SASEC2015 Thir d Southern African Solar Energy Conference

SASEC2015 Thir d Southern African Solar Energy Conference
SASEC2015
Third Southern African Solar Energy Conference
11 – 13 May 2015
Kruger National Park, South Africa
EFFECT OF DOPING ON ZnO BASED TRANSPARENT CONDUCTING OXIDES AND
DOWN/UP CONVERSION PHOSPHOR FOR SOLAR CELL APPLICATION
Vinod Ku mar1,* , H.C. Swart 1,* , Vijay Ku mar1 , Anurag Pandey 1 , L.P. Purohit 2 and O.M. Nt waeaborwa 1
1
Department of Physics, University of the Free State, Bloemfontein, ZA9300, South Africa
2
Department of Physics, Guru kula Kangri Un iversity, Harid war 249 404, India
E-mail: v [email protected] m
[email protected]
ABSTRACT
Boron doped ZnO (ZnO:B) films were g rown by the spin
coating method. The structure of the ZnO:B film has been
found to exhibit the hexagonal wurtzite structure. A min imu m
resistivity was obtained to be 7.9×10-4 Ω-cm at 0.6 at.% of B
concentration in the ZnO:B films. The optical interference
pattern in the transmittance spectra shows good homogeneity
with a transparency of ~91%, in the visible region. The
efficiencies of the dye sensitized solar cell (DSSCs) formed
using the ZnO:B window layers were obtained to be 1.56 %.
Rare earth doped oxide based nano-phosphors for up and down
conversion applications were also synthesised by a solution
combustion method. The efficiency of the DSSC can be
enhanced by using an extra layer of down and up conversion
phosphor material as a layer on top of the solar cell.
opened possibilities for using ZnO to improve the performance
of photovoltaic cells. For example, ZnO showed the first
experimental evidence of irreversible electron inject ion fro m
organic mo lecules into the conduction band of a wide band gap
semiconductor [11]. Nowadays, ZnO is emerg ing as an
efficient electron transport material in technologies such as
DSSCs and inverted polymer solar cells, QDSSCs, bio-med ical
applications and light emitting diodes [12-16].
Up-conversion (UC) and down-conversion (DC) of sunlight
are two possible routes for improving energy harvesting over
the whole solar spectrum. Via such processes it could be
possible to exceed the Shockley-Queisser limit for a singlejunction photovoltaic (PV) device [17]. The effect of adding
DC and UC layers to the front and rear of a solar cell,
respectively, is to modify the incident solar spectrum. One of
the materials more extensively studied for these propose have
been the lanthanides or rare-earth systems, due to the suitability
of their d iscrete energy levels for photon conversion inside a
wide variety of host materials. While high quantum yields of
200% have been demonstrated with DC materials, there remain
several barriers to realising such a layer that is applicable to a
solar cell. These are, firstly, weak absorption of the lanthanide
ions and, secondly, the competing loss mechanism of non radiative reco mbination. For UC, these two barriers still exist,
however an additional challenge is the non-linear nature of the
UC process, thus favouring operation under concentrated
sunlight. In this paper, we review the applicat ion of UC and DC
to the solar cell, d iscussing the material systems used and
optical characterisation [18].
In the paper, the effect of Boron (B) doping concentration
on the electrical and optical properties of ZnO:B is
investigated. The optimized thin film is used as a window layer
in DSSC. The PL of possible UP and DC materials are also
investigated.
