL
1.0
150000
100000
50000
0.1
0.0
300
400
500
600
700
800
0
500
900
600
700
800
900
1000
Wavelength (nm)
Wavelength (nm)
References:
[1] Florian Kretschmer, Martin D. Hager, Olesia Synooka, Harald Hoppe, Ulrich S. Schubert
Donor-acceptor ʌ-conjugated polymer libraries for polymer solar cells; submitted in 2012
List of Authors
A
Aernouts, T.
Aljarilla, A.
Andersson, Ersmann, P.
Andersson, L.M.
Andrae, G.
Anselmo, A.S.
Arwin, H.
Assche, G. van
98
115
19
95
84
125
143
98, 154
B
Balzer, F.
Bartelt, A.F.
Bartsch, J.
Bauer, S.
Bednorz, M.
Bergqvist, J.
Bertho, S.
Beverina, L.
Blankenburg, L.
Bodö, P.
Borchert, H.
Bozkurt, Z.
Brabec, C.J.
Brande, N. van den
Braun, S.
Budkowski, A.
11, 151
138
37
19, 119
134
143
154
151
106
19
151
119
134
98, 154
95
125
C
Caballero, R.
Como, E. da
Cruz, P. de la
115
122, 132
115
D
Deibel, C.
Demir, F.
Dobbertin, T.
Domann, G.
Dyakonov, V.
Dzwilewski, A.
135
154
117
19
135
125
E
Eberhardt, K.-R.
Edman, L.
Egbe, D.A.M.
Eichberger, R.
Eisenhawer, B.
Engmann, S.
162
72
153
138
84
159, 161
F
Fahlmann, M.
Feldmann, J.
Förtig, A.
Fromherz, T.
G
Georgakopoulos, S.
Glowacki, E.
Glowacki, E.D.
Gobsch, G.
Gobsch, G.
Gorenflot, J.
Goryachev, A.E.
Grobosch, M.
H
Hadipour, A.
Haen, J. D.
Hager, M.D.
Halik, M.
Hartmann, P.
Hauff, E. v.
Helbig, U.
Heljo, P.
Herrmann, F.
Himmerlich, M.
Hoppe, H.
95
122
135
134
31
119
134
124
159, 161, 162
163, 164, 165
135
153, 158
37
Hörselmann, I.
Hummelen, J.C.
98
98
165
113
19
122, 132
19
42
124, 164
84
124, 159, 161
162, 163, 164
165
33, 37, 146
1
I
Inganäs, O.
Irimia-Vladu, M.
9, 143
119
J
Jaeger, A.
Jensen, J.
Jong, de M.P.
117
151
95
K
Kesters, J.
Kirchmeyer, S.
Klemm, E.
Knupfer, M.
Koster, L.J.A.
Kraker, E.
Krause, M.
98
3
84
37
1
19
19
Kretschmer, F.
Kroll, M.
165
84
L
Langa, F.
Leeuw, D.M. de
Leonat L.
Li, M.
Lilja, K.
Liu, F.
Lopez-Arroyo, L.
Lorrmann, J.
Lupo, D.
Lüssem, B.
Lutsen, L.
Lützen, A.
115
98
119
42
42
6
115
135
42
58
98
11, 151
M
Maes, W.
Maltenberger, A.
Manca, J.
Marin, L.
Mateo-Alonso, A.
Matt, G. J.
May, C.
Mele, B. van
Meyer, F.
Mierlo, S. van
Mogck, S.
Mogessie, A.
Moons, E.
Muhsin, B.
Mukhacheva, O.A.
98
117
98
98
49
134
65
154
31
98
65
19
119
125
164
153
N
Nazmutdinova, G.
Nilsson, D.
Nunzi, J. M.
74
19
6
O
Opoku, C.
Osadnik, A.
89
11, 151
P
Paasch, G.
Parisi, J.
Pelado, B.
Perelaer, J.
Pertsch, T.
37
132, 151
115
130
84
Monkowius, U.
Petrukhina, A.
Pietsch, M.
Platt, D.
Presselt, M.
Pshenichnikov, M.
117
84
19
124, 164
1
R
Raabe, D.
Rauh, D.
Rauh, J.
Razumov, V.F.
Rösch, R.
Rubahn, H.-G.
Runge, E.
Ruttens, B.
Rysz, J.
74
135
16, 135
153, 158
162, 163
11
124
98
125
S
Sariciftci, N.S.
Sawatdee, A.
Scarpa, G.
Schache, H.
Scharber, M.
Scheinert, S.
Scheipl, G.
Scherf, U.
Schiek, M.
Schmid, G.
Schrödner, M.
Schubert, U. S.
Schultheis, K.
Schwabegger, E.
Seeland, M.
Sehati, P.
Sensfuß, S.
Shaheen, S.
Shkunov, M.
Shokhovets, S
Siebbeles, L.D.A.
Singh, C.-R.
Sio, A. de
Sitter, H.
Sparrowe, D.
Spijkman, M.J.
Stadlober, B.
Stam, J. van
Strothkämper, C.
Susarova, D.K.
Svensson, K.
Synooka, O.
119, 153
19
30
74, 84, 106
134
33, 37, 146
19
10, 122,
11, 151
117
74, 106
130, 165
106
119
163
95
84, 106
1
31, 89
84, 124
81
164
132
119
31
98
19
125
138
158
125
165
T
Tautz, R.
Trautwein, N.
Troshin, P. A.
Tunc, A.V.
Turkovic, V.
Tuukkanen, S.
122,
151
153, 158
132
159, 161
42
U
Urbani, B.
115
V
Vanderzande, D.J. M.
Verstappen, P.
Voss, G.
98, 154
98
119
W
Wagenpfahl, A.
Wemken, J.H.
135
117
Y
Z
Zou, W.
Zirkel, M.
Zhan, Y. Q.
1
19
95
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