mscThesis_I_A_Digdaya.

mscThesis_I_A_Digdaya.
Optical Enhancement for
Heterojunction Silicon Solar Cells
Master of Science Thesis
Ibadillah Ardhi Digdaya
Photovoltaic Materials and Devices
Optical Enhancement for
Heterojunction Silicon Solar Cells
Master of Science Thesis
For the degree of Master of Science in Sustainable Energy Technology
at Delft University of Technology
Ibadillah Ardhi Digdaya
November 14, 2012
Faculty of Applied Sciences · Delft University of Technology
c Photovoltaic Materials and Devices (PVMD)
Copyright All rights reserved.
Abstract
The heterojunction silicon solar cell is an innovative concept of crystalline silicon (c-Si) based
solar cell with several unique features. It makes use of the large band gap of the hydrogenated
amorphous silicon (a-Si:H) to form the emitter and the back surface field with the c-Si, so
that electrons or holes can flow in one direction only. The insertion is of intrinsic a-Si:H at
the interface between the p-type a-Si:H and the n-type c-Si has been a notable development
to passivate dangling bonds on the c-Si surface, reducing recombination losses. In comparison
with the conventional homojunction c-Si solar cell, the heterojunction silicon solar cell has
advantages of the low processing temperature and the shorter time required to complete the
fabrication processes.
In the heterojunction silicon solar cell, optical losses may occur due to the reflection
on the front surface and the parasitic absorption by the transparent conductive oxide (TCO)
and a-Si:H layers. The low optical performance can greatly reduce the short-circuit current
density (JSC ), thus lowering the efficiency. Therefore, anti-reflection is very important to
reduce the reflection and increase the absorption in the c-Si. Despite its ability to transport
charge carriers to the contacts, the transparent indium tin oxide (ITO) can also act as an
effective window for the incident light to enter the cell. Moreover, its refractive index match
with a-Si:H makes the ITO an excellent anti-reflective (AR) coating for the heterojunction
silicon solar cell. Further, to improve the anti-reflection effect, a silicon dioxide (SiO2 ) film is
added on top of the existing ITO coating to form a double-layer AR coating.
In this work, the optics of the flat heterojunction silicon solar cell is simulated by
Advance Semiconductor Analysis (ASA) program developed in Photovoltaic Materials and
Devices (PVMD) group. With the support of ASA program, the single- and the doublelayer AR coating applications on the heterojunction silicon solar cell are optimized. In the
optimization, the thicknesses of the two coating materials are treated as variables to map
the distribution of the total reflectance and the total absorptance in the c-Si with respect to
the coating thicknesses. In this way, the distribution map is used to determine the optimum
coating thicknesses with the tolerance range.
From the simulation, the total reflectance of the flat heterojunction silicon solar cell can
be reduced from 20.15% with the single-layer AR coating to 17.27% with the double-layer
AR coating. This implies to an increase of the total absorptance in the bulk c-Si from 73.2%
to 75.55%. This benefit from the double-layer AR coating is then experimentally observed,
Master of Science Thesis
Ibadillah Ardhi Digdaya
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indicating that the JSC is increased from 34.3 mA/cm2 with the single-layer AR coating to
35.7 mA/cm2 when the optimized double-layer AR coating applied. As a consequence, the
efficiency of the investigated device is increased from 17.1% to 17.6%.
For the textured heterojunction silicon solar cell, an advanced ray tracing model is used
to simulate the reflection and absorption profile in each layer. In order to reduce the reflection,
the simulation shows that the optimum AR coating is thinner than on a flat cell, resulting
in a less parasitic absorption in ITO, hence an increase of the absorption in the c-Si. From
the simulation, it is indicated that the total reflectance is further reduced from 4.06% with
the single-layer AR coating to 3.07% with the double-layer AR coating. This contributes to
an increase of the total absorptance in the c-Si from 84.61% to 86.72%. Experimentally, JSC
increases from 38.1 mA/cm2 to 40.5 mA/cm2 . As a result, an improvement of the efficiency
from 18% to 19% is achieved.
Ibadillah Ardhi Digdaya
Master of Science Thesis
Table of Contents
Preface
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1 Introduction
1-1
1-2
1-3
1-4
1-5
1
Current World Energy and the Photovoltaic Technology
Development of PV Technologies . . . . . . . . . . . .
Homojunction and Heterojunction Solar Cells . . . . .
Challenges of Heterojunction Silicon Solar Cells . . . .
Thesis Outline . . . . . . . . . . . . . . . . . . . . . .
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2 Fundamentals
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2-1 Optics . . . . . . . . . . . . . . . . . . . .
2-1-1 Reflection, refraction and absorption
2-1-2 Thin film interference . . . . . . . .
2-1-3 Antireflective coating . . . . . . . .
2-1-4 Double-layer antireflective coating .
2-2 Antireflective Coating Materials . . . . . . .
2-2-1 Indium tin oxide . . . . . . . . . . .
2-2-2 Silicon dioxide . . . . . . . . . . . .
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3 Equipment
3-1 Deposition . . . . . . . . . . . . . . . . .
3-1-1 RF magnetron sputtering . . . . .
3-1-2 Physical vapor deposition . . . . .
3-2 Characterization . . . . . . . . . . . . . .
3-2-1 Total integrating sphere (TIS) . .
3-2-2 Automated R/T Analyser (ARTA)
3-2-3 Mini-RT Setup . . . . . . . . . .
3-2-4 Spectroscopic ellipsometry . . . .
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Ibadillah Ardhi Digdaya
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Table of Contents
3-2-5
3-2-6
Illuminated I-V measurement . . . . . . . . . . . . . . . . . . . . . . . .
External quantum efficiency Measurement . . . . . . . . . . . . . . . . .
4 Antireflective Coating on the Heterojunction Silicon Solar Cell
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4-1 Optical Characterization of Individual Layers . . . . . . . . . . . . . . . . . . . .
4-1-1 Optical Properties of Semiconductor Materials Used in the Heterojunction
Silicon Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1-2 Optical Properties of Antireflective Coatings Materials and Encapsulants .
4-2 Optical Simulation on a Flat Surface . . . . . . . . . . . . . . . . . . . . . . . .
4-2-1 Single-layer AR coating on a flat surface . . . . . . . . . . . . . . . . . .
4-2-2 Double-layer AR coating on a flat Surface . . . . . . . . . . . . . . . . .
4-3 Optical Simulation on a Textured Surface . . . . . . . . . . . . . . . . . . . . .
4-3-1 Single-layer AR coating on a textured surface . . . . . . . . . . . . . . .
4-3-2 Double-layer AR coating on a textured surface . . . . . . . . . . . . . . .
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell . . . . .
4-4-1 Single-layer AR coating on an encapsulated flat solar cell . . . . . . . . .
4-4-2 Double-layer AR coating on an encapsulated flat solar cell . . . . . . . .
4-4-3 Single-layer AR coating on an encapsulated textured solar cell . . . . . .
4-4-4 Double-layer AR coating on an encapsulated textured solar cell . . . . . .
5 Experimental Results
5-1 Optical Measurement of Flat Heterojunction Solar Cells . . . . .
5-1-1 Single-layer AR coating on a flat device . . . . . . . . .
5-1-2 Double-layer AR coating on a flat device . . . . . . . . .
5-2 Optical Measurement of Textured Heterojunction Solar Cells . .
5-2-1 Single- and double-layer AR coating on a textured device
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6 Conclusion and Recommendation
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6-1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6-2 Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Bibliography
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Ibadillah Ardhi Digdaya
Master of Science Thesis
List of Figures
1-1 Growth in primary energy demand [1]. . . . . . . . . . . . . . . . . . . . . . . .
1-2 Three generations of solar cells used in different applications [2]. . . . . . . . . .
1-3 The p-n junction. The curve at the interface between the p-type and n-type
indicates the presence of internal electric field. The yellow-colored part represents
the depletion region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-4 Schematic energy band diagram of the heterojunction silicon solar cell (adapted
from [3]). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-5 Schematic diagram of HIT solar cell structure developed by Sanyo [4]. . . . . . .
1-6 Comparison of fabrication process between the conventional mono c-Si solar cell
and the heterojunction silicon solar cell [5]. . . . . . . . . . . . . . . . . . . . .
1-7 Losses in the heterojunction silicon solar cell [6]. . . . . . . . . . . . . . . . . . .
1-8 Optical losses in the heterojunction silicon solar cell. . . . . . . . . . . . . . . . .
1
3
2-1 The reflection and the refraction of light on a plane interface (adapted from [7]).
2-2 The superposition of two sinusoidal waves travelling to the same direction (adapted
from [8]). The amplitude of wave 1 is equal to the amplitude of wave 2 producing
a resultant amplitude of 2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-3 (a) The constructive interference of two sinusoidal waves. The solid black line
is the first wave and the dashed line is the second wave. The resultant wave
is represented by the solid red line (b) The destructive interference. A1 is the
amplitude given by wave 1 and A2 is the amplitude given by wave 2. Adapted
from [8]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-4 The light propagation on the substrate with a coating layer. . . . . . . . . . . .
2-5 The destructive interference in the double-layer coating. . . . . . . . . . . . . . .
12
3-1
3-2
3-3
3-4
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(a) Schematic diagram of RF magnetron sputtering. (b) Setup. . . . . . . . . .
(a) Schematic diagram of e-beam evaporation (b) Setup. . . . . . . . . . . . . .
Sketch of PerkinElmer Lambda/TIS setup for R/T measurement. . . . . . . . .
Sketch of the ARTA configuration. The angle represents the circular movement of
the detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-5 The Mini-RT setup at Delft University of Technology. . . . . . . . . . . . . . . .
Master of Science Thesis
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Ibadillah Ardhi Digdaya
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List of Figures
3-6 The skecth of ellipsometry configuration [9]. The linearly polarized light is reflected
from the sample surface. The change in polarization is measured to determine the
sample response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-7 J-V characteristics of a solar cell in the dark and under illumination [2]. . . . . .
3-8 The electric circuit diagram of Solar Cell. . . . . . . . . . . . . . . . . . . . . .
3-9 The illuminaed I-V setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-10 (a) EQE setup. (b) EQE user interface . . . . . . . . . . . . . . . . . . . . . . .
4-1 (a) Refractive indices and (b) absorption coefficients of semiconductor materials
as a function of wavelength. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-2 (a) Refractive indices of ITO (blue line), SiO2 (red line), Al2 O3 (green line), and
MgO (purple line) as a function of wavelengths. (b) Absorption coefficients of ITO
as function of wavelengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3 (a) Refractive indices of the glass and EVA as a function of wavelength. (b)
Absorption coefficients of the glass and EVA as a function of wavelength. . . . .
4-4 The spectral reflectance of the heterojunction solar cell without ITO (blue line),
the photon flux density of global AM1.5 spectrum (black curve), and the reflected
photon flux (red curve) as a function of wavelength. . . . . . . . . . . . . . . . .
4-5 The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-6 (a) The schematic structure of the heterojunction silicon solar cell. Except ITO,
other layer thicknesses will be kept constant for the entire simulations. (b) The
spectral reflectance and the absorptance profile of the simulated heterojunction
cell with an optimum ITO thickness. . . . . . . . . . . . . . . . . . . . . . . . .
4-7 (a) The total reflectance and (b) the total absorptance in the c-Si as a function of
ITO and SiO2 thicknesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-8 (a) The schematic structure of the heterojunction silicon solar cell with a doublelayer AR coating. (b) The spectral reflectance and absorptance profile of the simulated heterojunction solar cell with the optimum ITO and SiO2 thickness combination.
4-9 (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
the single- and the double-layer AR coatings. . . . . . . . . . . . . . . . . . . .
4-10 The measured spectral reflectance of a textured (solid red line) and a flat polished
(dashed black line) heterojunction silion solar cell without an AR coating. . . . .
4-11 (a) The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness. (b) The light propagation in ITO. . . . . . . . . . . . . . . . . . . . .
4-12 (a) The schematic structure of the textured heterojunction silicon solar cell with
a single-layer AR coating applied. (b) The spectral reflectance and absorptance
profile of the simulated textured heterojunction solar cell with an optimum ITO
thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-13 (a) The total reflectance and (b) the total absorptance in the c-Si as a function of
ITO and SiO2 thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-14 (a) The schematic structure of the textured heterojunction solar cell with a doublelayer AR coating applied. (b) The spectral reflectance and absorptance profile
of the simulated textured heterojunction solar cell with optimum ITO and SiO2
thickness combination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-15 (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
the single- and the double-layer AR coatings. . . . . . . . . . . . . . . . . . . .
Ibadillah Ardhi Digdaya
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List of Figures
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4-16 The schematic structure of an encapsulated heterojunction silicon solar cell where
the glass and EVA are incorporated. . . . . . . . . . . . . . . . . . . . . . . . .
4-17 The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness with a single-layer AR coating on an encapsulated flat heterojunction
solar cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-18 (a) The schematic structure of the encapsulated heterojunction silicon solar cell
with the single-layer AR coating applied. (b) The spectral reflectance and absorptance profile of the simulated heterojunction silicon solar cell with the optimum
ITO thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-19 The total absorptance in the c-Si as a function of (a) ITO and Al2 O3 thicknesses,
and (b) ITO and MgO thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . .
4-20 (a) The schematic structure of an encapsulated heterojunction silicon solar cell with
the ITO & MgO double-layer AR coating applied. (b) The spectral reflectance and
absorptance profile of the simulated encapsulated heterojunction solar cell with the
optimum ITO and MgO thicknesses. . . . . . . . . . . . . . . . . . . . . . . . .
4-21 (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
a single-layer AR coating (ITO) and a double-layer AR coating (ITO & MgO) on
an encapsulated heterojunction solar cell. . . . . . . . . . . . . . . . . . . . . .
4-22 The total reflectance and the total absorptance in the c-Si of an encapsulated
textured cell as a function of the ITO thickness. . . . . . . . . . . . . . . . . . .
4-23 (a) The schematic structure of an encapsulated textured heterojunction silicon
solar cell with the single-layer AR coating applied. (b) The spectral reflectance and
absorptance profile of the simulated encapsulated textured heterojunction solar cell
with an optimum ITO thickness. . . . . . . . . . . . . . . . . . . . . . . . . . .
4-24 The absorptance in the c-Si as a function of (a) ITO and Al2 O3 thicknesses, and
(b) ITO and MgO thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-25 (a) The schematic structure of an encapsulated textured heterojunction solar cell
with the ITO & MgO double-layer AR coating applied. (b) The spectral reflectance
and absorptance profile of the simulated encapsulated textured heterojunction silicon solar cell with the optimum ITO & MgO thickness combination. . . . . . .
4-26 (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
the single-layer AR coating (ITO) and the double-layer AR coating (ITO & MgO)
on an encapsulated textured heterojunction silicon solar cell. . . . . . . . . . . .
5-1 Measured (symbols) and simulated (solid line) reflectance spectra of heterojunction
solar cells with various ITO thickness. (b) EQE spectra. . . . . . . . . . . . . . .
5-2 (a) Measured (symbols) and simulated (solid line) reflectance spectra of heterojunction solar cells with two different double-layer AR coating thickness combinations. (b) EQE spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-3 (a) Spectral reflectance of heterojunction silicon devices with single-layer AR coating (measured: blue ◦ symbol, simulated: blue line) and with double-layer AR
coating (measured: green symbol, simulated: green line). (b) The EQE spectra
of heterojunction silicon devices with single-layer AR coating (blue line) and with
double-layer AR coating (green line). . . . . . . . . . . . . . . . . . . . . . . . .
5-4 The J-V characteristics of the flat heterojunction silicon solar cells with single-layer
and double-layer AR coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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List of Figures
5-5 The simulated 1-reflectance (blue line) and absorptance spectra in (p) a-Si:H (light
green), in (i) a-Si:H (yellow), in (n) c-Si (red line). The measured 1-reflectance
(blue symbol) and the measured EQE spectra (red ◦ symbol). . . . . . . . . .
5-6 (a) The reflectance spectra of devices with the single-layer AR coating (measured:
blue ◦ symbol, simulated: blue line) and with the double-layer AR coating (measured: green symbol, simulated: green line). (b) The EQE spectra of devices
with the single-layer AR coating (blue line) and with the double-layer AR coating
(green line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-7 The J-V characteristics of the textured heterojunction silicon solar cells with singlelayer and double-layer AR coatings. . . . . . . . . . . . . . . . . . . . . . . . . .
5-8 The textured heterojunction solar cell with: (a) a single-layer AR coating, (b) a
double-layer AR coating. The simulated 1-reflectance (blue line) and absorptance
spectra in (p) a-Si:H (light green), in (i) a-Si:H (yellow), in (n) c-Si (red line).
The measured 1-reflectance (blue symbol) and the measured EQE spectra (red
◦ symbol). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ibadillah Ardhi Digdaya
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List of Tables
2-1 List of materials with refractive indices lower than ITO. ITO data is given for
comparison [10, 11]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1 Comparative results of the total reflectance and the total absorptance in each
layer with different AR coating schemes of a flat heterojunction solar cell. The
thicknesses of AR coatings are optimum values. . . . . . . . . . . . . . . . . . .
4-2 Comparative results of the total reflectance and the total absorptance in each layer
with different optimum AR coating schemes on a textured heterojunction solar cell.
4-3 The comparative results of the total reflectance and the total absorptance in each
layer with different AR coating schemes on an encapsulated heterojunction silicon
solar cell. The thicknesses of AR coatings are optimum values. . . . . . . . . . .
4-4 Comparative results of the total reflectance and the total absorptance in each layer
with different AR coating schemes on unencapsulated textured heterojunction solar
cell. The thicknesses of AR coatings are optimum values. . . . . . . . . . . . . .
5-1 The total reflectance and JSC of the three devices with different ITO thicknesses.
5-2 The total reflectance and JSC of the two devices with different ITO & SiO2 thickness combinations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Master of Science Thesis
20
38
42
47
51
54
56
Ibadillah Ardhi Digdaya
x
Ibadillah Ardhi Digdaya
List of Tables
Master of Science Thesis
Preface
This thesis report is written as the final stage to complete the degree of Master of Science
in Sustainable Energy Technology (SET) at Delft University of Technology. The researh was
conducted in Photovoltaic Materials and Devices (PVMD) group at TU Delft. During my
thesis period, I was working under daily supervision oh PhD candidate, Dong Zhang and
supervised by Dr. René van Swaaij.
I came to Delft University of Technology with a high motivation and a deep passion to
pursue a Master’s degree in SET. I initially had no preference in the solar cell field as I was
aware that its technological development is somewhat stuck in a relatively low conversion efficiency. However, the ongoing research and the implementation of the solar energy technology
in the Europe continent leaves me a big curiosity. The annual solar irradiance in European
countries is incomparable with the tropical countries yet they are still eager and willing to
develop the solar cell technology. I soon realized that the solar energy is the most abundant,
clean and viable options of sustainable energy that the Earth can harness. Although the
current efficiency is far away from its thermodynamics limit, the source is real and will never
run out. Besides, several research groups, universities and laboratories have been through
research and development to establish new concepts for high efficiency solar cell technology.