INTRODUCTION
Transparent conducting window layer materials such as
Zinc o xide (Zn O), Indiu m tin o xide (ITO) and fluorine doped
tin o xide (FTO) play a very important ro le in solar cell
technologies such as dye sensitized solar cell (DSSCs),
quantum dot sensitized solar cells (QDSSCs), poly mer solar
cells and hybrid solar cells [1-4]. The advantages of DSSCs are
its low fabrication cost, flexibility and efficiency [1-4]. Usually,
TiO2 films are used as the transparent conducting oxide (TCO)
in DSSCs because of their adequate surface area and chemical
affinity for the dye adsorption as well as their suitable energy
band potential align ment for charge transfer. However, the
numerous grain boundaries between the TiO2 nano-particles
(NPs) restrict fast electron transport, which is detrimental to the
efficient energy conversion process [5, 6]. Recently, ZnO NPs
have been explored as alternatives to TiO2 as an electron
conductor. Bulk ZnO has a unique combination of electrical
and optical properties, includ ing relat ively high electron
mobility (more than 1 o rder of magnitude larger than anatase
TiO2 ) [5, 6]. In addition, ZnO has a rich family of
nanostructures with diverse applications in optoelectronics and
photovoltaics [7-10]. This unique combination of properties has
EXP ERIMENTAL DETAIL
Boron doped ZnO (ZnO:B) thin films have been
deposited on a microscopic corning glass substrate using the
476
spin coating technique. Zinc acetate dihydrate (Alfa Aesar) was
used as a starting material. Methanol (AR, Merck) and
monoethanolamine (M EA, Merck) were used as solvent and
stabilizer, respectively. The zinc precursor solution was
prepared by dissolving zinc acetate dihydrate in methanol so as
to prepare concentration of 0.2 mol/ l. Then MEA was dissolved
into the solution. The molar rat io M EA/Zn was fixed to 1 for all
samples. Trimethyl borate was also dissolved into the solution
to obtain ZnO:B solution. Boron concentration was varied from
0 to 1 at.%. The mixed solution was stirred by using a magnetic
stirrer at 25ºC for 2 h. The transparent and homogenous
solution was obtained after 72 h. The solution was dropped
onto the glass substrate, which was rotated at 2500 r/ min for 30
s by a spin coater. After deposition, the films were dried in air
at 230ºC for 10 min over a hot plate to evaporate the solvent
and remove organic residuals. The procedures for coating to
drying were repeated 15 times until the desired thickness. The
films were then inserted to a microprocessor controlled furnace
for annealing in air at 450ºC for 1 h.
Films of Zn O NPs (thickness ~10 μm) were deposited on
ZnO:B thin films using the doctor blade technique [19]. N3 dye
was used as a sensitizer. The deposited ZnO film was immersed
in a 0.3 mM solution of N3 dye in ethanol for 24 h. The dye
covered electrodes were then rinsed with ethanol to remove the
excess dye from surface and dried at room temperature. The
dye coated assembly was then dipped into gel electrolyte for 1
h. The gel electrolyte was prepared by using a 10%
polyethylene oxide (PEO 99%, Alfa Aesar) solution in
acetonitrile and carbon nanotubes (CNTs, 90 + %, Alfa Aesar)
with LiI 0.1 M (9.95%, Alfa Aesar), and I2 0.015 M (99.8 %,
Alfa Aesar). The whole mixture was placed for sonication to
disperse the CNTs into the polymer mat rix. The mixture was
stirred for 10 h with a magnetic stirrer in order to get a
complete mixing between the CNTs and the polymer
mo lecules. A thin plat inum sheet was used as counter electrode.
ZnO:Ce3+, ZnO:Tb 3+ and Er3+-Yb 3+ co-doped SrWO4
NPr were also synthesised for possible UC and DC materials by
using the solution combustion method. The source material and
urea were mixed and dissolved in distilled water. Dopant nitrate
was used as the dopant source in the solution. A homogeneous
solution was obtained after stirring for 20 min. The solution
was transferred to a pre-heated muffle furnace maintained at a
temperature of 450±10°C. All the liquid evaporated and a large
amount of heat was released which resulted into a flame that
decomposed the reagents further and released more gases. The
flame lasted for 60 s and the combustion process was
completed within 5 min . The resulting DC and UP powders
were cooled down to roo m temperature and ground gently
using a pestle and mortar.
competing processes; the increase of boron doping improves
the stoichiometry of the films and the crystal quality. This
indicates that boron ions are substituted at zinc ions sites up to
0.6 at.% after that B-B intra grain cluster is evaluated.
(002)
Intensity (a.u.)
1.0 at.%
0.8 at.%
0.6 at.%
0.4 at.%
0.2 at.%
0.0 at.%
30
32
34
36
38
2 (deg.)
40
Figure 1 XRD pattern of ZnO:B films as a function of
boron concentration
-3
2.0x10
-3
1.5x10
-3
1.0x10
-3
20
4.0x10
20
2.0x10
20
n

0.0
0.2
0.4
0.6
0.8
1.0
Boron Concentration (at.%)
477
6.0x10
0.0
-3
RESULTS AND DESSCUSION
X-ray diffraction (XRD) patterns of ZnO:B films prepared
by sol-gel process are shown in Figure 1. It indicates that most
of the grains in ZnO:B have a strong orientation along the caxis (002) plane as reported in JCPDS card No. 79-0206. The
intensity of the (002) peak has increased with increasing B
concentration up to 0.6 at.% and then the intensity of the (002)
peak has decreased. This behaviour can be understood by two
2.5x10
Carrier concentration (cm )
Resistivity (ohm-cm)
Figure 2 depicts the variation of electrical resis tivity and
carrier concentration in ZnO:B thin films as a function of B
concentration. The electrical resistivity of ZnO:B film has
decreased with increasing B concentration. A min imu m
resistivity was obtained to be 7.9×10-4 Ω-cm at 0.6 at.% of B.
