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Anti-Reflection coatings for highly efficient solar cells

Authors:
  • AcSIR-Central Scientific Instruments Organization

Abstract

Efficiency of solar cell is a big issue in the present time. Anti-Reflection Coating plays very important role in improving the efficiency of solar cell. Anti-Reflection coating is typically specified by either the maximum allowable reflectance at a single wavelength or by the average allowable reflectance over a specified wavelength range. Anti-Reflection coatings intended for a single wavelength or a single angle of incidence and very high performance can be obtained less than 0.1% reflectance per surface at visible wavelengths on glass substrates. A single layer of thin film of thickness around 100nm of Silicon Dioxide (SiO2) and Titanium Dioxide (TiO2), increases solar cell efficiencies by 3-4% and a triple-layer coating can improve its efficiency by 39-40%. The reflectance of solar cell can be reducing up to 3.2% by using Anti-reflection coating. So, multilayer coatings of SiO2 and TiO2 can be used for highly conversion of solar spectrum into electrical energy.
Anti-Reflection coatings for highly efficient solar cells
Vikas*, Surya Narain Diksit
Department of Applied Physics
Guru Jambheshwar University of Science & Technology,
Hisar 125001, Haryana, India
* vikas5293@gmail.com
ABSTRACT
Efficiency of solar cell is a big issue in the present time. Anti-Reflection Coating plays very important role in improving the efficiency of solar cell. Anti-Reflection coating is typically specified by either
the maximum allowable reflectance at a single wavelength or by the average allowable reflectance over a specified wavelength range. Anti-Reflection coatings intended for a single wavelength or a
single angle of incidence and very high performance can be obtained less than 0.1% reflectance per surface at visible wavelengths on glass substrates. A single layer of thin film of thickness around
100nm of Silicon Dioxide (SiO2) and Titanium Dioxide (TiO2), increases solar cell efficiencies by 3-4% and a triple-layer coating can improve its efficiency by 39-40%. The reflectance of solar cell can
be reducing up to 3.2% by using Anti-reflection coating. So, multilayer coatings of SiO2 and TiO2 can be used for highly conversion of solar spectrum into electrical energy.
Introduction
One of the main barriers of solar cell efficiency is reflection of light from
front of its surface. Because a bare silicon surface reflects the light about
30% and GaAs reflect the light about 32.58% light from their front
surface.
The Intensity of light transmitted and reflected between two medium is
depend upon change in refractive index of mediums and angle of incident
of light at interference . The intensity of reflected light is given by fresenl
equation of reflection is
IR =