My enthusiasm has led me to the TU Delft solar cell lab visit organized by Energy
Club. I was fascinated by the excellent facilities and research environment in Delft Institute
Microsystem and Nanoelectronics (Dimes). Several reasons have encouraged me to be further
involved in the solar cell research. The next question was, “what type of solar cell technology
I would like to learn?”. As far as I have concerned, the conventional c-Si solar was dominating
the market due to its high efficieny while at Delft, the main focus is the development of thin
film solar cell. Later, Marinda A. Putri who was the former master student in PVMD group
introduced me to the heterojunction silicon solar cell which combine the high efficiency of
c-Si solar cell and the low fabrication cost of thin film a-Si solar cell. This information then
brought me to the project on the optical enhancement for heterojunction silicon solar cells.
The aim of this project is to improve the optical performance of the heterojunction silicon solar cell. As the initial phase, simulations and optimizations were carried out to optimize
the application of anti-reflective coatings on this particular solar cell. Several experiments
were done to validate the reliability of the simulation and to evaluate the optical effect given
by anti-reflective coatings.
Looking back the ups and downs, this project would not have been possible with the
support and assistance of PVMD group. I would like to take this opportunity to thank Dr.
René van Swaaij who has supervised me throughout the project. I have always enjoyed our
Master of Science Thesis
Ibadillah Ardhi Digdaya
xii
Preface
weekly discussions. The knowledge and the advice you shared have taught me a lot and
enhanced my understanding to the scientific world. A special acknowledgement goes to my
daily supervisor, Dong Zhang M.Sc who have patiently guided me up to this point. I have
always appreciated the time and effort you have spent for teaching and supervising me. A
special recognition goes to Dr. Rudi Santbergen for introducing me to the ray-tracing model.
I have always enjoyed and gained much insight from our discussions. I would also like to
thank the master student coleague Joost Meerwijk and PhD student Dimitris Deligiannis.
Our discussions have left a profound impression. Another word of thanks goes to the current
and former occupants of LB.01.490 studentenkamer: Bert, Marc, Johan, Diego, Karthik.
Further I would like to thank Rian, Anti, Dindin, Mirza, Bhimo who have given me
moral supports through the thesis period. Lastly, I would like express a special gratitude to
my parents, Juli Satrio and Zaza Fauzah, and brother Sayyid Abdil Hakam Perkasa for the
love and support through all this time.
Delft, University of Technology
November 14, 2012
Ibadillah Ardhi Digdaya
Ibadillah Ardhi Digdaya
Master of Science Thesis
“The sun provides more energy in one hour than all humanity uses, in all forms,
in a single year. Sunlight can provide us with its own resolution to our energy
problems. The only transformation required is for humanity to reduce, or end,
consumption of stored solar (as fossil fuels) and, in its place, use freely available
‘fresh’ solar.”
— David S. Findley
Chapter 1
Introduction
1-1
Current World Energy and the Photovoltaic Technology
Over decades, the world has witnessed the importance of energy in terms of human civilization. The use of energy has been going upward due to the growth of population and
urbanization. Moreover, the progress of the economic and the technological developments in
most of developing countries have caused an increase in energy demand. The International
Energy Agency (IEA) has estimated that the global energy use will increase further by onethird from 2010 to 2035, with China and India accounting for 50% of the growth [1]. If the
trend continues, the current energy supply will not be sufficient to meet the requirement.
Figure 1-1: Growth in primary energy demand [1].
Globally, fossil fuels account for 74% of present primary energy sources [2]. However,
the world is now facing a major challenge that fossil-fuel reserves are constantly depleting
over time due to its extensive and continuous exploitation. With the growing population and
modernization, fossil fuels on Earth will be soon completely exhausted. Another challenge is
that fossil fuels are only available in certain countries. Due to an unequal distribution over
the world, fossil fuels have become a global strategic material. The oil-producing countries
Master of Science Thesis
Ibadillah Ardhi Digdaya
2
Introduction
may have ultimate authorities concerning the fossil fuel production levels and prices. Such
situation may lead to a social and a political tension and ultimately, will influence the world’s
economy. The extensive consumption of fossil fuels also draws a negative environmental
impact. Enormous quantities of greenhouse gasses released to the atmosphere by burning
fossil fuels are believed to be a dominant factor that causes the climate change. Although
there is a debate whether the fossil fuels are the main cause of the climate change or not, the
fact that their quantities depleted is not disputed.
Due to all-above mentioned reasons, the world can no longer rely on fossil-fuel based
energy sources. A transition to shift the current energy policy to sustainable energy economy
should be considered. Today, the renewable energy sources contribute merely around 19% to
the total energy production [2]. A large-scale application of renewable energy sources is not
economically feasible yet in industrialized countries due to its production cost. However, the
electricity from renewable energy sources is considered as an effective cost solution particularly
in remote areas which have no access to the electricity grid.
Solar energy is the most abundant energy source on earth. As much as 1.73 × 1014
kW of power from the sun is received by the Earth and which is 10,000 times more than
that consumed by all humans. Energy delivered by the sun in one hour is about the same of
energy consumed by all humans in a year. Photovoltaic (PV) solar cell is one of solar energy
technologies that works by direct conversion of light to electricity. At the moment the PV
solar cell contributes only 0.1% of total electricity production [12]. This number is very small,
considering the available total solar energy that can be utilized. However, IEA projected that
the PV solar cell application will grow very rapidly from 5 % of total energy consumption in
2030 to 11% in 2050 due to supporting policies and recent reduction of production cost [12].
1-2
Development of PV Technologies
The PV technology has attracted much attention due to the abundant resource of the solar
energy. In current trend, the first generation solar cell is dominating the market due to its
mature technology and high efficiency. It is generally the single crystal silicon solar cell that
uses silicon wafer as the substrate. The reported efficiency record is 25% for lab cells and
22.9% for the industrial scale [13]. However, in the production process, the crystalline silicon
solar cell requires a high processing temperature which may consume significant amounts of
energy. Moreover, the high material cost of the silicon wafer is also a major factor of the high
total cost of the first generation solar cell.
In order to cut down the production cost, the thin film solar cells, which represent
the second generation solar cells emerge as alternatives. The technology benefits from the
low processing temperature. The main characteristic of this type solar cells is the use of the
high absorption coefficient material which allows high photon absorption even in only 1 µm
thick absorber layer. Amorphous silicon (a-Si) solar cells are the most developed thin-film
solar cells. The a-Si can be deposited on a cheap glass substrate with continuous deposition
technique, thus reducing the production cost. The main disadvantage of a-Si solar cells is the
degradation of their performance under light exposure over time. Another notable concept
of a-Si solar cells is hydrogenated amorphous silicon (a-Si:H) combined with microcrystalline
silicon (µc-Si:H) forming the multi-junction thin-film silicon solar cell. The advantage of
µc-Si:H is that it absorbs light from the red and near infrared part of the spectrum, thus
increasing the efficiency. There are several other candidates of semiconductor materials for
Ibadillah Ardhi Digdaya
Master of Science Thesis
1-3 Homojunction and Heterojunction Solar Cells
3
thin film solar cells, namely copper indium gallium diselenide (CIGS) and cadmium telluride
(CdTe). The potential drawbacks for these types of solar cells are materials availability for
long-term use and issues with toxicity (CdTe) [14] and mass production has been proven to
be difficult (CIGS) [15]. The efficiency of the second generation solar cells varies from 11%
to 20% for lab scale and 9% to 12% for commercial scale [2].
Figure 1-2: Three generations of solar cells used in different applications [2].
Recently, there are attempts to boost further the conversion efficiency of solar energy
to electricity. The aim is to push the upper limit of 33% conversion efficiency for a single
junction solar cell up to the thermodynamics limit of 93% conversion efficiency. The concept
of this so-called third generation solar cell is to utilize advanced materials and new conversion
methods and processes.
1-3
Homojunction and Heterojunction Solar Cells
Homojunction crystalline silicon based solar cells
The direct conversion of solar energy into electrical energy is often referred as photovoltaic
effect. The processes involve the separation of charge carriers due to absorption of incident
light in semiconducting materials. Subsequently, photo-generated carriers are separated at
the junction followed by the collection of the electrons and holes at the terminals.
Photovoltaic cells consist of the p-type and n-type semiconducting materials. In the
conventional crystalline silicon solar cell, the p-type silicon is normally doped with boron while
the n-type silicon is doped with phosphorus. The introduction of phosphorus in the lattice
manipulates the concentration of electrons. Four of five valence electrons of phosphorus will
form bonds with neighboring silicon atoms. Consequently, one is not able to take part and
this single valence electron is easily liberated. Hence, these impurity atoms are called donors
due to its behavior of donating electrons to the lattice. In contrary, boron as the substitute
of Si atom has three valence electrons, leaving a vacant in the lattice and ready to capture
electron. In this way holes are created. The impurity atoms which accept electron are then
Master of Science Thesis
Ibadillah Ardhi Digdaya
4
Introduction
called acceptors. A material which is free of acceptors and donors has the same numbers of
electrons as holes called the intrinsic semiconductor.
Each semiconductor layer has different range of allowed and forbidden energy states for
electrons so called the energy band and the band gap respectively. Valence electrons bound
in covalent bonds are able to jump from valence band to conduction band with requirement
of certain energy. Only photons which have energy equal to or higher than the band gap can
excite electrons and create electron-hole pairs. The large difference of carrier concentrations
between the p-type and the n-type promotes the diffusion of majority carriers from one to
another. At the interface between p-type and n-type material, the p-n junction is established
by an internal electric field. In the electric field, the carriers having the opposite charges are
drifted apart to the opposite directions. For instance, the electrons which are the minority
carrier in the p-type material diffuse to the depletion region and are accelerated towards
the n-layer under the influence of internal electric field. Meanwhile, the wholes which are
the majority carriers are repulsed by the electric field. In this way, the electron-hole pair is
separated.
The p-n junction represents a system of charged particles in diffusive equilibrium in
which the electrochemical potential is constant and independent in position [2]. The electrochemical potential of electron system is called Fermi energy. The concentration of majority
and minority carriers in semiconductor material determine the position of Fermi-energy level.
In the n-type layer, the Fermi-energy position is close to the conduction band, while oppositely,
Fermi-energy position in the p-type layer is close to the valence band. Under equilibrium,
the Fermi level does not change in position but remains constant in the band diagram of the
p-n junction as shown in Figure 1-3. In the depletion region, the conduction and the valence
bands are not indicated by the straight lines but curved. This represents the presence of
electric field in the region.
p-type Si
n-type Si
EC
EF
EV
Figure 1-3: The p-n junction. The curve at the interface between the p-type and n-type indicates
the presence of internal electric field. The yellow-colored part represents the depletion region.
Under illumination, extra electron-hole pairs are generated and the concentration of
minority carrier in each layer (electrons in p-type and holes in n-type) increases significantly.
The increasing carrier concentrations makes the electrons in the p-type layer flow to n-type
layer and holes from the n-type into the p-type layer. The flow of photo-generated carriers is
called photo-generated current.
Ibadillah Ardhi Digdaya
Master of Science Thesis
1-3 Homojunction and Heterojunction Solar Cells
5
Heterojunction Silicon Solar Cells
The heterojunction solar cell was initially introduced in 1983 by Hamakawa et al [16]. At
that moment, the first heterojunction was fabricated in ITO/(n-i-p) a-Si/(n) a-Si/(p) poly
c-Si/Al structure. In 1991, Sanyo introduced a more developed structure of heterojunction
solar cells by sandwiching thin intrinsic a-Si layer between c-Si wafer and doped a-Si layer
[17], so called Heterojunction with Intrinsic Thin layer (HIT) solar cell. This development of
heterojunction silicon devices has attracted much attention and encouraged several research
groups to adopt and develop Sanyo’s HIT structure.
Principally, the heterojunction silicon solar cell makes use of the large band gap of the aSi:H to form the emitter and the back surface field with the c-Si, so that electrons or holes can
flow in one direction only. A thin layer a-Si:H is doped opposite the c-Si to form p-n junction
and create depletion region. The large band offset in the conduction band can only allow the
electron to travel to the n-type a-Si:H. The electrons that diffuse towards the junction are then
accelerated by the electric field to the back contact. At the front junction, however, the large
valence band offset creates a potential transport barrier for holes. The trapped holes may
be able to drift through narrow “spike” barriers by tunneling, trap-assisted tunneling and/or
thermionic emission [6]. At the backside, the large valence band offset serves as “mirror” for
holes. The (i/n) a-Si:H creates a barrier to prevent minority carrier holes from flowing to the
rear contact and recombining there. In the conduction band, the small band offset does not
much hinder the electron transport. The (i/n) a-Si:H provides a sufficient electron transport
and passivation repulsing holes from the back contact. The schematic energy band diagram
of the heterojunction silicon solar cell is shown in Figure 1-4.
Figure 1-4: Schematic energy band diagram of the heterojunction silicon solar cell (adapted from
[3]).
In the heterojunction structure, defects are established due to uncoordinated atom
bonds on the crystalline silicon surface. For instance on <111> oriented c-Si surface, each
Si atom is covalently bonded with three Si atoms, having one unpaired electron or dangling
bond. The dangling bonds at the interface become the trapping centers for the carriers.
The recombination losses can limit the performance of solar cell, hence several approaches
are required to manage these limitations. One strategy to passivate the surface is based on a
Master of Science Thesis
Ibadillah Ardhi Digdaya
6
Introduction
significant reduction of the excess electron or hole concentration at the interface by introducing
a built-in electric field [18]. The mechanism works by repulsion of minority carriers at the
interface by fixed charges. This so-called field-effect passivation can be achieved by putting
highly doped n-type a-Si:H onto the c-Si. Another strategy is to chemically passivate Si
dangling bonds with H atoms. This can be done by inserting an intrinsic thin a-Si:H film
at the interface between a-Si:H front emitter and c-Si. The a-Si:H layer passivate mainly
by hydrogenation of uncoordinated atoms (dangling bonds) on the c-Si surface, reducing the
defect density.
Figure 1-5 schematically shows the HIT solar cell structure developed by Sanyo. The
intrinsic a-Si:H layers are deposited upside down the n-type c-Si wafer to provide passivation
of the surface dangling bonds on the c-Si wafer. The p-type and n-type a-Si:H are deposited
as the front emitter and back surface field (BSF) respectively. The transparent conductive
oxide (TCO) films are deposited to compensate the high resistance of the a-Si:H surfaces and
enhance the carrier transport to the contacts.
Figure 1-5: Schematic diagram of HIT solar cell structure developed by Sanyo [4].
Heterojunction silicon solar cells hold several main advantages. The good passivation by
intrinsic a-Si:H materials is the cause of high minority carriers lifetime. Another advantage
is the relatively simple fabrication process in comparison to homojunction c-Si solar cells.
The lower processing temperature (<250 ◦ C) and shorter time required to form the junction
and BSF than conventional c-Si solar cell results in extensive reduction in fabrication cost.
Figure 1-6 demonstrates a comparison of processing temperature and duration between the
heterojunction solar cell and the typical homojunction c-Si solar cell.
Although the heterojunction silicon solar cell is composed by the a-Si:H and the c-Si
materials, it does not indicate a strong performance degradation under light exposure, as in
the case of thin-film a-Si:H solar cells. This is because, in the heterojunction silicon solar cells,
the photo-generated carriers are mainly produced in the bulk c-Si, while thin a-Si:H layers
(only several nanometers) in fact serve as emitter, BSF, and passivating layers. In this way,
the degradation effect of very thin a-Si:H materials is negligible, resulting in a high stability.
Besides, the heterojunction silicon solar cell does not exhibit a strong temperature dependence of performance as in the case of c-Si based solar cells. Sanyo has observed that their
heterojunction solar cell shows a smaller drop of performance with increasing temperature in
comparison the conventional homojunction c-Si solar cell [19].
Ibadillah Ardhi Digdaya
Master of Science Thesis
1-4 Challenges of Heterojunction Silicon Solar Cells
7
Figure 1-6: Comparison of fabrication process between the conventional mono c-Si solar cell and
the heterojunction silicon solar cell [5].
1-4
Challenges of Heterojunction Silicon Solar Cells
At present, the record efficiency 23.5% of the heterojunction silicon solar cell is achieved within
several years of research and development. This efficiency, indeed, can be advanced further
by doing several improvements. Such improvements are required to handle possible losses
in the heterojunction silicon solar cell, such as recombination losses which mainly influence
the open-circuit voltage (VOC ), the resistance losses that affect the fill factor (F F ), and the
optical losses which have direct impact to the short-circuit current density (JSC ).
Figure 1-7: Losses in the heterojunction silicon solar cell [6].
Several approaches have been considered to reduce the recombination losses. The first
step involves the cleaning of wafer surfaces prior to a-Si:H deposition to remove organic
particles and metallic contaminants. Next, the interface defect density can be suppressed
Master of Science Thesis
Ibadillah Ardhi Digdaya
8
Introduction
by partially passivating the dangling bonds with hydrogen termination on the wafer surface.
Another step is depositing high quality a-Si:H, resulting in an effective surface passivation.
The quality of these processes will determine the surface recombination, indicated by the
minority carrier lifetime measurement. The resistance losses can be reduced by decreasing
the series resistance of the device. The high conductive TCO and good ohmic contact at the
contact interfaces are important accordingly.
One major optical loss in the heterojunction silicon solar cells is the reflection losses at
the front surface due to the large refractive index of a-Si:H. Currently, two practical methods have been considered to reduce the reflection: (1) using an anti-reflective coating and
(2) texturing the wafer surface. The anti-reflective coating works based on the destructive
interference and the refractive-index matching between the ambient, the coating layer and
the cell. On the other hand, texturing the wafer can minimize the reflection due to multiple “bounces” of rays on the textured surface and provide light trapping. Another optical
loss is the parasitic absorption by TCO. The term “parasitic absorption” is used to refer
the absorption which does not contribute to the electron-hole pair generation. The TCO is
transparent in the visible region, but highly absorbing in the ultraviolet (UV) part of the
spectrum because of the large band gap (<3 eV) and in the infrared region because of the
free-carrier excitation. Despite its function to transport charge carriers to the contacts, the
transparent TCO can also serve as an anti-reflective coating. The optimum anti-reflection
effect is strongly dependent on the thickness of the coating material. Therefore, controlling
the thickness of TCO can effectively reduce the reflection.
Figure 1-8: Optical losses in the heterojunction silicon solar cell.
The a-Si:H is a strong absorbing material with a direct band gap and a high absorption
coefficient. Most of light absorbed in a-Si:H layers does not contribute to the current of the
solar cell, thus are considered as losses. In order to overcome these absorption losses, the
doped and the intrinsic a-Si:H layers should be as thin as possible to maximize the JSC .
However, the doped a-Si:H layer should be thick enough to avoid the complete depletion
region. Similarly, the intrinsic a-Si:H has a certain minimum thickness requirement to ensure
a high passivation quality, indicated by the VOC . Therefore the optimization of both doped
and intrinsic a-Si:H thicknesses are required to balance the JSC and the VOC . On the other
hand, the c-Si wafer absorbs light in a wide spectrum, ranging from 300 nm to 1200 nm.