The resistivity of ZnO:B films has decreased due to
contribution of extra free electrons of B3+ ions substituting into
the Zn 2+ ions sites. The resistivity was found to increase at
higher doping concentration of boron (0.8 and 1.0 at.%)
because excess B might result in intra grain forming B-B
cluster. Similar suggestions about intra grain cluster were
proposed by Lu et al.[20] and Yu et al.[21] for ZnO:Al and
ZnO:Y films, respectively. The carrier concentration has
increased with increasing B concentration up to 0.6 at.% and
the largest value of carrier concentration 6.26×1020 cm-3 at 0.6
at.% B was obtained. Then the carrier concentration has
decreased with a further increase in B concentration due to intra
grain cluster scattering [22].
Figure 2 Effect of boron doping on the resistivity and carrier
concentration of ZnO:B films,
band gap (Eg) was obtained by extrapolat ing the linear part of
500
600
700
3.6
(h) (cm eV)
-1
3.3
3.4
h (eV)
2
800
5
DSSC-1
Current density (mA/cm 2)
Transmittance %
0.0 at.%
0.2 at.%
0.4 at.%
0.6 at.%
0.8 at.%
1.0 at.%
Wavelength (nm)
3.2
I-V MEAURMENT OF SOALR CELL
As pointed out above, the sandwich type DSSCs were
fabricated using the ZnO:B window layer films as front
electrodes (topmost layer in schematic diagram). The
synthesized ZnO NPs was used as host material (bigger
- spheres), the N3 dye as active layer (smaller spheres), I /I3
electrolyte for charge separation and Pt as counter electrode
(bottom layer). The cell was illu minated under one sun
illu mination (AM 1.5, 100 mW/cm2 ) and I-V characteristics of
the cells were obtained using a Keithley source meter. I-V
curves of the DSSCs with different window layers are shown in
Figure 5. The efficiency of the ZnO:B based DSSC was
obtained as 1.56%.
60
400
3.1
Figure 4 band gap variation of Zn O:B films with boron
concentration
80
0
300
3.5
0.0 at.%
0.2 at.%
0.4 at.%
0.6 at.%
0.8 at.%
1.0 at.%
3.0
100
20
2
2
Figure 3 shows the optical transmittance spectra in the
wavelength range of 300 to 800 n m of the ZnO:B thin films at
different doping concentration. Optical interference in the
transmittance spectra shows a homogenous nature of the films.
The transmittance of the film has increased with increasing
doping concentration and a maximu m t ransmittance was
obtained of 88% at 0.6 at.% of B. The increase in the
transmittance at 0.6 at.% of B may be due to decreasing optical
scattering caused by the densification of grains followed by
grain growth and the reduction of grain boundary density as
demonstrated. Since we are mainly interested in measuring the
optical band gap of films, many criteria have been used to
define the onset of inter band transitions in direct band gap
semiconductors. The transition energy could be simp ly deduced
fro m the zero crossing of the second derivative of the
absorption spectrum. The Fermi exclusion principle, optical
transitions can only occur for higher photon energies to make
vertical transitions from the valence band up to the state with
Fermi mo mentum in the conduction band. A blue shift in
ultraviolet (UV) absorption edge is observed with increasing
concentration, indicating the broadening of the optical band
gap.
40

the Tauc’s plot to intercept the energy axis at h  0 . The
band gap of the films was found to increase continuously from
3.24 to 3.35 eV with an increase in the concentration. The
change in the optical band gap was analyzed in terms of
Burstein- Moss (B-M) band gap widening [22].