Fresnel reflection at the front surface of the solar cell can be reduce by
using another optical phenomenon known as “Interference of light in thin
film.
Fig :- fresnel reflection
IR
Design of Anti-Reflection Coating
Antireflection coatings (ARCs) are applied to PV cells to ensure a gradual increase of the
refractive index as incident photons traverse from air through to the absorber so as to reduce the
backward reflection losses.
We deposit a slab of material of thickness d and refractive index nf and expect that R3 and R5
interferes destructively and transfer energy inside slab.
d = /4nf IR = [ 
 nf =   
Fig:- basic principle of ARC Fig:- Physical arrangement of Photovoltaic solar cells with ARC
Discussion of Deposition Technique
The SiO2 has good passivation and scratch resistant properties and chemically stable at elevated temperatures . Another material is TiO2 which has suitable refractive index and a
low absorption throughout the visible region. In addition TiO2 is known for its chemical stability mechanical hardness, less moisture absorption, and comparatively smooth
fabrication process .
Sol-gel spin-coating method applied extensively in preparing lots of different kinds of coated films has become a popular technique for the past thirty years due to its high process
speed, low-cost, continuous production, and suitability for the large-area process.
The sol-gel process is a method for producing solid materials from small molecules. This method is used for the fabrication of metal oxides, especially the oxides of Silicon and
Titanium.
It provide uniformly distribution of anti reflection material on substrate than plasma enhanced chemical vapor deposition. Due to centrifugal force appears on four corners of
substrate non-uniformity appear. It can control by adjusted spin rate of spin coater and concentration of sol-gel.
Sol-Gel method provide high efficient ARCs on large area multicrystalline wafers.
The SiO2-TiO2 colloid solutions contained varying volume ratios of SiO2 and TiO2 solutions.
The SiO2 solution can be prepared by reacting metal alkoxide with a mixture of a critical amount of water and hydrochloric acid (HCl) catalyst in a medium diluted with ethyl
alcohol solvent. Tetraethyl orthosilicate (Si(OC2H5)4) was then added into the resultant solutions and stirred with a magnetic stirrer.
Similarly TiO2 solution can be prepared except that titanium isopropoxide (Ti(OC3H9)4) was added instead of (Si(OC2H5)4).The SiO2 and TiO2 were then mixed separately in
different volume ratios. Finally, a small amount of DMF was added into the SiO2-TiO2 synthetic colloid solution.
Various parameter of solar cells with coating and without coating is given in table where Jsc gives short circuit current density, Voc gives open circuit voltage and efficency of solar
cell.
AR Coating Jsc (mA/cm2) Voc(V) η (%)
No ARC 21.60 .5717 8.92
TiO2 25.80 0.5766 10.78
ZnO 28.31 0.5792 11.77
ZnO/TiO2 31.91 0.5824 13.37
SiO2/TiO2 32.51 0.5829 13.59
Table:- Parameters of Solar cells with different Coating Fig:- Refractive index spectrum for pure TiO2 and SiO2-TiO2 composite films deposited
Results and Discussion
Application of thin films of SiO2 and TiO2 using the sol-gel process were utilized
as antireflection coating on monocrystalline silicon wafers.
For deposition of Anti-Reflection Coating on substrate Sol-Gel technique provide
uniform distribution of material. Refractive index of composition decreases from
2.1 to 1.45 as ratio of SiO2 to TiO2 (SiO2/TiO2) decreases as shown in figure at a
wavelength of 630 nm.
It is expected that sol-gel-ARC device should absorb more incident light resulting in
higher current density. It improve the quality of sol-gel ARCs that could help
increase the efficiency of solar cells.
It could enables a more efficient and a continuous mass production of Si solar cells
compared to the use of PECVD method, which requires an expensive vacuum
process and dangerous process gases. SiO2-TiO2 (sol-gel)
Fig:- Sol-Gel anti-reflection coated device
References
[1] M. Moradi, Z. Rajabi Efficiency Enhancement of Si Solar Cells by Using Nanostructured Single and Double Layer Anti-Reflective Coatings JNS 3 (2013) 365-369.
[2] Khuram Ali*, Sohail A. Khan, M. Z. Mat Jafri Effect of Double Layer (SiO2/TiO2) Anti-reflective Coating on Silicon Solar Cells Int. J. Electrochem Sci., 9 (2014) 7865 - 7874 .
[3] Chih-Hsiang Yang, Shui-Yang Lien, Chia-Ho Chu, Chung-Yuan Kung, Tieh-Fei Cheng, Pai-Tsun Chen Effectively Improved SiO2-TiO2 Composite Film Applied in Commercial
Multicrystalline Silicon Solar Cells International Journal of Photoenergy Volume (2013).
[4] S.-M. Jung, Y.-H. Kim, S.-I. Kim, and S.-I. Yoo, Design and fabrication of multi-layer antireflection coating for III-V solar cell, Current Applied Physics, vol. 11, no. 3, pp.,538541, 2011.
[5] Yi-Yu Lee, Wen-Jeng Ho, Jhih-Kai Syu, Quan-Ru Lai, and Cheng-Ming Yu 17.9% Efficiency Silicon Solar Cells by Using Spin-on Films Processes PIERS Proceedings, Suzhou, China, September
12-16, (2011)
... In this aspect electrical and optical properties of Solar cell have been introduced by some cost effective technique. There are different types of antireflection coating in the commercial production scale to reduce the reflectance [2,3]. Although the coating shows good optical property but the budget is high due to sophisticated fabrication unit. ...
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Rajabi Efficiency Enhancement of Si Solar Cells by Using Nanostructured Single and Double Layer Anti-Reflective
  • M Moradi
M. Moradi, Z. Rajabi Efficiency Enhancement of Si Solar Cells by Using Nanostructured Single and Double Layer Anti-Reflective Coatings JNS 3 (2013) 365-369.