However, the long wavelength light (>1200 nm) with low photon energy cannot be absorbed
Ibadillah Ardhi Digdaya
Master of Science Thesis
1-5 Thesis Outline
9
further in the bulk c-Si layer due to the optical band gap limitation.
Of all the above-mentioned losses, this thesis work will only focus on the optical improvement of the heterojunction silicon solar cells.
1-5
Thesis Outline
This thesis consists of six chapters. The first chapter covers the general description of the
photovoltaic effect in conventional homojunction and heterojunction silicon solar cells. The
challenges which cover possible losses in the heterojunction silicon solar cell are briefly explained.
The second chapter serves as the theoretical background related to the main work in
this thesis. In this work, the interference effect of thin film is the main focus. This includes
the influence of interference to construct anti-reflective coating for solar cells. Furthermore,
some materials which are employed as anti-reflective coatings are addressed.
The third chapter deals with equipment involved in this work. The equipment are
divided into two parts. Firstly, the working principle of setups used to deposit different
coating materials is introduced. Secondly, various equipment to characterize optical properties
of different solar coating materials and the solar-cell performance are also introduced.
Optical simulations are used to determine the optimum thickness of anti-reflective coatings. Firstly, for the single-layer coating, the thickness of anti-reflective coating is varied to
obtain the minimum total reflectance and the maximum total absorptance in the c-Si. Afterwards, for the double-layer coating, optimizations are done to achieve optimum thicknesses.
In this work, both flat and textured solar cells are simulated. In addition, to reflect the real
commercial solar cell, optical simulations of encapsulated heterojunction silicon solar cell are
conducted.
Experiments are conducted and reported in the fifth chapter. The results from experiment are then compared with simulations which have been done previously. This chapter will
determine the effect of anti-reflective coating on the heterojunction silicon devices.
Lastly, this thesis is wrapped up in conclusion. Possible recommendations will also be
given for future improvements.
Master of Science Thesis
Ibadillah Ardhi Digdaya
10
Ibadillah Ardhi Digdaya
Introduction
Master of Science Thesis
Chapter 2
Fundamentals
In this chapter, firstly, some fundamental optics knowledge will be discussed. Secondly, the
antireflective coating materials to be applied in this work are introduced.
2-1
2-1-1
Optics
Reflection, refraction and absorption
When a light wave reaches an interface between two mediums with different refractive indices,
the wave will be partially reflected, leaving the surface and partially will be transmitted into
the second medium. When passing through the medium, a fraction of light is absorbed in
the material. The optical interface is therefore characterized by the behavior of the light
associated with the reflectance, the absorptance, and the transmittance.
Reflection and transmission at flat interface
The optical properties of a material are often characterized by its complex refractive indices,
denoted by ñ. By definition, the real part of the refractive index n is the ratio of the speed of
light in the medium and in the vacuum, while the imaginary part κ is the extinction coefficient
associated with the absorption loss when the light propagates through the medium.
ñ = n − iκ =
√
ε1 + iε2
(2-1)
where ε is the complex dieletric constant or the relative permittivity. When a light wave
propagates in a medium, its intensity will be reduced exponentially. The reduced intensity
occurs due to the absorption in the medium. The Lambert-Beer law describes how light
intensity changes when light passes through a medium.
I(x) = I0 e−αx
(2-2)
where I0 represents the initial intensity of the incident radiation and I(x) is the light intensity
of the light passes through a distance x, while α is the absorption coefficient. The absorption
coefficient α refers to a fraction of light absorbed in the medium and is related to the extinction
coefficient.
Master of Science Thesis
Ibadillah Ardhi Digdaya
12
Fundamentals
4πκ
(2-3)
λ
According to the electromagnetic wave theory, reflection on a perfectly smooth and flat
surface is always specular [20]. This means upon hitting the surface, the angle of the incident
light is equal to the reflected light. A fraction of light also penetrates the medium, changing
its direction due to the change of the medium, called refraction. The incoming light from
a medium with a refractive index n1 might come to an interface from any directions and is
refracted in certain direction depending on the refractive index of the medium n2 . The angle
of incidence is denoted as θi and the angle of refraction is denoted as θt . This phenomenon is
shown in Figure 2-1.
α=
inc
id
en
t li
gh
t
θi θr
t
igh
dl
te
ec
refl
l
ted
rac
ref
θt
smooth interface
n1
n2
t
igh
Figure 2-1: The reflection and the refraction of light on a plane interface (adapted from [7]).
The relationship between the direction of the incoming and the transmitted light is given by
Snell’s law.
sin θt
n1
=
sin θi
n2
(2-4)
Light travelling in free space is an electromagnetic wave, consisting of the electric field
and the magnetic field. Every electromagnetic wave has a property called state of polarization.
Generally, when considering the polarization of light, the magnetic field can be ignored since
it is perpendicular and proportional to the electric field. The electric field is a resultant vector
of x− and y− components. In this way the light propagates in z− direction. A distinction
can be made for polarization in accordance with its orientation of propagation towards the
plane of incidence. If the vector is parallel (k) to the plane, then it is called p−polarization
and if the vector is perpendicular (⊥) to the plane, it is s−polarization. An expression to
represent the behavior of moving light with varying indices of refraction is given by Fresnel’s
equation. For s−polarized light, Fresnel’s equations are expressed as:
r̃⊥ =
ñ1 cos θi − ñ2 cos θt
ñ1 cos θi + ñ2 cos θt
and
t̃⊥ =
2ñ1 cos θi
ñ1 cos θi + ñ2 cos θt
(2-5)
while for p−polarized light, Fresnel’s equations are represented as follows:
r̃k =
ñ2 cos θi − ñ1 cos θt
ñ1 cos θt + ñ2 cos θi
and
t̃k =
2ñ1 cos θi
ñ1 cos θt + ñ2 cos θi
(2-6)
It should be noted that r̃ and t̃ are the reflectance and the transmittance amplitude coefficient
respectively. The reflectance itself is a fraction of intensity and is expressed by taking square
of the absolute value of r̃. The reflectance of each polarization is represented as follows:
Ibadillah Ardhi Digdaya
Master of Science Thesis
2-1 Optics
13
R = |r̃|2
(2-7)
while the overall transmittance for each polarization is evaluated as the remaining amount of
the intensity upon being subtracted by the reflectance. In another way, the transmittance is
evaluated by considering the difference in the refractive indices and the direction of incoming
and transmitted light propagation.
n2 cos θt 2
t =1−R
(2-8)
n1 cos θi
The term reflectance is a fraction of incident radiation reflected by the surface while
the transmittance is a fraction of radiation passing through medium. It should be noted
that “reflection” and “transmission” refer to the physical process, whereas “reflectance” and
“transmittance” are mathematical ratios.
T =
System with two flat interfaces
A system is considered as a single layer if it consists of two interfaces with certain layer
thickness. If the thickness is much larger than the coherence length of the incident light, the
layer is considered to be incoherent. In contrary, the layer is considered to be coherent if
the thickness is comparable to the coherence length of light. In case of the coherent layer
at normal incidence, the overall reflectance and transmittance are obtained using Fresnel
amplitude coefficients r̃ and t̃.
r̃ = r̃1 +
t̃1 t̃01 r̃2 e−2iδ̃1
1 − r̃2 r̃10 e−2iδ̃1
and
t̃ =
t̃1 t̃2 e−iδ̃1
(2-9)
1 − r̃2 r̃10 e−2iδ̃1
where δ̃1 is the complex phase difference of light. The real part of δ̃1 represents the phase
shift of the light wave and the imaginary part is the absorption when the light passes through
the layer with a thickness of d1 .
2π
2π
d1 ñ1 =
d1 (n1 − iκ1 )
(2-10)
λ
λ
A single layer as one effective interface is described by the reflectance R using Eq. (2-7) and
the transmittance T using Eq. (2-8). Finally the fraction of intensity, A, which is absorbed
in the layer, can be calculated as follows:
δ̃1 =
A=1−R−T
(2-11)
Similarly, for an incoherent layer, the reflectance and the transmittance are calculated by
substituting the amplitude coefficients with energies (the square of the amplitude coefficients).
R = R1 +
T1 T10 R2 e−2αd1
1 − R2 R10 e−2αd1
and
T =
T1 T2 e−α1 d1
1 − R2 R10 e−2α1 d1
(2-12)
where α1 is the absorption coefficient (cf. Eq. (2-3)). In addition, the normalized absorption
can also be calculated using Eq. (2-11).
In case of a multilayer system, a layer is considered as one effective interface, and depending on the coherence length of the layer, Eq. (2-9) or Eq. (2-12) can be applied iteratively.
The whole system is then calculated using Eq. (2-7) and Eq. (2-11).
Master of Science Thesis
Ibadillah Ardhi Digdaya
14
Fundamentals
2-1-2
Thin film interference
The interference effect proceeds by two coherent waves travelling in the same direction superimpose with each other, resulting in greater or lower amplitude of the resultant wave. To
further understand the interference mechanism, the principle of superposition explains that
when two or more waves move in the same region of space, each wave proceeds each other
resulting in a total wave displacement which is equal to the vector sum of displacements of
individual waves. For example, two waves travel at the same point causing displacements
Y1 and Y2 for wave 1 and wave 2 respectively. According to the superposition theory, the
two waves produce a resultant wave displacement of YR which is the vector sum of the two
displacements.
YR = Y1 + Y2
(2-13)
Figure 2-2 shows an example of superposition. Wave 1 (solid black line) moves along with
wave 2 (dashed black line) in the same direction. The two waves are out of phase, causing
resultant wave indicated by the solid red line. By applying Eq. (2-13), the resultant wave is
obtained at any point along x− direction.
YR
2A
y
Wave 2
A
Wave 1
x
Figure 2-2: The superposition of two sinusoidal waves travelling to the same direction (adapted
from [8]). The amplitude of wave 1 is equal to the amplitude of wave 2 producing a resultant
amplitude of 2A.
Figure 2-3 (a) shows two identical waves which are in phase and equal in frequency.
Consequently, the crests and troughs of each wave meet at same points, resulting in the
magnitude of displacement which is the sum of the amplitude of both waves. This phenomenon
is called the constructive interference. In contrary, if the crest of one wave meet the trough
of another wave, the destructive interference occurs, as illustrated in Figure 2-3 (b).
Depending on the purposes, a coating can be constructed to greatly enhance the reflection (high-reflective coating) or to minimize the reflection (anti-reflective coating). If a light
wave travels from a medium with lower refractive index to a medium with higher refractive
index, the reflected wave will undergo a phase change of π or 180◦ . In contrast, if a light wave
travels from a medium with high refractive index to a medium with low refractive indices,
there is no phase change upon reflection. The wavelength in the medium n upon refraction
also changes, following λn = λ0 /n, where λ0 is the light wavelength in the vacuum.
For a better understanding, the influence of the optical path difference with respect
to the interference is discussed. As shown in Figure 2-4, a light wave in air impinges onto
Ibadillah Ardhi Digdaya
Master of Science Thesis
2-1 Optics
15
YR
2A
y
A
x
(a)
y
YR
A1
A2
x
(b)
Figure 2-3: (a) The constructive interference of two sinusoidal waves. The solid black line is
the first wave and the dashed line is the second wave. The resultant wave is represented by the
solid red line (b) The destructive interference. A1 is the amplitude given by wave 1 and A2 is the
amplitude given by wave 2. Adapted from [8].
material stacks with increasing index order, nair < nc < ns , where nair , nc and ns are
refractive indices of the air, the coating layer and the substrate. Then, the light hit upon
A will experience reflection and refraction. The reflected light from a light coming from a
lower-index medium nair towards a higher-index medium nc undergoes a phase shift of π. On
the other hand, the refracted lights travel to the interface B also experiences the reflection
and the refraction. Similarly, the light going from a lower index of nc to a higher index of ns
shifts its phase by 180◦ . In order to keep the two reflected waves in phase with each other,
the optical path difference should be equal to λ, leading to the constructive interference. We
denote δ as the optical path difference. For clarity, a formula is constructed to annotate the
constructive interference as follows:
δ = mλ,
where m = 0, 1, 2, . . .
(2-14)
where m is an integer, meaning the phase shift is a multiple of 2π to induce constructive
interference. If the path difference is equal to a multiple of half of the wavelength, the reflected
waves will be completely out phase with each other, leading to the destructive interference.
1
δ = m+
λ,
2
2-1-3
where m = 0, 1, 2, . . .
(2-15)
Antireflective coating
The incident light that impinges on the solar cell is in the form of electromagnetic wave in
the solar spectrum range. The absorption of light in the silicon favorably occurs in the range
of visible light, where high irradiances of the sun are emitted. However, the high refractive
index of silicon can cause significant reflection losses at the surface. Therefore, antireflective
Master of Science Thesis
Ibadillah Ardhi Digdaya
16
Fundamentals
coatings are required to minimize the overall reflection losses and enhance the absorption at
the bulk layer, particularly within the visible light spectrum.
1
in
cid
2
en
tl
ig
ht
A
nair
C
tc Coating B
nc
Substrate
ns
Figure 2-4: The light propagation on the substrate with a coating layer.
Antireflective coatings for solar cells have been widely applied for optical improvement
of solar cells. The mechanisms are based on the destructive interference occurring among the
reflected light and refractive index matching to minimize the reflectance. A single antireflective coating gives destructive interference effect so that the reflectance can be reduced to a
minimum value at a specific wavelength, as given by the following equation:
R=
n2c − nair ns
n2c + nair ns
!2
(2-16)
In order to achieve zero or minimum reflectance at specific wavelength (λ0 ) the index ratio
between the reflected wave on the top surface and the reflected wave at the bottom interface
should be equal. The index ratio relationship is expressed as follows:
nair
nc
=
nc
ns
(2-17)
If the thickness in such that the path difference of the two reflected waves is λ/2, the
reflections will interfere destructively. The incoming light might come from any directions.
We introduce angle of incidence θ into Eq. (2-15):
δ = d sin θ = m +
1
λ0 ,
2
where m = 0, 1, 2, . . .
(2-18)
At the near normal incidence, the distance that the wave travels in the coating medium is
twice the thickness of the coating.
d = nc (2tc )
(2-19)
where nc and tc are the refractive index and the thickness of the coating layer respectively.
Thus the relevant coating thickness to develop destructive interference is given by:
tc =
λ0
4nc
(2-20)
or so called one-quarter wavelength. By applying such thickness, the reflected light will have
a phase difference of 180◦ and interfere destructively. For a single antireflective coating,
Ibadillah Ardhi Digdaya
Master of Science Thesis
2-1 Optics
17
the coating layer thickness is chosen in such that the reflection is minimal at the desired
wavelength. From this understanding, a fine coating thickness can be determined to obtain
minimum reflection.
2-1-4
Double-layer antireflective coating
The fundamental issue associated with the single-layer antireflective coating is the availability of coating materials with suitable refractive indices. Moreover, the single-layer coating
exhibits minimum or zero reflectance in only a narrow spectral range. The multilayer coating
is an alternative to further reduce reflectance in a wider wavelength range. Principally, twolayer coating cancels the first reflection out through destructive interference with two weaker
out-of-phase reflections at the underlying interfaces.
Wave 1
Wave 2
y
Resultant
Wave 3
x
Figure 2-5: The destructive interference in the double-layer coating.
When considering the double-layer antireflective coating, the films are arranged in such
a way that the refractive indices are in the order, nair < n1 < n2 < ns , where n1 and n2 the
refractive indices of the top and the bottom coating layers respectively. In this configuration,
the reflectance at λ0 is given by:
R=
n21 ns − n22 nair
n21 ns + n22 nair
!2
(2-21)
From Eq. (2-21) we can conclude that if n21 ns = n22 nair , the reflectance R at λ0 will be zero.
In this sense, a broad minimum reflectance can be achieved. While one quarter-wavelength
thickness of the single antireflective coating achieves minimum reflectance at λ0 , the doublelayer antireflective coating will result in a broad minimum reflectance through the existence
of two reflectance minima [21]. The relevant refractive indices of both layers can be derived
by the following equation:
n1 2
nair
=
ns
n2
with optical thicknesses of both coating layers, given by
t1 =
Master of Science Thesis
λ0
,
4n1
and
t2 =
(2-22)
λ0
4n2
(2-23)
Ibadillah Ardhi Digdaya
18
Fundamentals
where t1 and t2 are the thicknesses of the top and the bottom coating layers. When the optical
thicknesses of both layers equal quarter-wavelength, it is called quarter/quarter-wavelength
coatings. The aim of double-layer antireflective coating is to achieve zero or minimum reflectance at a broader spectral range. Particularly in this work, the minimum reflectance is
desirable in the wavelength range of 300 nm to 1200 nm.
2-2
2-2-1
Antireflective Coating Materials
Indium tin oxide
A very thin a-Si:H emitter in the heterojunction silicon solar cell is highly resistive. Therefore, a conductive material is necessary to improve charge transport to the contacts. Such
material should have a capability to provide access for incoming light to pass through to the
active layers. In this sense, TCO which is both transparent and conductive is important in
heterojunction silicon devices. One of mostly used TCO is indium tin oxide (ITO or tin-doped
indium oxide), commonly 90 wt% In2 O3 and 10 wt% SnO2 , due to its high conductivity as
well as high transparency. The electrical conductivity (σ) of TCO can be expressed by the
proportional relationship between the free carrier concentration in the conduction band and
the electron mobility.
σ = µne
(2-24)
where µ is the electron mobility, n and e are the electron density and the electron charge
respectively. The electron mobility has dependence on the crystallinity and the defect concentration. The deposited RF-sputtered ITO on a substrate at room temperature exhibits
high defect densities and small grain sizes. However, high deposition temperature can increase
the grain sizes and post-annealing can result in polycrystalline structure, resulting in a high
conductivity.
Other important factors are the dopant concentration and the number of oxygen vacancy in ITO film. The replacement of In3+ with the substitutional multivalent Sn4+ forms
interstitial bond with oxygen. The addition of Sn dopant leads to an increase in populating carriers in the conduction band, resulting in the formation of donor states just below the
conduction band. As the doping density is increased, these eventually merge with the conduction band [22]. Consequently, a higher dopant concentration and a larger number of oxygen
vacancies lead to an increase in the carrier concentration, thus increasing film conductivity
[23].
Despite increasing the conductivity, the higher carrier concentration can also enhance
the parasitic absorption of ITO, so there is a trade-off between the conductivity and the
transparency. ITO exhibits a high absorption in the UV part of the spectrum and shorter
wavelengths due to the large band gap, while in the near infrared part, it is fairly absorbing
particularly for free carrier excitations.
In heterojunction solar cells, ITO plays a key role to compensate the high resistance
of front emitter and promote carrier transport to the contacts. Moreover, ITO can serve
as a good antireflective coating for the high refractive index substrate which is excellent for
photovoltaic device application. Among the available TCO materials, ITO is chosen due to
its lower resistivity compared to aluminium-doped zinc oxide (ZnO:Al or AZO). The reported
resistivity of ITO and AZO are 6 × 10−4 Ω-cm and 1 × 10−3 Ω-cm respectively [24].