Figure 3 Optical transmittance spectra of ZnO:B films for
different boron concentration
4
The optical band gap was calculated by using Tauc’s plot
method [22]
h  Bh  E g  2
3
1
(9)
Where,  is the absorption coefficient, h is the plank’s
constant and  is frequency of incident photon, Eg is the
optical band gap and B is a constant, α is evaluated using
1 1
(10)
  ln  
d T 
Where, T is the transmittance of the film and d is the

2
1
0
0.0
2
thickness of film. The plot of h versus h for ZnO:B at
different concentration of B is shown in Figure 4. The optical
0.2
0.4
0.6
Voltage (V)
0.8
Figure 5 I-V characteristic of Zn O:B window layer based
DSSC
478
UP CONVERSION MATERIAL
The UC emission spectra of the Er3+-Yb 3+ co-doped
SrWO4 phosphor recorded in the 400-900 n m range upon a 980
nm excitation is shown in figure 8. Two do minant green UC
emission bands have been marked along with the co mparatively
weak blue, red and NIR bands. These emission bands are
peaking at about 409, 525, 547, 658 and 800 n m and assigned
for the 2 H9/2 →4 I15/2 , 2 H11/2 →4 I15/2 , 4 S3/2 →4 I15/2 , 4 F9/2 →4 I15/2 and
4
I9/2 →4 I15/2 transitions of the Er3+ ion and about 489 n m due to
the 2 F5/2 →2 F7/2 transition of the Yb 3+ion, respectively [26, 27].
It may therefore be used as a UC material.
DOWN CONVERSION MATERIAL
ZnO:Tb 3+ and ZnO:Ce 3+ materials were synthesized for
possible DC material. The PL emission curve of the ZnO:Tb 3+
material excited by 325 is shown in Figure 6. Fo r the ZnO:Tb 3+,
a major green emission peak at 543 n m and a few minor peaks
at 489, 586, 622, 444 and 420 n m were detected. These peaks
represent the 5 D4 -7 F5 , 5 D4 -7 F6 , 5 D4 -7 F4 , 5 D4 -7 F3 , 5 D3 -7 F4 and 5 D3 7
F5 transitions of Tb 3+, respectively [23, 24].
3+
Intensity (a.u.)
ZnO:Tb
Excited at 325 nm
3+
700
Wavelength (nm)
Figure 6
800
3+
I15/2
4
F7/2
2
I15/2
4
S3/2
exi = 980nm
4
F9/2
4
I15/2
4
I9/2
500
600
700
4
I15/2
800
Wavelength (nm)
900
Intensity (a.u.)
ACKNOWLEDGEMENT
This work is based on the research supported by the South
African Research Chairs Initiative of the Department of
Science and Technology, and the National Research
Foundation of South Africa. The PL system used in this study
is supported both technically and financially by the rental pool
programme o f the National Laser Centre. The financial support
fro m the University of the Free State is highly recognized.
Authors are grateful to Prof. J.R. Botha, Nelson Mandela
Metropolitan University, Po rt Elizabeth, South Africa for
providing the PL measurement facility.
300 350 400 450 500 550 600 650
Wavelength (nm)
PL emission curve of ZnO:Ce
248 n m laser.
3+
CONCLUSION
DSSCs were fabricated using highly transparent and
conducting ZnO:B thin films as front window electrodes with
crystalline ZnO NPs as host material. The efficiencies of the
DSSCs fabricated using the ZnO:B as window layers were
obtained to be 1.56 %. These results demonstrate that these
highly transparent and conducting ZnO:B thin films are
promising candidates for window layer application in DSSC.
The UC and DC materials for enchantment of the efficiency of
DSSC were successfully synthesized.
ZnO:ce
Excited at 248 nm
Figure 7
I15/2
Figure 8 PL emission curve of SrWO4 :Ce3+ -Yb 3+ NPr excited
by 980 n m laser.
The PL emission curve of ZnO:Ce3+ material excited by 248.6
nm line of a Ne Cu laser is shown in Figure 7. According to the
reported values in literature [25], the transitions in a Ce3+ doped
ZnO latt ice may be attributed to (i) 399-404n m exciton
emission (ii) 417n m (5 D0 –4F) (characteristic emission of Ce3+)
(iii)426-428 n m (5 D4 −4 F1 ) (iv ) 448-452 n m (5 D0 −7 F1 ) (v) 468
nm (5 D2 −7 F0 ) (vi) 553-566 n m (5 D4 –Fj ) (v ii) 601n m (5 D0 -7 F1 )
(viii) 629 n m (5 D0 −7 F2 ). Although not very efficient in the case
of the Ce doped ZnO both materials maybe used as possible DC
materials.
3+
4
400
PL emission curve of ZnO:Tb 3+ NPr excited by
325 n m laser.
SrWO4:Er -Yb
4
2
600
2 F5/2
500
2
400
H11/2
H 9/2
Intensity (a.u.)
2
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