Ibadillah Ardhi Digdaya
Master of Science Thesis
2-2 Antireflective Coating Materials
19
High quality ITO can be deposited using various techniques such as RF magnetron
sputtering [25, 26], DC sputtering [27, 28], pulsed laser deposition [29, 30], spray pyrolysis
[31, 32], sol gel [33, 34], reactive evaporation [35]. Especially in this work, ITO is prepared
by RF magnetron sputtering on the a-Si:H layer of the heterojunction silicon solar cell. A
careful attention should be paid as a very thin a-Si:H material can easily degrade due to
ion bombardment during the sputtering process, resulting in a low minority carrier lifetime
which directly affects VOC . Zhang et al. [25] has investigated the ITO sputtering processing
parameters and post-treatment in order to maintain the passivation quality of the a-Si:H
layer.
2-2-2
Silicon dioxide
Silicon dioxide (silica or SiO2 ) is a universal substance which has a wide range of applications from daily utilities to advanced opto-electronic devices. SiO2 is formed by tetrahedral
coordination of Si atom with the surrounding four oxygen atoms located at the corners of a
tetrahedron, yielding a net chemical formula SiO2 . Despite its major production for glasswares, SiO2 films have also been useful for many microelectronic applications due to its unique
properties such as high chemical and thermal stability and fine electrical insulation.
As optical coatings, SiO2 holds several key features. It is an excellent transparent
material with high optical transmittance in the UV, visible, and near infrared spectrum.
In the visible range, SiO2 has a low refractive index of 1.46 and an extinction coefficient
of zero which lead to high transmittance. These optical properties of SiO2 is beneficial for
antireflective coatings on substrates with low refractive indices or to form a double-layer
antireflective coating on substrates with higher refractive indices.
Table 2-1 shows a list of materials with refractive index lower than ITO. In order to
obtain the benefit from a double-layer antireflective coating, two materials should have a large
refractive index gap. This has been discussed in Section 2-1-4. The presence of ITO in the
heterojunction solar cell is necessary and irreplaceable (although AZO can be the substitute
yet AZO has a similar refractive index to ITO). Therefore, the material to be coupled with ITO
should have the lowest refractive index. Magnesium flouride (MgF2 ) and SiO2 are materials
with lowest refractive indices which can be a fine match with ITO. Another material with a
moderate refractive index is aluminium oxide (Al2 O3 ). The Al2 O3 film can be deposited by
atomic layer deposition (ALD) [36] and electron beam evaporation [37]. The MgO can also be
a good alternative for material with a refractive index between SiO2 and ITO. The Mgo film
can be deposited by plasma-enhanced chemical vapor deposition (PECVD) [38] and electron
beam evaporation [39]. In this work, SiO2 is used to form the double AR coating with ITO.
The technological development has led to various techniques of material coating deposition. The SiO2 film can be deposited by electron beam evaporation [40, 41], PECVD
[42, 43], and ALD [44, 45]. In this work, thin SiO2 film is deposited by e-beam evaporation
from granulates.
Master of Science Thesis
Ibadillah Ardhi Digdaya
20
Fundamentals
Table 2-1: List of materials with refractive indices lower than ITO. ITO data is given for comparison [10, 11].
Material
Magnesium flouride
MgF2
Silicon dioxide
SiO2
Aluminium oxide
Al2 O3
Magnesium Oxide
MgO
Indium tin oxide (ITO)
In2 O3 −SnO2
Ibadillah Ardhi Digdaya
Range of transparency
(µm)
0.12−8
Refractive index
(λ = 0.55 µm)
1.38−1.42
0.2−9
1.45−1.47
0.2−7
1.63
0.2−7
1.74
0.4−1.5
1.95−2.00
Master of Science Thesis
Chapter 3
Equipment
The construction of the heterojunction silicon solar cell consists of different materials with
different properties. Various fabrication steps are required to deposit different layers. In the
post-fabrication scheme, the heterojunction silicon solar cell needs to be characterized with
different characterization methods.
In this chapter, the equipment used in this work will be introduced. Firstly, in Section 3-1, deposition methods of coating materials on the heterojunction silicon solar cell are
explained. Secondly, in Section 3-2, various mechanisms for electrical and optical characterizations are generally discussed.
3-1
3-1-1
Deposition
RF magnetron sputtering
The sputtering deposition is a thin film deposition technique by ejecting atoms from solid
materials so-called the “target” onto a “substrate”. The deposition involves the acceleration
of positive ions (Ar+ ) from inert gas, i.e. Argon, to bombard a target material. If the binding
energy of the target is lower than the energy of bombarding ions, target atoms are ejected out
of the surface of the material and travel to the substrate. This also determines the sputtering
rate. The greater the binding energy of a target is, the more energetic ions are required to
release atoms from the material. Magnetic fields are applied to trap the secondary electrons
in the plasma, thus increasing the ionizing effect.
In the RF magnetron sputtering method, the glow discharge from positive gas ions is
generated by the radio frequency electromagnetic field. As a result, the RF power determines
the ion bombardment energy, hence the sputtering rate. Figure 3-1 schematically shows (a)
the diagram of RF magnetron sputtering and (b) the setup in PVMD group at Delft University
of Technology.
In this work, the RF magnetron sputtering is used to deposit ITO onto the heterojunction silicon solar cell.
Master of Science Thesis
Ibadillah Ardhi Digdaya
22
Equipment
Film
Ejected
atom
Argon
atom
Target
N
S
S
N
N
S
RF generator
(a)
(b)
Figure 3-1: (a) Schematic diagram of RF magnetron sputtering. (b) Setup.
3-1-2
Physical vapor deposition
One of the common physical vapor deposition methods is the electron beam (e-beam) evaporation. It is a deposition technique that a target is bombarded by the e-beam under vacuum
condition. The electron beams are discharged due to the high DC voltage applied to the
tungsten filament. The generated electrons then strike the targeted solid material, releasing
vapors which subsequently travel to the substrate. As they reach the substrate, they condense and form a film. A schematic diagram and the setup of e-beam evaporation is shown
in Figure 3-2.
Substrate
Material
Vapor
Electron
Beam
Water cooled
Target
Material
Crucible
Filament
+
-
-
+
(a)
(b)
Figure 3-2: (a) Schematic diagram of e-beam evaporation (b) Setup.
The e-beam evaporation is used to deposit the metal contacts onto the heterojunction
silicon solar cell. In this work, the SiO2 film coating is also deposited using the e-beam
evaporation.
Ibadillah Ardhi Digdaya
Master of Science Thesis
3-2 Characterization
3-2
3-2-1
23
Characterization
Total integrating sphere (TIS)
The optical characterization is mostly done with the PerkinElmer Lambda 950 UV/VIS. It is a
spectrophotometer equipped with a deuterium and a tungsten lamp which enables to handle
measurement in a broad wavelength range of 175−3300 nm. The setup is equipped with
two separate modules for the types of measurements. For the reflectance and transmittance
(R/T) measurement, the total integrating sphere (TIS) accessory is mounted onto the setup.
Figure 3-3 schematically shows the arrangement of the TIS module.
Reference
beam
This part is removed
for measuring only
the diffuse part of
reflectance
This part is removed
for measuring only
the diffuse part of
transmitance
Light
Source
Detector
Position of the sample
for transmittace measurement
Position of the sample
for reflectance
measurement
Figure 3-3: Sketch of PerkinElmer Lambda/TIS setup for R/T measurement.
The TIS module is an optical component consisting a spherical cavity with white diffusereflection surfaces, two channels for the light source and the reference beam and detectors.
The surface of the cavity is highly reflective, allowing the light to diffuse everywhere in the
cavity, so that the light flux becomes uniform by the detector.
The TIS has the ability to measure both total R/T and diffuse R/T. The total transmittance is measured by fixing a sample at the front port and attaching a spectralon at
the back port. To measure the diffuse transmittance, the spectralon at the back port is removed, so that the specularly transmitted light escape from the system and only the diffusely
transmitted light is received by the detectors. On the other hand, the total reflectance is
measured by replacing the spectralon at the back of the sphere with the sample. The plane
where the sample is located is in fact not perpendicular to the incident light, but tilted by 5◦
from the normal. This arrangement enables to measure the diffuse reflectance by removing
small part of the sphere wall, so that the specularly reflected light can escape and only the
diffusely reflected light is measured by the detectors. For clarity, the illustration can be seen
in Figure 3-3.
3-2-2
Automated R/T Analyser (ARTA)
The Lambda Perkin Elmer 950 can also be equipped with an automated R/T analyzer (ARTA)
module. ARTA is built with an automatically adjustable rotating sample holder and a circular
moving detector. This allows ARTA to conduct measurement at different angles. The sample
Master of Science Thesis
Ibadillah Ardhi Digdaya
24
Equipment
holder can be directed at desired angle from 15◦ to 345◦ (cf. Figure 3-4). The small angle
is the minimum gap required by the detector to prevent blocking of the incoming light. The
configuration of ARTA is illustrated in Figure 3-4.
θ = 90°
moving
detector
θ = 15°
b
incoming le
light hable
θ = 345°
slit
sample
not re
acha
not re
θ
θ = 180°
ac
rotating
sample holder
θ = 270°
Figure 3-4: Sketch of the ARTA configuration. The angle represents the circular movement of
the detector.
The ARTA works mostly for variable angle spectroscopy. Besides, it can also be used to
analyze accurately the refractive index and the extinction coefficient of a thin layer material.
For the sample characterization, a film deposited on a glass substrate is placed in the sample
holder. The adjustable rotating sample holder and the moving detector can be set to a series
of angles, allowing R/T measurement at different angles of incidence which are 0◦ , 15◦ , 30◦ ,
45◦ and 60◦ for both p- and s- polarizations. At 0◦ , the R/T can only be measured with one
polarization. This is due to the fact that at the normal incidence the R/T spectra do not
differ for both polarizations. The measured data are then fitted with the predicted model
available in the SCOUT software to extract the optical constants of the film.
3-2-3
Mini-RT Setup
The Mini-RT setup is a device to measure the spectral R/T of a thin film on a transparent
substrate. The setup consists of a 50 W halogen lamp, a monochromator and filters. The
specular R/T are measured both on the same spot by two separate photodiodes. Figure 3-5
shows the Mini-RT setup at Delft University of Technology.
Unlike the Lambda/TIS module, one major limitation of Mini-RT is its inability to
measure the diffuse R/T. Moreover, the spectrum is limited to the range of 375 nm to 1060
nm. The Mini-RT setup at Delft University of Technology is from ETA-optik. The device is
integrated with the SCOUT software to fit measured R/T to determine the thickness, optical
constants, the band gap and other parameters of the film.
3-2-4
Spectroscopic ellipsometry
The spectroscopic ellipsometry is a useful tool to determine the thickness of a film and its refractive indices or dielectric function. The measuring principle is based on the variation in polarizations upon reflection or transmission of polarized light. The incident light is linear with
Ibadillah Ardhi Digdaya
Master of Science Thesis
3-2 Characterization
25
Figure 3-5: The Mini-RT setup at Delft University of Technology.
both of p− and s−components (the p−component is oscillating parallel and s−component is
oscillating perpendicular to the plane of incidence). Upon reflection on a sample surface, the
linearly polarized light undergoes phase and amplitude changes. In most common cases, the
two components are not in phase nor 90◦ out of phase and arbitrary in amplitude, so called
elliptically polarized. Figure 3-6 shows the sketch of the ellipsometry configuration.
Figure 3-6: The skecth of ellipsometry configuration [9]. The linearly polarized light is reflected from the sample surface. The change in polarization is measured to determine the sample
response.
In ellipsometry, the change of polarization (ρ) is represented by the amplitude ratio (Ψ)
and the phase difference (∆) and related to each other by the following expression:
ρ=
r̃k
= tan(Ψ)ei∆ .
r̃⊥
(3-1)
The reflection coefficients (r̃k and r̃⊥ ) are obtained from Fresnel’s equations (Eq. (2-5) and
Eq. (2-6)). The data analysis proceeds through a fitting procedure where the dielectric model
and the measurement are compared.
It is worth to mention that the spectroscopic ellipsomtery offers more complex optical
analysis. While the single wavelength ellipsomtery can only measure two parameters (Ψ and
∆) per measurement at one wavelength, the spectroscopic ellipsometry has an ability to measure in a broad spectrum which covers a wide range from the UV, visible, and infrared spectral
Master of Science Thesis
Ibadillah Ardhi Digdaya
26
Equipment
region. The strong advantages of the spectroscopic ellipsometry are its non-destructive character, its high sensitivity due to the measurement of the phase of the reflected light, its large
measurement range (from nanometers to micrometers), and the possibilities to control in a
real time complex processes. It is an excellence setup which allows an analysis of complex
layer properties such as multilayer structures, interface roughness and inhomogeneous layers.
3-2-5
Illuminated I-V measurement
Solar cell external parameters
The solar cell output performance is characterized by four main external parameters that are
the JSC , the VOC , the F F , and the maximum power Pmax . By plotting a J-V curve, these
parameters can be determined. The J-V curve is illustrated in Figure 3-7.
The short circuit current is the current when zero voltage across the solar cell is applied
or when the terminals are short circuited. In other words, the JSC means the maximum
current the solar cell can deliver. It is dependent on the number of absorbed photons over
the incident photon flux, thus dependent on the optical properties of the solar cell. On the
other hand, when there is no current flow through the external circuit, the voltage reaches
its maximum value. This occurs when the two electrodes are in open circuit condition, thus
denoted as VOC . In practice, VOC is mainly limited by the recombination losses through the
defects present at the bulks and interfaces.
Figure 3-7: J-V characteristics of a solar cell in the dark and under illumination [2].
According to Figure 3-7, the Pmax is obtained by the maximum square area inside the
J-V curve. Another important parameter, the FF, is a ratio between the actual maximum
power produced by the solar cell and the product of JSC and VOC .
FF =
JM P VM P
JSC VOC
(3-2)
Technically, the F F is strongly influenced by the series resistance (Rs ) and the shunt
resistance (Rp ). The series resistance is the sum of the resistances, including resistances of
each solar-cell component and contact resistances. The shunt resistance is the representative
of the leakage current. Another additional factor causing the lower F F is the recombination
in the junction of a non-ideal solar cell.
Ibadillah Ardhi Digdaya
Master of Science Thesis
3-2 Characterization
27
Figure 3-8: The electric circuit diagram of Solar Cell.
Illuminated I-V setup
The illuminated I-V measurement is carried out with Oriel Solar Simulator. The measurement
setup makes use of the xenon arc lamp to approximate sun illumination. The standard solar
spectrum is defined under illumination on a bright clear day with irradiance of 1000 W/m2 .
Two electrodes are connected at the front and the back contacts of the solar cell. The
illuminated I-V setup at Delft University of Technology is shown in Figure 3-9.
Figure 3-9: The illuminaed I-V setup.
In this work, the solar simulator is mainly used to determine the VOC and F F . To avoid
miscalculation due to indefinite solar-cell area, the JSC is confirmed with external quantum
efficiency (EQE) measurement.
3-2-6
External quantum efficiency Measurement
Quantum efficiency
In the solar cell, the conversion efficiency strongly depends on the quantum efficiency (QE).
Two types of QE are often considered, namely the external quantum efficiency (EQE) and
the internal quantum efficiency (IQE). The EQE is the ratio between the number of collected
electron-hole pairs to the number of incident photons impinges on the solar cell. The EQE
corresponds to the spectral response (SR) at wavelength (λ). The SR is the ratio of the
current generated by the solar cell to the power incident on the solar cell. The relationship
between EQE and SR is expressed as follows
Master of Science Thesis
Ibadillah Ardhi Digdaya
28
Equipment
EQE(λ) = SR(λ) ·
hv
qλ
(3-3)
here h is the Planck constant, v is the speed of light, and q is the electron charge. The EQE
involves both the electrical and optical properties of solar cell. By calculating the EQE, one
can determine the short-circuit current (ISC ).
Z λmax
ISC =
SR(λ) · AM 1.5(λ) ∂λ
(3-4)
λmin
where AM 1.5 is the standard solar spectral irradiance at the Earth’s surface atmosphere.
Depending on the types of solar cells, the λ range differs according to the optical band gap
of the absorber material.
On the other hand, the IQE is the ratio between the number of collected electronhole pairs to the number of photons absorbed in the solar cell. By taking into account the
reflectance R at the front side, the IQE can be simply defined as:
IQE =
1
EQE
1−R
(3-5)
External quantum efficiency setup
The EQE setup is a tool to measure the current of the solar cell illuminated by the chopped
monochromatic light. The light source comes from the xenon arc lamp. The light then passes
through the optical chopper and crosses monochromator. Through adjustable focusing lense,
the light spot can be controlled at the desired size, allowing to measure the EQE of a small
sample area. The EQE setup is shown in Figure 3-10.
(a)
(b)
Figure 3-10: (a) EQE setup. (b) EQE user interface
Prior to the measurement, the calibration is done using a reference silicon photodiode.
It is a UV enhanced photodiode with a wavelength range of 270−1200 nm. The setup is
integrated with the LabView software for the user interface. The user interface provides
parameter sets, such as the measurement wavelength and increment steps.
The ISC of the solar cell is calculated from the EQE spectrum using Eq. (3-3) and 3-4.
In order to eliminate the dependence of the solar cell area, the JSC is used instead of simply
ISC .
Ibadillah Ardhi Digdaya
Master of Science Thesis
Chapter 4
Antireflective Coating on the
Heterojunction Silicon Solar Cell
This chapter will highlight the design of the antireflective coating for the heterojunction silicon
solar cell. The optical properties of each component involved in the heterojunction silicon solar
cell structure will be addressed. Optical simulations include the single- and the double-layer
antireflective coating for both lab scale solar cells and commercial modules. Optimizations are
done to determine the prime coating thickness combination of the double-layer antireflective
coating. Furthermore, comparative illustrations of the spectral reflectance and absorptance
given by different antireflective coating schemes will be presented and discussed.
4-1
Optical Characterization of Individual Layers
The heterojuction silicon solar cell consists of materials with different optical properties. In
order to execute accurate optical simulations, the complex refractive indices of each layer in
the heterojunction silicon solar cell are characterized.
4-1-1
Optical Properties of Semiconductor Materials Used in the Heterojunction
Silicon Solar Cell
Crystalline silicon
Figure 4-1 (a) demonstrates different refractive indices and (b) absorption coefficients of
semiconductor materials in the heterojunction silicon solar cell. The complex refractive indices
of the c-Si are obtained from the R/T measurement by PerkinElmer Lambda 950 UV/VIS
spectrophotometer and fitted by SCOUT software [47].
The optical characteristic of the semiconductor material is the high absorption coefficients which vary in orders of magnitude at different wavelengths. As shown in the figure,
the absorption coefficient of the bulk c-Si decreases at increasing wavalengths. Photons with
higher energy than the c-Si band gap (1.1 eV) are able to create electron-hole pairs. On the
other hand, long-wavelength photons (λ > 1200 nm) with energy higher than the band gap
are hardly absorbed by the c-Si material.
Master of Science Thesis
Ibadillah Ardhi Digdaya
30
Antireflective Coating on the Heterojunction Silicon Solar Cell
7.0
6
Refractive index (-)
-1
(p) a-Si:H
6.0
Absorption coefficient (cm )
10
(n) c-Si
(i) a-Si:H
(n) a-Si:H
5.0
4.0
3.0
(n) c-Si
(p) a-Si:H
5
10
(i) a-Si:H
(n) a-Si:H
4
10
3
10
2
10
1
10
0
10
-1
2.0
10
400
600
800
1000
Wavelength (nm)
(a)
1200
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-1: (a) Refractive indices and (b) absorption coefficients of semiconductor materials as
a function of wavelength.
Hydrogenated amorphous silicon
The complex refractive indices of the p-type, n-type, and intrinsic a-Si:H are obtained from
Woollam Spectroscopic Ellipsomtery measurement of individual layers deposited on glass
substrates. Figure 4-1 (b) illustrates absorption coefficients of a-Si:H and c-Si as a function of
wavelength. As can be seen, in the wavelength range of 450 nm to 700 nm, the a-Si:H absorbs
almost 100 times more than the c-Si. The higher absorption is because the disordered atomic
structure that makes a-Si:H behave like a semiconductor with a direct band gap [2].
4-1-2
Optical Properties of Antireflective Coatings Materials and Encapsulants
Indium tin oxide and silicon dioxide
The R/T of the ITO layer are measured by PerkinElmer Lambda 950 UV/VIS spectrophotometer and fitted by SCOUT software to obtain the complex refractive indices. The complex
refractive indices of SiO2 is extrapolated by fitting the spectroscopic ellipsometry measurement of the SiO2 layer on a silicon wafer with the Cauchy layer model incorporated in the
setup software. Figure 4-2 (a) illustrates the refractive indices of ITO (blue line) and SiO2
(red line). As shown, the refractive indices of SiO2 are constant throughout the spectrum
while the refractive indices of ITO gradually decrease with increasing wavelengths.
From Figure 4-2 (b), it can be seen that SiO2 has absorption coefficients of zero throughout the spectrum. This is the reason why that SiO2 is a highly transparent and a nonabsorbing material. With this optical properties, one can expect that light will be entirely
transmitted when passing through SiO2 . For comparison, the absorption coefficients of ITO
are shown. The high absorption coefficients of ITO gradually decrease, then after 500 nm
start to increase with increasing wavelengths. The increase of absorption coefficients at long
wavelengths is explained by the free carrier absorption in the near infrared.
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-2 Optical Simulation on a Flat Surface
31
Refractive index (-)
ITO
SiO
2.2
2
Al O
2
3
MgO
2.0
1.8
1.6
1.4
400
600
800
1000
1200
Absorption coefficient (cm-1)
2.4
106
ITO
105
SiO
2
Al O
2
3
MgO
104
103
102
101
400
Wavelength (nm)
(a)
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-2: (a) Refractive indices of ITO (blue line), SiO2 (red line), Al2 O3 (green line), and
MgO (purple line) as a function of wavelengths. (b) Absorption coefficients of ITO as function
of wavelengths.
Aluminium Oxide and magnesium oxide
Figure 4-2 also shows the refractive indices of Al2 O3 (green line) and MgO (purple line) as a
function of wavelength. In Section 4-4-2, Al2 O3 and MgO will be used to form the double-layer
antireflective coating with ITO in the encapsulated solar cell. As illustrated, the refractive
indices of MgO are slightly higher than the refractive indices of Al2 O3 . Both Al2 O3 and MgO
have absorption coefficients of zero. This indicates that the two materials are transparent
and non-absorbing throughout the entire spectrum. The complex refractive indices of both
Al2 O3 and MgO are derived from Bass et al. [11].
Glass and EVA
The refractive indices of the glass and EVA are illustrated in Figure 4-3. Both the glass
and ethylene vinyl acetate (EVA) have refractive indices of about 1.5 throughout the spectrum. Figure 4-3 (b) indicates the absorption coefficients of the glass and EVA. As shown,
the absorption coefficients of the glass and EVA are apparently low, yet relatively high at
short wavelengths, which may result in absorption losses. The optical constants of the glass
are obtained from the Energy research Center of the Netherlands (ECN), while the optical
constants of EVA are derived from the R/T measurement by Santbergen [7] and Hylton [48].
4-2
Optical Simulation on a Flat Surface
In the heterojunction silicon solar cell, the high refractive index of the a-Si:H causes high
reflection losses. As shown in Figure 4-4, nearly half of the incident photon flux is reflected
back out of the solar cell. Therefore, anti-reflection is very important to reduce the reflection
losses, hence increasing the light absorption in the c-Si. An anti-reflective (AR) coating is
commonly applied to minimize the reflection of solar cells. In case of heterojunction silicon
solar cells, the ITO as a contact is also designed for the AR purpose.
Master of Science Thesis
Ibadillah Ardhi Digdaya
32
Antireflective Coating on the Heterojunction Silicon Solar Cell
6
10
Absorption coefficient (cm )
1.60
EVA
Refractive index (-)
Glass
-1
Glass
1.56
1.52
1.48
EVA
4
10
2
10
0
10
-2
10
-4
10
400
600
800
1000
400
1200
Wavelength (nm)
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-3: (a) Refractive indices of the glass and EVA as a function of wavelength. (b)
Absorption coefficients of the glass and EVA as a function of wavelength.
18
1.0
5x10
0.8
4x10
18
Reflectance
0.6
18
3x10
AM1.5
Reflected flux
18
0.4
2x10
0.2
18
1x10
Total reflectance
2
44.07 %
0
0.0
400
600
Photon flux density (photon/s/m /nm)
Reflectance (-)
The front ITO absorbs the light at short wavelenghts due to its large band gap and at
long wavelenghts due to excitation of free-carriers. In addition, considerable absorption takes
place in the wavelength range of 400 nm to 500 nm due to the high absorption coefficient of aSi:H layer. The parasitic absorption in the ITO and a-Si:H are considered as loss mechanisms
and can reduce the benefits gained by the AR coating. Therefore, to design AR coating, it
is more relevant considering the absorption in the c-Si rather than simply minimizing the
reflection.
800
1000
1200
Wavelength (nm)
Figure 4-4: The spectral reflectance of the heterojunction solar cell without ITO (blue line), the
photon flux density of global AM1.5 spectrum (black curve), and the reflected photon flux (red
curve) as a function of wavelength.
For optical evaluations, it is important to consider the standard photon flux density of
the solar spectrum. The reference solar spectrum used is the international standard AM1.5
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-2 Optical Simulation on a Flat Surface
33
spectrum, defined in ASTM G173-03 reference spectra [49]. The global AM1.5 spectrum has
an integrated solar irradiance of 1000 W/m2 . In this work, the calculation for reflectance and
absorptance spectra is restricted to the wavelength range of 300 nm to 1200 nm. The range
is chosen because photons with wavelengths below 300 nm are absorbed by the atmosphere
while wavelengths above 1200 nm are negligible for c-Si absorption due to the optical band
gap limitation. Here, the total reflectance is introduced. The total reflectance of the solar cell
is defined as a fraction of the reflected photon flux in the photon flux density of the AM1.5
spectrum.
R 1200
Total reflectance (%) =
R(λ)φ(λ) dλ
300 φ(λ) dλ
300
(4-1)
R 1200
where R(λ) is the reflectance of the solar cell as a function of wavelength and φ(λ) is the
photon flux density of the AM1.5 spectrum. An illustration to represent Eq. (4-1) can be
seen in Figure 4-4. From this, total reflectance can be interpreted as the ratio between the
area under the red curve (reflected flux) and the black curve (incident AM1.5 solar spectrum).
The reflected flux is derived by multiplying the spectral reflectance of the solar cell (blue line)
with the photon flux density of the AM1.5 spectrum.
The minimum total reflectance, nevertheless, does not guarantee the maximum absorptance in the c-Si. Therefore, the total absorptance in the c-Si is introduced as follows
R 1200
Total absorptance in c-Si (%) =
300
Ac-Si (λ)φ(λ) dλ
300 φ(λ) dλ
R 1200
(4-2)
where Ac-Si (λ) is the fraction of incident photons with a wavelength of λ absorbed in the c-Si.
For a complete optical evaluation, thereby, it is worth to inspect the effect of the AR coating
on the heterojunction silicon solar cell by its total reflectance and total absorptance in the
c-Si.
In this work, optical simulations of the heterojunction silicon solar cell have been carried
out using the Advance Semiconductor Analysis (ASA) program developed by PVMD group
at Delft University of Technology. The input parameters for optical simulations in the ASA
program are the layer thicknesses, optical constants of each material, and the AM1.5 spectrum.
4-2-1
Single-layer AR coating on a flat surface
When an AR coating is applied, the thickness of the coating material is important to establish
destructive interference. The ITO thickness is designed in such a way that the reflectance is
suppressed at wavelength where the photon flux of the AM1.5 spectrum is high. According
to Figure 4-4, the photon flux density reaches its maximum value at approximately 670 nm.
Therefore, applying a quarter-wave coating thickness (Eq. (2-20)) which corresponds to this
wavelength is expected to reduce the reflectance optimally. However, determining the ITO
thickness based on the total reflectance minimization (Eq. (4-1)) is more reliable. In this way,
not only the reflected photon flux at one specific length is minimized, but the total reflected
photon flux at the entire spectrum is reduced.
The total absorptance in the c-Si is also the main concern to design the optimum AR
coating thickness. Figure 4-5 shows the total reflectance and the total absorptance in the c-Si
as a function of ITO thickness. As shown, the total reflectance and the total absorptance
Master of Science Thesis
Ibadillah Ardhi Digdaya
34
Antireflective Coating on the Heterojunction Silicon Solar Cell
26
74
25
73
24
72
23
71
Total reflectance
Absorptance in c-Si
22
70
21
69
20
68
Absorptance in c-Si (%)
Total reflectance (%)
in the c-Si indicate exactly opposite trends. The optimum thickness to obtain the minimum
total reflectance and the maximum total absorptance is approximately 80 nm.
67
19
50 55 60 65 70 75 80 85 90 95 100
ITO thickness (nm)
Figure 4-5: The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness.
ITO
80 nm
(p) a-Si:H
(i) a-Si:H
5 nm
5 nm
(n) c-Si
300 μm
(i) a-Si:H
(n) a-Si:H
5 nm
5 nm
Metal
(a)
Spectral reflectance and absorptance (-)
Figure 4-6 schematically shows the structure of the simulated heterojunction silicon
solar cell with the optimum ITO thickness. The spectral reflectance and absorptance profile
of the corresponding configuration is shown in Figure 4-6 (b).
1.0
Reflected
reflected
20.15%
abs. ITO
0.8
abs. a-Si:H
abs. c-Si
abs. metal
0.6
ITO
0.4
c-Si
2.52%
73.20%
a-Si:H
0.2
3.5%
Metal
0.63%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-6: (a) The schematic structure of the heterojunction silicon solar cell. Except ITO, other
layer thicknesses will be kept constant for the entire simulations. (b) The spectral reflectance and
the absorptance profile of the simulated heterojunction cell with an optimum ITO thickness.
We can observe that the minimum total reflectance is 20.15%. The absorptance in the
ITO, which is about 2.52%, mostly occurs in the UV region. A fraction of light absorption in
the ITO also takes place in the near infrared region, which is due to the free-carrier absorption.
Another notable optical loss is caused by the parasitic absorption in front a-Si:H layers. Their
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-2 Optical Simulation on a Flat Surface
35
optical band gap (1.7 eV) is responsible for the absorption in the wavelength range of 300 nm
to 700 nm. Nevertheless, very thin a-Si:H layers (5-nm thick p-type and 5-nm thick intrinsic
a-Si:H) cause only a small absorptance of 3.5%.
Most photons in the wavelength range of 400 nm to 1100 nm are absorbed in the c-Si
layer. The small absorptance in the near infrared also occurs in the rear metal, which is about
0.63%. On the other hand, above 1100 nm most photons are reflected back out of the cell.
4-2-2
Double-layer AR coating on a flat Surface
As has been discussed previously, a single-layer AR coating can bring down the reflectance
at one single wavelength. However, this is still somewhat problematic as a large part of the
spectrum cannot be harnessed. The double-layer AR coating is able to realize a minimum
reflectance in a broader spectral range. The details about the double-layer AR coating mechanisms have been addressed in Section 2-1-4. The challenge is now to find a suitable material
which can satisfy Eq. (2-22). The presence of the ITO in the heterojunction silicon solar
cell is necessary and irreplaceable. If one includes ITO in a double-layer AR coating system,
therefore, the optimum refractive index of the other material should be less than unity (<1),
which is not possible. However, this does not limit the application of the double-layer AR
coating on the heterojunction silicon solar cell. One argument is that it is still advantageous
to apply a material with a lower refractive index in combination with ITO. The larger the
index difference between the two materials is, the lower reflectance can be achieved. In this
work, SiO2 is chosen among the available low refractive index materials and paired with the
ITO to form a double-layer AR coating.
In designing a double-layer AR coating, quarter/quarter wavelength thicknesses can be
used by fulfilling Eq. (2-23). The wavelength (λ0 ) can be varied, ranging from 400 nm to 1100
nm. Thereafter, the total reflectance and the total absorptance in the c-Si can be calculated
as a function of quarter/quarter wavelength coating thicknesses. Principally, it is similar
to the method that is used to design the optimum thickness of the single-layer AR coating,
but in this case the thicknesses of both ITO and SiO2 are set according to quarter/quarter
wavelength.
However, this approach happens to be ineffective in finding the optimum AR coating
thickness combination. One solution is to use the computational optimization technique.
The total reflectance and the total absorptance in the c-Si are used as the cost functions for
the total reflectance minimization and the total absorptance maximization in the c-Si. At
each iteration, a new point with a certain thickness combination is randomly generated. The
distance of the new point from the current point is based on a probability that depends on
the difference between the corresponding cost function values. The algorithm accepts all new
points that lower the objective, but also, with a certain probability, points that raise the
objective. By accepting points that raise the objective, the algorithm avoids being trapped
in a local minima in early iterations and is able to explore globally for better solutions.
Since the double-layer AR coating system involves two variables (i.e., the layer thicknesses), the optimization should compute a large number of iterations. By computing a large
number of iterations, the optimization generates a large number of scattered data, consisting
of the two coating thickness variables and the corresponding cost functions. The distribution
of scattered data can be used to map the total reflectance and/or the total absorptance in the
c-Si with respect to ITO and SiO2 thicknesses. Figure 4-7 (a) demonstrates the change of the
total reflectance when ITO and SiO2 thicknesses vary independently. The dark blue region
Master of Science Thesis
Ibadillah Ardhi Digdaya
36
Antireflective Coating on the Heterojunction Silicon Solar Cell
represents the ITO & SiO2 thickness combination which result in the lowest total reflectance
and the red region represents the higher total reflectance. As shown, the optimum thicknesses
of ITO and SiO2 to obtain the minimum total reflectance are 60 nm and 70 nm respectively.
20
60
19
ITO thickness (nm)
21
70
Total reflectance (%)
ITO thickness (nm)
22
80
90
74
80
73
70
72
60
71
50
50
18
40
60
80
100
SiO2 thickness (nm)
(a)
120
Absorptance in c−Si (%)
75
23
90
70
40
60
80
100
SiO2 thickness (nm)
120
(b)
Figure 4-7: (a) The total reflectance and (b) the total absorptance in the c-Si as a function of
ITO and SiO2 thicknesses
As indicated in Figure 4-7 (b), the highest total absorptance in the c-Si can be achieved
when the ITO and the SiO2 thicknesses meet in the dark red region. Obviously, Figure 4-7 (a)
and (b) show identical patterns, i.e., the same thickness combination of 60-nm thick ITO and
70-nm thick SiO2 gives both a minimum total reflectance and a maximum total absorptance
in the c-Si.
Figure 4-8 (a) schematically shows the structure of the simulated heterojunction silicon
solar cell with the optimum ITO and SiO2 thicknesses. The spectral reflectance and absorptance profile of the corresponding configuration is shown in Figure 4-8 (b). Numerical results
indicating the total reflectance and the total absorptance in each layer are also presented. As
can be seen, the reflectance is suppressed in a broader spectral range. The total reflectance
given by this heterojunction silicon solar cell configuration with a double-layer AR coating
is about 17.27%. This implies an absolute reduction of almost 3% compared to the total
reflectance on the device with a single-layer AR coating. The addition of SiO2 on the top
surface does not indicate any absorptance in this particular layer. This is because SiO2 is
a perfectly transparent and non-absorbing medium throughout the entire spectrum. About
2.33% of the photon flux in the UV region is parasitically absorbed in the ITO. Due to the
wider minimum reflectance, the parasitic absorption in a-Si:H layers is higher than that with
the single-layer AR coating. In total, the parasitic absorptance in a-Si:H layers is 4.52%. The
broader minimum spectral reflectance also leads to a higher total absorptance in the c-Si.
Compared to the single-layer AR coating, the total absorptance in the c-Si is increased from
73.2% to 75.6%.
The reflectance difference between the optimum single- and double-layer AR coatings
is shown in Figure 4-9 (a). As indicated, with the double-layer AR coating, the reflectance is
reduced across almost the entire spectrum. From Figure 4-9 (b) we see that improvements of
the total absorptance in the c-Si given by the double-layer AR coating occur in the wavelength
range of 300 nm to 500 nm and 650 nm to 1050 nm. The increase at these wavelengths
Ibadillah Ardhi Digdaya
Master of Science Thesis
SiO2
Spectral reflectance and absorptance (-)
4-2 Optical Simulation on a Flat Surface
70 nm
ITO
60 nm
5 nm
5 nm
(p) a-Si:H
(i) a-Si:H
(n) c-Si
300 μm
(i) a-Si:H
(n) a-Si:H
5 nm
5 nm
Metal
37
1.0
0.8
Reflected
reflected
17.27%
abs. ITO
ITO
abs. a-Si:H
2.33%
0.6
abs. c-Si
abs. metal
a-Si:H
4.52%
c-Si
0.4
75.55%
0.2
Metal
0.33%
0.0
400
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-8: (a) The schematic structure of the heterojunction silicon solar cell with a double-layer
AR coating. (b) The spectral reflectance and absorptance profile of the simulated heterojunction
solar cell with the optimum ITO and SiO2 thickness combination.
corresponds to wavelengths at which the reflectance is reduced. On the other hand, the
absorptance in the c-Si is reduced in the wavelength range of 500 nm to 650 nm due to the
increased reflectance.
1.0
1.0
Total reflectance (ITO & SiO ) = 17.27%
Absorptance in c-Si (-)
Spectral reflectance (-)
Total reflectance (ITO) = 20.15%
2
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
Abs. in c-Si (ITO) = 73.2%
Abs. in c-Si (ITO & SiO ) = 75.55%
2
0.0
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-9: (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
the single- and the double-layer AR coatings.
The complete comparison of numerical results between the single- and the double-layer
AR coating on the simulated flat heterojunction silicon solar cell can be seen in Table 4-1.
Master of Science Thesis
Ibadillah Ardhi Digdaya
38
Antireflective Coating on the Heterojunction Silicon Solar Cell
Table 4-1: Comparative results of the total reflectance and the total absorptance in each layer
with different AR coating schemes of a flat heterojunction solar cell. The thicknesses of AR
coatings are optimum values.
AR coating
ITO
ITO & SiO2
4-3
Reflected
flux (%)
20.15
17.27
ITO
2.52
2.33
Absorbed in (%)
a-Si:H c-Si metal
3.5
73.20 0.63
4.52
75.55 0.33
Optical Simulation on a Textured Surface
For solar cell applications, in general, the textured c-Si wafer offers two optical benefits
compared to the flat polished c-Si wafer. The first benefit is that when incident light hits a
steep facet feature on a textured surface, a fraction of light will be reflected and directed to
the neighboring facet. In this way, the initial reflected light will have a second opportunity
to enter the wafer. Secondly, when entering the wafer, the light travels in oblique directions,
increasing the optical path length to enhance the light absorption in the wafer, particularly for
the weakly absorbed light. Figure 4-10 shows the comparison between the measured spectral
reflectance of the textured (red line) and the flat (dashed black line) heterojunction silicon
solar cell without AR coatings. As indicated, without the AR coating, the reflectances of the
textured device are approximately 20% lower than the flat device across almost the entire
spectrum. The total reflectance given by the textured surface is about 17.63%.
1.0
Spectral reflectance (-)
Flat surface
Textured surface
0.8
0.6
0.4
0.2
Total reflectance
17.62 %
0.0
400
600
800
1000
1200
Wavelength (nm)
Figure 4-10: The measured spectral reflectance of a textured (solid red line) and a flat polished
(dashed black line) heterojunction silion solar cell without an AR coating.
For the textured heterojunction silicon solar cell, optical simulations are carried out
using the GENPRO4 model, the improved version of OPTICALCULATE model developed
by Santbergen [7]. In the model, the extended net-radiation method is used to treat sub-rays
at each interface of multilayer systems in the matrix formalism. The ray tracing model is
incorporated to generate scatter matrices using periodic boundary conditions defined in a unit
cell with fixed dimensions and facet angles. The three-dimensional representation of a single
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-3 Optical Simulation on a Textured Surface
39
pyramid is used to capture both external and internal reflections. In the model, the complex
refractive indices, the thickness of each layer and the pyramid steepness of 54.7◦ are used as
the input parameters. The interface is represented by predefined periodic three-dimensional
pyramid shapes.
In the model, the thin p-type and intrinsic a-Si:H layers and ITO are treated as coherent
layers (i.e., interference effect is considered) that homogeneously cover and follow the shape
of large pyramid features on the c-Si surface. The thickness of each layer is defined as the
thickness from the surface normal direction.
4-3-1
Single-layer AR coating on a textured surface
Figure 4-11 (a) shows the minimum total reflectance and the total absorptance in the c-Si
as a function of ITO thickness. For textured surfaces, the optimum ITO thickness is smaller
than that for flat surfaces. The reason is that, due to the tilt sidewall of the pyramid, the
incoming light from the normal incidence has a larger optical path from the layer thickness,
as shown in Figure 4-11 (b).
7.5
Total reflectance (%)
7.0
84
6.5
83
6.0
Total reflectance
Absorptance in c-Si
5.5
82
5.0
81
4.5
4.0
40
Absorptance in c-Si (%)
85
ITO
a-Si:H
80
50
60
70
80
90
100
110
c-Si
120
ITO thickness (nm)
(a)
(b)
Figure 4-11: (a) The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness. (b) The light propagation in ITO.
Figure 4-12 (a) schematically shows the structure of the textured heterojunction silicon
with the optimum ITO thickness that is used in the simulation. The spectral reflectance
and absorptance profile of the corresponding configuration is shown in Figure 4-12 (b). As
indicated, the reflectance is greatly suppresed, particularly in the wavelength range of 500
nm to 1000 nm. The total reflectance given by the optimum thickness of the single-layer AR
coating is 4.06%. In the wavelength range of 300 nm to 500 nm, about 8% of the light is
absorbed by the ITO, p-type and intrinsic a-Si:H layers. This parasitic absorptance on the
textured surface is more than that on the flat surface. This is due to the lower reflectance
at short wavelengths, resulting in an enhancement of the absorptance in ITO. In the near
infrared region, free carriers in the ITO also absorb a larger fraction of light.
For the absorptance in the c-Si, a notable improvement occurs in the near infrared
region. Due to the light scattering on the textured surface, the light travels obliquely in the
c-Si, increasing the optical path length and enhancing the light absorption at long wavelengths.
Master of Science Thesis
Ibadillah Ardhi Digdaya
40
Antireflective Coating on the Heterojunction Silicon Solar Cell
ITO
(p) a-Si:H
(i) a-Si:H
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
75 nm
Spectral reflectance and absorptance (-)
This leads to an increase in the total absorptance in the c-Si of 84.61%. Inevitable optical
losses occur in the metal in the wavelength range of 1100 nm to 1200 nm spectrum. This is
due to the fact that the steep texture makes the light hardly escape from the back surface
while the absorption in the c-Si is limited in that spectral range. In this way, the transmitted
light penetrates the wafer and is absorbed in the metal.
1.0
0.8
Reflected
reflected
4.06%
abs. ITO
abs. a-Si:H
ITO
abs. c-Si
3.97%
abs. metal
0.6
c-Si
a-Si:H
0.4
84.61%
4.15%
0.2
Metal
3.21%
0.0
400
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-12: (a) The schematic structure of the textured heterojunction silicon solar cell with
a single-layer AR coating applied. (b) The spectral reflectance and absorptance profile of the
simulated textured heterojunction solar cell with an optimum ITO thickness.
4-3-2
Double-layer AR coating on a textured surface
Although the optical performance given by the single-layer AR coating on a textured surface
shows very low reflectances in a broad spectral range, it is also worth to inspect the effectiveness of the double-layer AR coating. The low-refractive-index material, SiO2 , is also used in
combination with the ITO. When the double-layer AR coating is employed, the traditional
calculation of coating thicknesses is more complex as the incident light enters from an oblique
direction relative to the wafer surface due to the tilt sidewalls of the pyramid. When entering,
the light passes through the two coating mediums, refracted two times before coming into the
Si layers. However, with the aid of an optimization technique, the thicknesses of coating films
can be optimized computationally.
Figure 4-13 (a) demonstrates the distribution map of the total reflectance with respect to
various ITO and SiO2 thickness combinations. The minimum total reflectance can be achieved
by pairing approximately 60-nm thick ITO with 90-nm thick SiO2 . However, in Figure 4-13
(b), it is shown that to obtain the maximum total absorptance in the c-Si, the combinations
differ to some extent from combinations for the minimum total reflectance, particularly for the
ITO thickness. It requires approximately 35-nm thick ITO film in combination with 90-nm
thick SiO2 .
The schematic structure of the textured heterojunction silicon solar cell with the optimum ITO and SiO2 thicknesses that is used in the simulation is shown in Figure 4-14 (a). The
spectral reflectance and absorptance profile for the corresponding combination is illustrated
Ibadillah Ardhi Digdaya
Master of Science Thesis
41
6
40
5
4
20
ITO thickness (nm)
7
60
Total reflectance (%)
ITO thickness (nm)
8
80
86
80
60
85
40
84
20
83
3
60
80
100
120
SiO2 thickness (nm)
140
60
80
100
120
SiO2 thickness (nm)
(a)
Absorptance in c−Si (%)
4-3 Optical Simulation on a Textured Surface
140
(b)
Figure 4-13: (a) The total reflectance and (b) the total absorptance in the c-Si as a function of
ITO and SiO2 thicknesses.
SiO2
90 nm
ITO
(p) a-Si:H
(i) a-Si:H
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
(a)
35 nm
Spectral reflectance and absorptance (-)
in Figure 4-14 (b). As indicated, the reflectance is greatly suppressed in the wavelength range
of 300 nm to 1100 nm. Although there is a small deviation at 400 nm, the reflectances in most
parts of the spectrum are nearly zero. However, compared to the single-layer AR coating on
a textured surface, the fraction of parasitic absorption is lower. This is because the ITO layer
thickness to form an optimum double-layer AR coating is much reduced, resulting in a lower
absorptance in the ITO and consequently higher absorptance in the c-Si, particularly in the
UV region. The absorptance in the SiO2 is only 0.2% (not indicated in the figure). The
overall improvement given by the double-layer AR coating on a textured surface leads to a
total absorptance in the c-Si of 86.72%.
1.0
ITO
0.8
Reflected
reflected
3.07%
abs. ITO
2.9%
abs. a-Si:H
abs. c-Si
a-Si:H
0.6
abs. metal
4.20%
c-Si
86.72%
0.4
0.2
Metal
3.73%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-14: (a) The schematic structure of the textured heterojunction solar cell with a doublelayer AR coating applied. (b) The spectral reflectance and absorptance profile of the simulated
textured heterojunction solar cell with optimum ITO and SiO2 thickness combination.
Figure 4-15 (a) shows the comparison of the reflectance spectra between the single- and
Master of Science Thesis
Ibadillah Ardhi Digdaya
42
Antireflective Coating on the Heterojunction Silicon Solar Cell
the double-layer AR coating on the textured heterojunction silicon solar cell. As indicated,
the double-layer AR coating is able to bring down further the reflectance from 4.06% by the
single-layer AR coating to 3.07%. The improvement of the absorptance in the c-Si given by
the double-layer AR coating is shown in Figure 4-15 (b). In the wavelength range of 300 nm
to 400 nm, the absorptance in the c-Si is increased of approximately 10% compared to the
single-layer AR coating. A slight enhancement also occurs in the wavelength range of 600 nm
to 1100 nm. This corresponds to the region where the reflectance is reduced.
1.0
0.8
0.6
Total reflectance (ITO) = 4.10%
Total reflectance (ITO & SiO ) = 3.12%
2
0.4
0.2
Absorptance in c-Si (-)
Spectral reflectance (-)
1.0
0.8
0.6
0.4
0.2
Abs. in c-Si (ITO) = 84.61%
Abs. in c-Si (ITO & SiO ) = 86.72%
2
0.0
0.0
400
600
800
1000
1200
Wavelength (nm)
400
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-15: (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by
the single- and the double-layer AR coatings.
In Table 4-2, the complete numerical results of the simulated textured heterojunction
silicon solar cell with optimum thicknesses of the single-layer AR coating and the double-layer
AR coating schemes are compared.
Table 4-2: Comparative results of the total reflectance and the total absorptance in each layer
with different optimum AR coating schemes on a textured heterojunction solar cell.
AR coating
ITO
ITO & SiO2
4-4
Reflected
flux (%)
4.06
3.07
Absorbed in (%)
ITO a-Si:H c-Si metal
3.97
4.15
84.61 3.21
2.09
4.20
86.72 3.73
Optical Simulation of an Encapsulated Heterojunction Silicon
Solar Cell
In PV module manufacturing processes, the solar cell is encapsulated to provide protection
for long term mechanical and electrical stability. The thin solar cell is fragile and easy to
break, thus encapsulation is required. The encapsulation ensures the complete protection
against water or moisture penetration which can result in the risk of short circuits that can
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell
43
damage the sensitive electronic components. For optical purposes, the encapsulation material
is designed in such that it has sufficient transparency to transmit the light into the solar cell.
The polymer based material, EVA, is widely used for encapsulant due to its transparency,
hard-wearing, corrosion resistance.
Previously, the laboratory solar cells were investigated, which means the solar cell is
exposed to air. However, this does not reflect real industrial PV applications. In practice,
the commercial solar cell is encapsulated with two extra optical slabs, that are the glass
and the encapsulant, forming a PV module structure. Certainly, the additional glass and
encapsulant on the top surface will greatly influence the optical performance of the solar cell.
This is because in the real module, the light passes through the glass and the encapsulant
first before entering the solar cell. A fraction of light is reflected at the front glass surface,
and some are refracted to the bottom layers. In a PV module, the thickness of the glass
and the encapsulant are normally constant. Thus in this section, the optical impacts of the
glass and the encapsulant on the solar cell are highlighted. The effects of the single- and the
double-layer AR coatings are simulated in the heterojunction silicon solar cell with EVA and
glass on top as demonstrated in Figure 4-16. The thickness reference of the glass and EVA
are 3 mm and 200 µm respectively.
Glass
EVA
TCO
(p) a-Si:H
(i) a-Si:H
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Semiconductor
Metal
Figure 4-16: The schematic structure of an encapsulated heterojunction silicon solar cell where
the glass and EVA are incorporated.
4-4-1
Single-layer AR coating on an encapsulated flat solar cell
The total reflectance and the total absorptance in the c-Si as a function of ITO thickness are
plotted as shown in Figure 4-17. The two curves give an indication that the optimum ITO
thickness is 72 nm. This also implies that changing the ITO thickness of the encapsulated
solar cell can influence both the total reflectance and the total absorptance in the c-Si as it
does without the presence of encapsulation.
Figure 4-18 (a) schematically shows the structure of the encapsulated heterojunction
solar cell with the optimum ITO thickness that is used in the simulation. Except the reoptimized ITO and the added glass and EVA on top, the thicknesses of other layers in this
solar cell are kept the same as in the unencapsulated cell. The back sheet is neglected,
considering within the range of interest (300 nm to 1200 nm), no light can pass through the
back metal contacts. The spectral reflectance and absorptance profile of the corresponding
configuration is shown in Figure 4-18 (b). The total reflectance caused by the encapsulated
cell is about 20.81%, which is similar to the total reflectance by the unencapsulated cell (cf.
Figure 4-6 (b)). For the wavelength range of 300 nm to 350 nm, most light is absorbed by
Master of Science Thesis
Ibadillah Ardhi Digdaya
44
Antireflective Coating on the Heterojunction Silicon Solar Cell
70
69
24
68
23
67
Total reflectance
Absorptance in c-Si
22
66
65
21
40
50
60
70
80
90
Absorptance in c-Si (%)
Total reflectance (%)
25
64
100 110 120
ITO thickness (nm)
Figure 4-17: The total reflectance and the total absorptance in the c-Si as a function of ITO
thickness with a single-layer AR coating on an encapsulated flat heterojunction solar cell.
Glass
3 mm
EVA
200 μm
ITO
(p) a-Si:H
(i) a-Si:H
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
(a)
72 nm
Spectral reflectance and absorptance (-)
the glass. The absorption coefficient of the glass is in fact small, however, considering the
great thickness of the glass (3 mm) through which light must travel, it causes optical losses
of 1.90%. The EVA, another medium the light must pass through, causes an absorption loss
of 1.47% by blocking the light in the wavelength range of 350 nm to 400 nm. The ITO, ptype and intrinsic a-Si:H are responsible for an important fraction of the parasitic absorption,
which amounts to 5.69% in the wavelength range of 400 nm to 700 nm.
1.0
Glass
Reflected
1.90%
20.81%
0.8
abs. glass
abs. EVA
ITO
abs. ITO
1.73%
0.6
abs. a-Si:H
abs. c-Si
c-Si
a-Si:H
0.4
0.2
reflected
abs. metal
69.75%
3.96%
EVA
1.47%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-18: (a) The schematic structure of the encapsulated heterojunction silicon solar cell
with the single-layer AR coating applied. (b) The spectral reflectance and absorptance profile of
the simulated heterojunction silicon solar cell with the optimum ITO thickness.
In theory, the interference effect is dependent on the optical thickness and the coherent
propagation of the light. From the figure, it is noticed that the spectral reflectance of the
encapsulated cell does not indicate zero reflectance at any wavelenghts. This is mainly due
to the great thickness of the glass (3 mm) and EVA (200 µm), causing no occurrence of
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell
45
interference on top layers. All in all, the total absorptance in the c-Si drops to 69.75%
compared to the unencapsulated cell.
4-4-2
Double-layer AR coating on an encapsulated flat solar cell
The glass and EVA have a high transparency with a refractive index of 1.5 (at λ = 550 nm) for
both materials. However, their low refractive indices are fairly similar to the refractive index
of the SiO2 (n = 1.47 at λ = 550 nm), which in the previous section was used in combination
with ITO on the unencapsulated cell. Thus, when SiO2 is added to form a double-layer AR
coating at the interface between EVA and the ITO, the light will not experience a changing
medium, instead, the light is assumed to simply travel in the same medium with an additional
thickness. Consequently, applying the ITO & SiO2 double-layer AR coating will result in a
similar characteristic to the single-layer AR coating of ITO in the encapsulated cell.
In order to achieve an effective optical interface, a selective material with a refractive
index between 1.5 (EVA) and 2.04 (ITO) should be considered. One available material that
can be used is Al2 O3 which has a refractive index of 1.63 (at λ = 550 nm). Another candidate
material within the index range is MgO which has a slightly higher refractive index of 1.74
(at λ = 550 nm).
For a double-layer AR coating on an encapsulated cell, simulations and optimizations
are carried out to select a compatible material to couple with ITO. In this section, the total
absorptance in the c-Si is the main focus, therefore, only thickness combinations of ITO &
Al2 O3 and ITO & MgO are optimized and compared for the total absorptance maximization
in the c-Si while the total reflectance minimization are not considered. Figure 4-19 depicts
the optimization for the total absorptance in the c-Si as a function of (a) ITO and Al2 O3
thicknesses and (b) ITO and MgO thicknesses. It can be observed that optimum thicknesses
of both ITO and Al2 O3 and ITO and MgO combinations are 60 nm and 40 nm. The two
plots show similar trends where the total absorptance in the c-Si can be maximized up to a
similar value of approximately 70% with the same thicknesses for both combinations.
67
60
66
65
40
64
20
63
62
20
40
60
Al2O3 thickness (nm)
(a)
80
ITO thickness (nm)
68
Absorptance in c−Si (%)
ITO thickness (nm)
80
90
69
80
68
70
67
60
50
66
40
65
30
64
20
63
10
Absorptance in c−Si (%)
70
69
62
20
40
60
MgO thickness (nm)
80
(b)
Figure 4-19: The total absorptance in the c-Si as a function of (a) ITO and Al2 O3 thicknesses,
and (b) ITO and MgO thicknesses.
Now, the MgO is added to form a double-layer AR coating on the encapsulated cell,
of which a schematic is shown in Figure 4-20 (a). As indicated in Figure 4-20 (b), the
Master of Science Thesis
Ibadillah Ardhi Digdaya
46
Antireflective Coating on the Heterojunction Silicon Solar Cell
Glass
3 mm
EVA
200 μm
MgO
40 nm
ITO
(p) a-Si:H
(i) a-Si:H
60 nm
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
(a)
Spectral reflectance and absorptance (-)
spectral reflectance and absorptance profile of the encapsulated cell with a double-layer AR
coating shows a similar pattern to the cell with a single-layer AR coating (cf. Figure 418 (b)). The reduction of the reflectance with the double-layer AR coating happens to be
insignificant. The small index difference between the two coating materials does not lead to a
major improvement for the reflectance minimization. The large thickness of the glass on the
top allows no occurrence of interference on the top surface, resulting in no zero reflectance
achieved in any part of the spectrum. A relatively large fraction of the light is reflected first
by the glass before entering the solar cell. The addition of MgO does not absorbed light as
it is a transparent and non-absorbing throughout the spectrum. The absorptance in other
layers indicate similar results.
1.0
Glass
Reflected
1.89%
20.59%
0.8
abs. glass
abs. EVA
ITO
abs. ITO
1.46%
0.6
abs. a-Si:H
abs. c-Si
c-Si
a-Si:H
0.4
0.2
reflected
abs. metal
70.23%
4.02%
EVA
1.43%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-20: (a) The schematic structure of an encapsulated heterojunction silicon solar cell with
the ITO & MgO double-layer AR coating applied. (b) The spectral reflectance and absorptance
profile of the simulated encapsulated heterojunction solar cell with the optimum ITO and MgO
thicknesses.
For better illustration, Figure 4-21 shows the effect of the double-layer AR coating in
comparison with the single-layer AR coating on the encapsulated heterojunction silicon solar
cell with respect to (a) the reflectance and (b) the absorptance spectra. As shown, the doublelayer AR coating can only reduce the reflectance at 400 nm yet increase in the wavelength
range of 450 nm to 600 nm compared to the single-layer AR coating. A closer inspection
shows that a minor enhancement in the total absorptance in the c-Si in the wavelength range
of 900 nm to 1200 nm, leading to a slight increase from 69.75% with a single-layer AR coating
to 70.25% with a double-layer AR coating.
In summary, the two options of using Al2 O3 and MgO are compared as shown in Table
4-3. The thicknesses of both ITO & Al2 O3 combination and ITO & MgO combination are
optimum values.
4-4-3
Single-layer AR coating on an encapsulated textured solar cell
The applications of the AR coating on a textured encapsulated solar cell are also investigated.
As already mentioned in Section 4-3, the textured surface provides an extra path for light
arriving on a groove facet as the first incidence. Upon arriving, the light is reflected sideways
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell
Spectral absorptance in c-Si (-)
1.0
Spectral reflectance (-)
Total reflectance (ITO) = 20.81%
Total reflectance (ITO & MgO) = 20.59%
0.8
0.6
0.4
0.2
0.0
400
600
800
1000
1200
Wavelength (nm)
47
1.0
0.8
0.6
0.4
0.2
Abs. in c-Si (ITO) = 69.75%
Abs. in c-Si (ITO & MgO) = 70.23%
0.0
400
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-21: (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by a
single-layer AR coating (ITO) and a double-layer AR coating (ITO & MgO) on an encapsulated
heterojunction solar cell.
Table 4-3: The comparative results of the total reflectance and the total absorptance in each
layer with different AR coating schemes on an encapsulated heterojunction silicon solar cell. The
thicknesses of AR coatings are optimum values.
AR coating
ITO
ITO & Al2 O3
ITO & MgO
Reflected
flux (%)
20.81
20.74
20.59
glass
1.90
1.89
1.89
Absorbed in (%)
EVA ITO a-Si:H
1.47 1.73
3.96
1.42 1.46
4.09
1.43 1.46
4.02
c-Si
69.75
70.00
70.23
and re-directed towards the neighboring texture facet, having a second chance to enter to
the cell. In case of the encapsulated cell, the encapsulation provides an additional indirect
path. The reflected light escaping out of the textured surface is confined by the glass/air
interface and reflected back towards the textured cell. In this way the light will have second
incident chance to enter the cell. However, this is greatly influenced by the steepness of the
texture surface. As investigated by Santbergen [7], the steeper texture under encapsulation
will result in no occurrence of the total internal reflection. Moreover, the textured cell under
encapsulation suffers from the low absorption in the solar cell because the glass surface reflects
the light.
Figure 4-22 demonstrates the deviation between the two optimum ITO thicknesses to
achieve a minimum reflectance and a maximum absorptance in the c-Si. The total reflectance
does not change drastically with varying thickness. However, the larger thickness of ITO
can be a major cause for the lower total absorptance in the c-Si. This is because the larger
ITO thickness implies more high absorption losses in the ITO. Since in the heterojunction
silicon solar cell it is more desirable to achieve a maximum total absorptance in the c-Si rather
than the minimum total reflectance, therefore the optimum ITO thickness for this particular
module configuration is 50 nm.
Master of Science Thesis
Ibadillah Ardhi Digdaya
48
Antireflective Coating on the Heterojunction Silicon Solar Cell
7.25
81.0
Total reflectance (%)
7.00
80.5
6.75
80.0
6.50
Total reflectance
79.5
Absorptance in c-Si
6.25
79.0
6.00
78.5
5.75
Absorptance in c-Si (%)
81.5
78.0
40
60
80
100
120
ITO thickness (nm)
Figure 4-22: The total reflectance and the total absorptance in the c-Si of an encapsulated
textured cell as a function of the ITO thickness.
Glass
3 mm
200 μm
EVA
ITO
(p) a-Si:H
(i) a-Si:H
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
(a)
50 nm
Spectral reflectance and absorptance (-)
The schematic structure of the encapsulated textured heterojunction silicon solar cell
with the optimum ITO thickness is shown in Figure 4-23 (a). The spectral reflectance and
absorptance profile of the corresponding configuration is shown in Figure 4-23 (b). The
spectral reflectance shows a constant value of about 5% in the wavelength range of 300 nm to
1000 nm, resulting in a total reflectance of 6.23%. The a-Si:H absorbs a larger fraction of light
in the wavelength range of 350 nm to 450 nm compared to the flat surface (cf. Figure 4-20
(b)). A considerable loss takes place in the metal in the near infrared region as the near
infrared light, which the solar-cell absorber cannot absorb efficiently, is trapped in the back
of the solar cell due to the back texture. The optical loss in the back metal is about 3.65%.
The overall optical losses lead to a maximum absorptance in the c-Si of 81.21%.
1.0
Glass
Reflected
1.25%
6.23%
0.8
abs. glass
abs. EVA
ITO
abs. ITO
1.37%
0.6
abs. a-Si:H
abs. c-Si
c-Si
a-Si:H
0.4
0.2
reflected
abs. metal
81.21%
4.88%
EVA
1.40%
Metal
3.65%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-23: (a) The schematic structure of an encapsulated textured heterojunction silicon solar
cell with the single-layer AR coating applied. (b) The spectral reflectance and absorptance profile
of the simulated encapsulated textured heterojunction solar cell with an optimum ITO thickness.
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell
4-4-4
49
Double-layer AR coating on an encapsulated textured solar cell
The effect of the double-layer AR coating in an encapsulated textured cell is also investigated.
MgO and Al2 O3 are chosen among the available materials with similar refractive indices due
to their relatively simple deposition processes. The optimizations are carried out both for
combinations of ITO & MgO and ITO & Al2 O3 . Figure 4-24 shows the distribution map of
the total absorptance in the c-Si as a function of (a) ITO and Al2 O3 thicknesses and (b) ITO
and MgO thicknesses. As can be seen, both of two different combinations demonstrate similar
trends that the maximum absorptance in the c-Si is achieved at a very small ITO thickness
or without ITO. This can be understood that reducing the ITO thickness implies a reduction
in the absorptance in ITO, thus increasing the absorptance in the c-Si. However, a very small
ITO thickness is not applicable since the electrical properties of the ITO is dependent on
its thickness [50]. In this way, thereby, the ITO thickness is fixed to 30 nm, a reasonable
value of a thin TCO film which is assumed to still have sufficient conductance for the charge
transport.
81
100
80.5
80
80
60
79.5
40
79
20
78.5
20
40
60
80
100
Al2O3 thickness (nm)
(a)
120
140
ITO thickness (nm)
120
140
Absorptance in c−Si (%)
ITO thickness (nm)
81.5
82
120
81.5
100
81
80
80.5
60
80
40
79.5
79
20
Absorptance in c−Si (%)
82
140
78.5
20
40
60
80
100
MgO thickness (nm)
120
140
(b)
Figure 4-24: The absorptance in the c-Si as a function of (a) ITO and Al2 O3 thicknesses, and
(b) ITO and MgO thicknesses.
Figure 4-25 (a) schematically shows the structure of the encapsulated textured heterojunction silicon solar cell with the optimum ITO and MgO thicknesses that is used in the
simulation. The spectral reflectance and absorptance profile of the corresponding configuration is shown in Figure 4-25 (b). The total reflectance does not exhibit significant changes
compared to the single-layer AR coating. A small reduction of approximately 1% occurs in
the a-Si:H. The lower parasitic absorption losses in the ITO and a-Si:H layers implies a higher
total absorptance in the c-Si, which now equals to 82.15%.
The difference between the optical effects given by the single- and the double-layer AR
coatings on the simulated encapsulated textured cell is illustrated in Figure 4-26. The spectral
reflectance given by the double-layer AR coating does not significantly differ from that given
by the single-layer AR coating throughout the spectrum. Similar to the encapsulated flat
solar cell, this indicates that the applied double-layer AR coating does not contribute to an
optical improvement. One possible reason is due to the small index difference between ITO
and MgO, resulting in an ineffective anti-reflection.
Table 4-4 lists the absorptance in each layer of the simulated encapsulated textured
Master of Science Thesis
Ibadillah Ardhi Digdaya
Antireflective Coating on the Heterojunction Silicon Solar Cell
Glass
Spectral reflectance and absorptance (-)
50
3 mm
200 μm
EVA
MgO
100 nm
ITO
(p) a-Si:H
(i) a-Si:H
30 nm
(n) c-Si
(i) a-Si:H
(n) a-Si:H
Metal
1.0
Glass
reflected
Reflected
1.24%
abs. glass
6.10%
0.8
abs. EVA
ITO
abs. ITO
1.15%
0.6
abs. a-Si:H
abs. c-Si
c-Si
a-Si:H
0.4
0.2
abs. metal
82.15%
3.91%
EVA
1.42%
Metal
4.03%
0.0
400
600
800
1000
1200
Wavelength (nm)
(a)
(b)
Figure 4-25: (a) The schematic structure of an encapsulated textured heterojunction solar
cell with the ITO & MgO double-layer AR coating applied. (b) The spectral reflectance and
absorptance profile of the simulated encapsulated textured heterojunction silicon solar cell with
the optimum ITO & MgO thickness combination.
Spectral absorptance in c-Si (-)
1.0
Spectral reflectance (-)
Total reflectance (ITO) = 6.23%
Total reflectance (ITO & MgO) = 6.10%
0.8
0.6
0.4
0.2
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
1.0
0.8
0.6
0.4
0.2
Abs. in c-Si (ITO) = 81.21%
Abs. in c-Si (ITO & MgO) = 82.15%
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 4-26: (a) The spectral reflectance and (b) the spectral absorptance in the c-Si given by the
single-layer AR coating (ITO) and the double-layer AR coating (ITO & MgO) on an encapsulated
textured heterojunction silicon solar cell.
heterojunction silicon cell with different AR coating schemes. As shown, the ITO & MgO
double-layer AR coating can only contribute to a small increase of the absorptance in the
c-Si, from 81.21% by the single-layer AR coating to 82.15%.
Ibadillah Ardhi Digdaya
Master of Science Thesis
4-4 Optical Simulation of an Encapsulated Heterojunction Silicon Solar Cell
51
Table 4-4: Comparative results of the total reflectance and the total absorptance in each layer
with different AR coating schemes on unencapsulated textured heterojunction solar cell. The
thicknesses of AR coatings are optimum values.
AR coating
ITO
ITO & Al2 O3
ITO & MgO
Master of Science Thesis
Reflected
flux (%)
6.23
6.25
6.10
glass
1.25
1.25
1.24
Absorbed in (%)
EVA ITO a-Si:H
1.40 1.37
4.88
1.42 1.14
3.92
1.42 1.15
3.91
c-Si
81.21
82.00
82.15
Ibadillah Ardhi Digdaya
52
Ibadillah Ardhi Digdaya
Antireflective Coating on the Heterojunction Silicon Solar Cell
Master of Science Thesis
Chapter 5
Experimental Results
In this chapter, different AR coating schemes are implemented and tested through experiments. The accuracy of the optical simulation program is verified by the spectral reflectance
measurements of actual heterojunction silicon devices. In order to gain insight of the direct
impact of the optical performance on the electrical current, EQE measurement is carried out.
The optical performance of both flat and textured solar cells are experimentally characterized.
In addition, the solar-cell external parameters are served to calculate the overall efficiency of
the devices. In this work, only lab cells are investigated, therefore, optical influences of the
encapsulation in PV modules are not included.
During the initial stage of the solar-cell fabrication process, the silicon wafer is prepared
by cleaning and drying processes. After the wafer is preheated at 180 ◦ C, 5-nm thick intrinsic
a-Si:H and 5-nm thick n-type a-Si:H are deposited onto the back side of n-type c-Si wafer by
RF PECVD, followed by the deposition of 5-nm thick intrinsic a-Si:H and 5-nm thick p-type
a-Si:H onto the front side. The solar cell is finally finished by ITO and metal grids deposition
on the front side and metal deposition on the back side.
5-1
5-1-1
Optical Measurement of Flat Heterojunction Solar Cells
Single-layer AR coating on a flat device
The validation of the optical simulation is carried out by comparing the numerical results
from the simulation to the optical characterization of experimental devices. This involves
spectral reflectance measurements by PerkinElmer Lambda 950 UV/VIS spectrophotometer.
The spectral transmittance is neglected since no light can pass through the back contact.
In this section, only applications of the ITO based single-layer AR coating are considered. A set of heterojunction solar cells are prepared with different ITO thicknesses of 53,
71 and 94 nm. In order to understand the optical effect caused by AR coatings, the thicknesses of semiconductor layers are kept constant. The measured reflectance spectra of the
corresponding devices are shown in Figure 5-1 (a).
Measurement results are indicated by symbols (, ◦ and .) and the simulation results
are indicated by solid lines. As illustrated, the simulation can fit the spectral reflectance
measurement of each device quite precisely. This confirms the accuracy of optical model and
the optical constants used as the input parameters. It is noticed that the zero reflectance
Master of Science Thesis
Ibadillah Ardhi Digdaya
54
Experimental Results
1.0
1.0
meas. tot. reflectance = 24.86%
0.8
meas. tot. reflectance = 21.13%
sim. ITO = 53 nm
sim. ITO = 71 nm
0.6
EQE (-)
Spectral reflectance (-)
meas. tot. reflectance = 21.03%
0.8
sim. ITO = 94 nm
0.4
0.6
0.4
53 nm ITO. J
= 31.5 mA/cm
71 nm ITO. J
= 34.3 mA/cm
94 nm ITO. J
= 34.2 mA/cm
SC
0.2
0.2
SC
SC
2
2
2
0.0
0.0
400
600
800
1000
1200
400
600
800
1000
1200
Wavelength (nm)
Wavelength (nm)
(a)
(b)
Figure 5-1: Measured (symbols) and simulated (solid line) reflectance spectra of heterojunction
solar cells with various ITO thickness. (b) EQE spectra.
shifts to the right with increasing ITO thickness. This is mainly because, for the single-layer
AR coating, a certain ITO thickness is responsible for zero or minimum reflectance at specific
wavelength, as expressed by Eq. (2-20). As demonstrated in Figure 4-5, the lowest total
reflectance can be achieved when the ITO thickness is close to 80 nm. This is in agreement
with the solar cell with ITO thickness of 71 nm that reaches the lowest total reflectance among
other investigated devices.
The measured EQE spectra of the corresponding devices are shown in Figure 5-1 (b).
For the device with 53 nm thick ITO, the EQE peak is quite low, although the reflectance
reaches nearly zero at 450 nm. This is due to the strong parasitic absorption of a-Si:H in
visible part of the spectrum, resulting in a low EQE. However, the device with ITO thickness
of 94 nm shows a EQE of almost unity at approximately 725 nm. The reason is that the light
is hardly absorbed by a-Si:H at this wavelength due to its band gap limitation. As expected,
the highest JSC between devices with three different ITO thicknesses is achieved by the device
with ITO thickness of 71 nm. This agrees with the trend given by Figure 4-5, so that a lower
total reflectance implies a higher absorptance in the c-Si, hence higher JSC .
The summary of the optical performances of the three devices with different ITO thicknesses associated with the JSC is given by Table 5-1
Table 5-1: The total reflectance and JSC of the three devices with different ITO thicknesses.
ITO thickness
(nm)
53
71
94
Ibadillah Ardhi Digdaya
Total reflectance
(%)
24.86
21.03
21.13
JSC
(mA/cm2 )
31.5
34.3
34.2
Master of Science Thesis
5-1 Optical Measurement of Flat Heterojunction Solar Cells
5-1-2
55
Double-layer AR coating on a flat device
Section 4-2-2 has summed up that the double-layer AR coating can lead to lower reflectance
in a wider spectral range. The experiments, therefore, are carried out in order to verify
the beneficial effect of the double-layer AR coating on a flat device over the single-layer AR
coating.
Two different double-layer AR coating scenarios are deposited onto separate heterojunction silicon solar cells. Figure 5-2 (a) shows the measured ( and ◦ symbols) and the
simulated (solid lines) reflectance spectra.
1.0
1.0
meas. tot. reflectance = 17.95%
sim. ITO = 53 nm & SiO
sim. ITO = 57 nm & SiO
2
2
0.8
= 71 nm
= 85 nm
0.6
EQE (-)
Spectral reflectance (-)
meas. tot. reflectance = 18.47%
0.8
0.4
0.6
0.4
53 nm ITO & 71 nm SiO
J
sc
0.2
J
sc
= 35.5 mA/cm
2
2
57 nm ITO & 85 nm SiO
0.2
0.0
= 35.7 mA/cm
2
2
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 5-2: (a) Measured (symbols) and simulated (solid line) reflectance spectra of heterojunction solar cells with two different double-layer AR coating thickness combinations. (b) EQE
spectra.
The validity of optical simulations is confirmed by the fitting between the measured
(symbols and ◦) and the simulated (solid lines) reflectance spectra of the two solar cells.
It is indicated that 53 nm thick ITO in combination with 71 nm thick SiO2 achieves a lower
reflectance. This also confirms the reliability of the distribution map of the total reflectance
as a function ITO & SiO2 thickness combination as given by Figure 4-7 (a).
Figure 5-2 (b) shows the EQE of two devices with different ITO & SiO2 thickness
combinations. Similar to the reflectance, the EQE shifts to the right with increasing ITO and
SiO2 thicknesses. The thinner ITO & SiO2 thickness (green line) leads to a higher EQE than
the thicker one (purple line). The reason is that the EQE is comparable to the absorptance
in the c-Si, from which most of photo-generated carriers which can contribute to the current
come. Recalling Figure 4-7 (b), it is noticed that the absorptance in the c-Si is higher for the
device with ITO & SiO2 thickness combination of 53 nm and 71 nm than the device with the
other combination.
When compared to the other combination, the devices with ITO & SiO2 thickness
combination of 53 nm and 71 nm gains EQE in the wavelength range of 450 nm to 650 nm,
which is the region with the higher photon flux density, but loses some in the wavelength
ranges of 350 nm to 450 nm and 850 nm to 950 nm. The lower EQE at short wavelengths
is due to the higher reflectance and lower absorptance in the c-Si at 400 nm. Meanwhile, for
Master of Science Thesis
Ibadillah Ardhi Digdaya
56
Experimental Results
the ITO & SiO2 thickness combination of 57 nm and 85 nm, the higher reflectance occurs in
the wavelength range of 500 nm to 700 nm, resulting in a lower EQE. In short, the JSC of
35.7 mA/cm2 can be achieved by our device with 53-nm thick ITO and 71-nm thick SiO2 .
The comparison between the two thickness combinations can be seen in Table 5-2.
Table 5-2: The total reflectance and JSC of the two devices with different ITO & SiO2 thickness
combinations.
ITO thickness
(nm)
53
57
SiO2 thickness
(nm)
71
85
JSC
(mA/cm2 )
35.7
35.5
Total reflectance
(%)
17.95
18.47
Single- vs. double-layer AR coating on a flat device
In the previous section, the optical performances of the heterojunction silicon solar cells with a
single-layer and a double-layer AR coating have been shown separately. Now, two investigated
cells with the optimum single- and double-layer AR coatings are compared as illustrated in
Figure 5-3 regarding their (a) reflectance and (b) EQE spectra.
1.0
1.0
meas. tot. reflectance = 21.03 %
0.8
sim. ITO = 71 nm
sim. ITO = 53 nm & SiO
2
=71 nm
0.6
EQE (-)
Spectral reflectance (-)
meas. tot. reflectance = 17.95%
0.8
0.4
0.6
0.4
71 nm ITO
J
SC
0.2
0.2
= 34.3 mA/cm
2
53 nm ITO & 71 nm SiO
J
SC
= 35.7 mA/cm
2
2
0.0
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 5-3: (a) Spectral reflectance of heterojunction silicon devices with single-layer AR coating
(measured: blue ◦ symbol, simulated: blue line) and with double-layer AR coating (measured:
green symbol, simulated: green line). (b) The EQE spectra of heterojunction silicon devices
with single-layer AR coating (blue line) and with double-layer AR coating (green line).
As shown, the application of the double-layer AR coating can reduce the reflectance
in the wavelength range of 300 nm to 450 nm as well as 700 nm to 1000 nm. However the
reflectance in the wavelength range of 500 nm to 600 nm is increased. Overall, the total
reflectance is reduced from ∼21 % to ∼18%. The lower reflectance obtained by applying a
double-layer AR coating results in a JSC increase of 1.6 mA/cm2 , as indicated by the EQE
measurement (Figure 5-3 (b)). The wavelengths at which the EQE response increases is in
line with those at which the reflectance is reduced.
Ibadillah Ardhi Digdaya
Master of Science Thesis
5-1 Optical Measurement of Flat Heterojunction Solar Cells
57
In Figure 5-4, the J-V characteristics of these two heterojunction silicon solar cells with
an optimum single-layer and double-layer AR coatings are shown. Due to the JSC increase
of the flat solar cell with a double-layer AR coating, the efficiency is increased from 17.1% to
17.6%.
2
Current density (mA/cm )
40
30
ITO
ITO & SiO
2
20
ITO
VOC (mV)
2
JSC (mA/cm )
10
FF (%)
Efficiency
0
0
200
ITO & SiO2
668
662
34.1
35.7
74.6
74
17.1%
17.6%
400
600
Voltage (mV)
Figure 5-4: The J-V characteristics of the flat heterojunction silicon solar cells with single-layer
and double-layer AR coatings.
Absorptance in the c-Si vs. EQE
In Section 4-2-1, the thickness of ITO is designed to obtain a maximum absorptance in the
c-Si. Since the EQE is comparable to the absorptance in the c-Si, therefore, it is worth to
evaluate their exact relationship. Figure 5-5 (a) demonstrates the comparison of the simulated
spectral absorptance with the measured spectral EQE (red ◦ symbol) of the device with the
single-layer AR coating. Here the spectral reflectance is plotted oppositely and is indicated
as 1-reflectance. The measured and simulated 1-reflectance spectra are given to confirm the
validity of the optical simulation with the actual measurement.
As shown, the plots (red ◦ symbol and red line) give an indication that the absorptance
in the c-Si is equal with the EQE in the wavelength range of 600 nm to 1200 nm. However,
a deviation occurs in the wavelength range of 300 nm to 600 nm and in this range the EQE
surpasses the absorptance in the c-Si. This observation indicates that the collected current
generated by the short-wavelength light is slightly higher than the c-Si can produce. This
leads to the hypothesis that the absorption in the intrinsic a-Si:H is not entirely parasitic.
Similarly for the device with the double-layer AR coating in Figure 5-5 (b), the accurate
fitting between the simulated spectral absorptance in the c-Si and the EQE spectra can only
be achieved in the wavelength range of 500 nm to 1200 nm. The small deviation, however,
occurs in a more narrow spectral range of 300 nm to 500 nm and the EQE is slightly higher
than the absorptance in the c-Si. The change in the deviation between the EQE and the
absorptance in the c-Si is due to the fact that in the wavelength range of 500 nm to 600 nm,
more light is reflected by the double-layer AR coating rather than absorbed by the a-Si:H
layers.
Master of Science Thesis
Ibadillah Ardhi Digdaya
58
Experimental Results
1.0
EQE, absorptance spectra (-)
EQE, absorptance spectra (-)
1.0
0.8
0.6
sim. 1-reflectance
sim. abs. (p) a-Si:H
0.4
sim. abs. (i) a-Si:H
sim. abs. (n) c-Si
meas. 1-reflectance
0.2
meas. EQE
0.0
400
600
800
Wavelength (nm)
1000
1200
0.8
0.6
sim. 1-reflectance
sim. abs. (p) a-Si:H
0.4
sim. abs. (i) a-Si:H
sim. abs. (n) c-Si
meas. 1-reflectance
0.2
meas. EQE
0.0
400
600
800
1000
1200
W avelength (nm)
(a)
(b)
Figure 5-5: The simulated 1-reflectance (blue line) and absorptance spectra in (p) a-Si:H (light
green), in (i) a-Si:H (yellow), in (n) c-Si (red line). The measured 1-reflectance (blue symbol)
and the measured EQE spectra (red ◦ symbol).
A similar phenomenon has been reported by Holman et al. [51]. They stated that the
EQE can be written as the sum of contributions from each layer in the solar cell. In their
case, the extra contribution of the current is only provided by the intrinsic a-Si:H layer.
EQE(λ) = Ac-Si (λ) + A(i) a-Si:H (λ)β(i) a-Si:H
(5-1)
where EQE(λ) is the EQE spectrm, the Ac-Si (λ) and A(i) a-Si:H (λ) are the fractions of incident
light with a wavelength of λ absorbed in the c-Si and in the intrinsic a-Si:H layer respectively,
and the β(i) a-Si:H is the probability that light absorbed in intrinsic layer will result in charge
collection. Their study is conducted by comparing the simulation results with the EQE
spectra of individual layers (p-type and intrinsic a-Si:H) with various thicknesses deposited
on the c-Si wafer.
However, especially in our case, there is a small contribution by the p-type a-Si:H in the
UV part of the spectrum. Therefore, for our flat heterojunction silicon solar cell, the fraction
of light absorbed in the p-type a-Si:H (A(p) a-Si:H ) and the probability of light absorbed in the
p-type a-Si:H that can contribute to the current (β(p) a-Si:H ) are introduced into Eq. (5-1)
EQE(λ) = Ac-Si (λ) + A(i) a-Si:H (λ)β(i) a-Si:H + A(p) a-Si:H (λ)β(p) a-Si:H
5-2
(5-2)
Optical Measurement of Textured Heterojunction Solar Cells
Texturing the wafer surface has been proven to be an effective way to reduce the surface
reflection and provide a light trapping, hence increasing the JSC . In this work, the effect of
AR coatings on textured heterojunction silicon solar cells is experimentally investigated. The
texture is produced by the alkaline etching of c-Si wafers with (100) orientation. The etch
exposes the <111> facets, resulting in pyramidal features with a well-defined steepness of
54.7◦ .
Ibadillah Ardhi Digdaya
Master of Science Thesis
5-2 Optical Measurement of Textured Heterojunction Solar Cells
5-2-1
59
Single- and double-layer AR coating on a textured device
In this work, the investigated textured heterojunction solar cells are restricted to one with 41
nm thick ITO for the single-layer AR coating and one with 41 nm thick ITO in combination
with 65 nm thick SiO2 for the double-layer AR coating. Figure 5-6 (a) depicts the spectral
reflectance given by the two investigated textured heterojunction silicon solar cells. As shown,
the simulations (blue line for the single-layer AR coating and green line for the double-layer
AR coating) can fit the measured reflectance spectra of textured cells with a single-layer
AR coating (blue ◦ symbol) and with a double-layer AR coating (green symbol) almost at
the entire spectrum although a considerable deviation of 4% occurs at wavelength 300 nm.
This is because the model uses the interpretation of periodic pyramid features with definite
dimension and steepness. However in the real case, the pyramids are randomly distributed
over the surface with different sizes [52].
1.0
1.0
meas. tot. reflectance = 3.86%
0.8
0.8
sim. ITO = 41 nm
sim. ITO = 41 nm & SiO
2
= 65 nm
0.6
EQE (-)
Spectral reflectance (-)
meas. tot. reflectance = 7.02%
0.4
0.6
0.4
41 nm ITO
J
SC
0.2
0.2
= 38.1 mA/cm
2
41 nm ITO & 65 nm SiO
2
J
SC
= 40.5 mA/cm
2
0.0
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 5-6: (a) The reflectance spectra of devices with the single-layer AR coating (measured:
blue ◦ symbol, simulated: blue line) and with the double-layer AR coating (measured: green symbol, simulated: green line). (b) The EQE spectra of devices with the single-layer AR coating
(blue line) and with the double-layer AR coating (green line).
As indicated, the reflectance is much reduced at a broad wavelength range by the singlelayer AR coating, compared to the textured cell without any AR coating (cf. Figure 4-10
(b)). This ITO thickness leads zero reflectance at 380 nm. This yields the total reflectance
of 7.02%. On the other hand, the addition of SiO2 layer can greatly suppress the reflectance
in the wavelength range of 450 nm to 1050 nm. However, the reflectance at wavelength of
about 400 nm is slightly increased. Overall, the additional SiO2 on the ITO leads to further
reduction of the total reflectance from 7.02% by the single-layer AR coating to 3.86% by the
double-layer AR coating.
The comparison between the EQE of two devices with the single- and the double-layer
AR coating is also given in Figure 5-6 (b). By applying a double-layer AR coating, the EQE
is increased of about 8% in the wavelength range of 450 nm to 1050 nm. The wavelengths at
which the EQE response increases are in line with those at which the reflectance is reduced.
Overall, this leads to a JSC increase from 38.1 mA/cm2 with a single-layer AR coating to
40.5 mA/cm2 with a double-layer coating.
Master of Science Thesis
Ibadillah Ardhi Digdaya
60
Experimental Results
2
Current density (mA/cm )
In Figure 5-7, the J-V characteristics of the two textured heterojunction silicon solar
cells with the single-layer and the double-layer AR coatings are shown. Due to the JSC
increase with the double-layer AR coating, the efficiency of the textured solar cell is increased
from 18% to 19%.
40
ITO
30
ITO & SiO
2
ITO
20
VOC (mV)
638
JSC (mA/cm )
38.1
40.5
FF (%)
73.9
73.7
Efficiency
18%
19%
2
10
0
ITO & SiO2
638
0
200
400
600
Voltage (mV)
Figure 5-7: The J-V characteristics of the textured heterojunction silicon solar cells with singlelayer and double-layer AR coatings.
Absorptance in the c-Si vs. EQE
In section 5-1-2, we have discussed how a fraction of absorbed light in the a-Si:H in a flat
device can contribute to the current. Now, it is also worth to examine the correlation between
the absorptance in the c-Si and the EQE in textured devices.
Figure 5-8 illustrate the comparison between absorptance in the c-Si and the EQE
spectra of textured heterojunction solar cells with (a) the single-layer and (b) the double-layer
AR coating. For the sake of the validity of the simulation, the measured reflectance spectra
are given and fitted by our simulation. As shown in Figure 5-8, a deviation apparently occurs
in the wavelength range of 950 nm to 1050 nm and absorptance in the c-Si is higher than the
corresponding EQE. The reason is the bad passivation of the c-Si of our textured devices. The
light with long wavelengths are weakly absorbed and a larger amount of electron-hole pairs is
generated in the back of the c-Si which is far away from the depletion region. Subsequently,
minority carriers need to diffuse a longer path to the junction. The bad passivation of the
c-Si causes recombination of minority carriers, which is not considered by the optical model.
Similar to the flat devices, EQE spectra surpass the absorptance in the c-Si both for the
two textured devices in the wavelength range of 300 nm to 500 nm. The apparent contribution
of p-type a-Si:H is most probably due to imperfect fittings from 300 nm to 400 nm, which
can be seen from the measured and the simulated reflectance spectra.
Ibadillah Ardhi Digdaya
Master of Science Thesis
5-2 Optical Measurement of Textured Heterojunction Solar Cells
1.0
EQE, absorptance spectra (-)
EQE, absorptance spectra (-)
1.0
61
0.8
0.6
sim. 1-reflectance
sim. abs. (p) a-Si:H
sim. abs. (i) a-Si:H
0.4
sim. abs. (n) c-Si
meas. 1-reflectance
meas. EQE
0.2
0.0
400
600
800
Wavelength (nm)
(a)
1000
1200
0.8
0.6
sim. 1-reflectance
sim. abs. (p) a-Si:H
sim. abs. (i) a-Si:H
0.4
sim. abs. (n) c-Si
meas. 1-reflectance
meas. EQE
0.2
0.0
400
600
800
1000
1200
Wavelength (nm)
(b)
Figure 5-8: The textured heterojunction solar cell with: (a) a single-layer AR coating, (b) a
double-layer AR coating. The simulated 1-reflectance (blue line) and absorptance spectra in (p)
a-Si:H (light green), in (i) a-Si:H (yellow), in (n) c-Si (red line). The measured 1-reflectance
(blue symbol) and the measured EQE spectra (red ◦ symbol).
Master of Science Thesis
Ibadillah Ardhi Digdaya
62
Ibadillah Ardhi Digdaya
Experimental Results
Master of Science Thesis
Chapter 6
Conclusion and Recommendation
6-1
Conclusion
In this work, optical simulations for flat and textured heterojunction silicon solar cells are
presented. The ASA program has the ability to simulate the spectral reflectance and absorptance in each layer of the solar cell. From optical simulation on a flat surface, the optimum
double-layer AR coating exhibits a further reduction of the total reflectance and an increase
of the total absorptance in the c-Si compared to the optimum single-layer AR coating. The
optimization of coating thickness based on the total reflectance does not guarantee maximum
light absorptance in the c-Si, where absorbed light can contribute to the current almost completely. Therefore, the optimization of coating thickness(es) to obtain maximum absorptance
in the c-Si is more straightforward. For the double-layer AR coating, the distribution map of
the total absorptance in the c-Si as a function of the two coating layer thicknesses can indicate
the optimum combination of coating thicknesses with the appropriate tolerance range.
Particularly for textured heterojunction silicon solar cells, optical simulations are done
with the GENPRO4 model. The addition of SiO2 to form a double-layer AR coating on a
textured device can effectively minimize the reflectance even further to very low values in a
wide spectral range. This results in a higher total absorptance in the c-Si.
The optical simulations for encapsulated heterojunction silicon solar cells are presented
both for flat and textured cells. The application of the double-layer AR coating in solar cells
does not show significant improvements in the absorptance in the c-Si both for encapsulated
flat and textured heterojunction silicon solar cells. The reason is that most of reflection losses
are caused by the glass. In addition, the small refractive-index difference of the two coating
materials leads to an inffective inteterference effect.
Accurate fittings are achieved between the reflectance spectra from optical simulation
results and the reflectance spectra from measurements of real devices, confirming the validity
of the optical simulations. The investigated heterojunction solar cell on the flat surface shows
an increase of JSC from 34.3 mA/cm2 with the single-layer AR coating to 35.7 mA/cm2
with the double-layer AR coating. The JSC increase leads to the increase of the solar-cell
efficiency from 17.1% to 17.6%. For textured devices, the JSC increases from 38.1 mA/cm2
to 40.5 mA/cm2 . This JSC increase leads to the increse of the solar-cell efficiency from 18%
to 19%.
Master of Science Thesis
Ibadillah Ardhi Digdaya
64
Conclusion and Recommendation
In heterojunction silicon solar cell, the light that passes through the ITO and the a-Si:H
layers is absorbed by the c-Si and generates electron-hole pairs with an efficiency of about
100%. This means that the absorptance in the c-Si is comparable to the EQE. From the
comparison between the simulated spectral absororptance and the EQE spectra, it is shown
that that the absorption in the intrinsic a-Si:H is not entirely parasitic. The probability of the
absorption in the intrinsic a-Si:H which contribute to the current increases with decreasing
wavelength.
6-2
Recommendation
The optimization of coating thicknesses for obtaining maximum total absorptance in the c-Si
has been done. However, in practice, the thickness of deposited coating materials is hard
to control due to the inhomogeneity and morphology dependent deposition rate. Therefore
additional investigations to achieve accurate deposition rates and homogeneous coating layers
are required.
The applications of double-layer AR coatings in encapsulated heterojunction solar cell
with both flat and textured surfaces do not show an obvious increase of the absorptance in the
c-Si, comparing to the single-layer AR coating. Therefore, more suitable AR coating materials
can be pursued further in order to evaluate the benefit of the double-layer AR coating in an
encapsulated heterojunction solar cell. One suggestion is to develop a TCO material with
higher refractive index (n > 2). In this way, a greater refractive-index difference between the
TCO and the additional coating material can be obtained, hence increasing the effectiveness
of the double-layer AR coating.
For AR coatings on a textured surface, only one device with a single-layer AR coating
and one device with a double-layer AR coating are experimentally fabricated. From the
thickness optimization of the double-layer AR coating on a textured heterojunction silicon
devices, it is noticed that the optimum AR coating layer thicknesses for minimum total
reflectance is different from the AR coating thicknesses for maximum total absorptance in
the c-Si. For future evaluation, it is worth to deposit the optimum AR coating thicknesses
for both purposes.
A deep investigation into the absorption in the intrinsic a-Si:H layer should be carried
out. The verified contribution of the absorbed light in the intrinsic a-Si:H to the current
might lead to the change of optimization parameters.
Ibadillah Ardhi Digdaya
Master of Science Thesis
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Master of Science Thesis
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