Solar Cells Thin Film Technologies Part 1 docx

Kosyachenko Chapter 2 Enhanced Diffuse Reflection of Light by Using a Periodically Textured Stainless Steel Substrate 39 Shuo-Jen Lee and Wen-Cheng Ke Chapter 3 Low Cost Solar Cells Ba

SOLAR CELLS – THIN-FILM TECHNOLOGIES

Edited by Leonid A Kosyachenko

Solar Cells – Thin-Film Technologies

Edited by Leonid A Kosyachenko

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Notice

Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher No responsibility is accepted for the accuracy of information contained in the published chapters The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book

Publishing Process Manager Sandra Bakic

Technical Editor Teodora Smiljanic

Cover Designer Jan Hyrat

Image Copyright inacio pires, 2011 Used under license from Shutterstock.com

First published October, 2011

Printed in Croatia

A free online edition of this book is available at www.intechopen.com

Additional hard copies can be obtained from orders@intechweb.org

Solar Cells – Thin-Film Technologies, Edited by Leonid A Kosyachenko

p cm

ISBN 978-953-307-570-9

free online editions of InTech

Books and Journals can be found at

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Contents

Preface IX

Chapter 1 Thin-Film Photovoltaics

as a Mainstream of Solar Power Engineering 1

Leonid A Kosyachenko Chapter 2 Enhanced Diffuse Reflection of Light by Using

a Periodically Textured Stainless Steel Substrate 39

Shuo-Jen Lee and Wen-Cheng Ke Chapter 3 Low Cost Solar Cells Based on Cuprous Oxide 55

Verka Georgieva, Atanas Tanusevski and Marina Georgieva Chapter 4 Application of Electron Beam Treatment

in Polycrystalline Silicon Films Manufacture for Solar Cell 77

L Fu Chapter 5 Electrodeposited Cu 2 O Thin Films

for Fabrication of CuO/Cu 2 O Heterojunction 89

Ruwan Palitha Wijesundera Chapter 6 TCO-Si Based Heterojunction Photovoltaic Devices 111

Z.Q Maand B He Chapter 7 Crystalline Silicon Thin Film Solar Cells 137

Fritz Falk and Gudrun Andrä Chapter 8 Architectural Design Criteria for Spacecraft Solar Arrays 161

Antonio De Luca Chapter 9 Power Output Characteristics

of Transparent a-Si BiPV Window Module 187

Jongho Yoon Chapter 10 Influence of Post-Deposition

Thermal Treatment on the Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 209

Nicola Armani, Samantha Mazzamuto and Lidice Vaillant-Roca

VI Contents

Chapter 11 Chemical Bath Deposited CdS for CdTe

and Cu(In,Ga)Se 2 Thin Film Solar Cells Processing 237

M Estela Calixto, M L Albor-Aguilera, M Tufiño-Velázquez,

G Contreras-Puente and A Morales-Acevedo Chapter 12 Innovative Elastic Thin-Film Solar Cell Structures 253

Maciej Sibiński and Katarzyna Znajdek Chapter 13 Computer Modeling of Heterojunction

with Intrinsic Thin Layer “HIT” Solar Cells:

Sensitivity Issues and Insights Gained 275

Antara Datta and Parsathi Chatterjee Chapter 14 Fabrication of the Hydrogenated Amorphous Silicon

Films Exhibiting High Stability Against Light Soaking 303

Satoshi Shimizu, Michio Kondo and Akihisa Matsuda Chapter 15 Analysis of CZTSSe Monograin Layer Solar Cells 319

Gregor Černivec, Andri Jagomägi and Koen Decock Chapter 16 Large Area a-Si/μc-Si Thin Film Solar Cells 335

Fan Yang Chapter 17 Novel Deposition Technique for Fast

Growth of Hydrogenated Microcrystalline Silicon Thin-Film for Thin-Film Silicon Solar Cells 359

Jhantu Kumar Saha and Hajime Shirai Chapter 18 Chemical Surface Deposition of CdS

Ultra Thin Films from Aqueous Solutions 381

H Il’chuk, P Shapoval and V Kusnezh Chapter 19 Development of Flexible Cu(In,Ga)Se 2

Thin Film Solar Cell by Lift-Off Process 405

Yasuhiro Abe, Takashi Minemoto and Hideyuki Takakura Chapter 20 What is Happening

with Regards to Thin-Film Photovoltaics? 421

Bolko von Roedern Chapter 21 Spectral Effects on CIS Modules

While Deployed Outdoors 441

Michael Simon and Edson L Meyer

Preface

Solar cells are optoelectronic devices that convert the energy of solar radiation directly into electricity by the photovoltaic (PV) effect Assemblies of cells electrically connected together are known as PV modules, or solar panels The photovoltaic effect was first recognized in the 19th century but the modern PV cells were developed in the mid-1950s The practical application of photovoltaics started to provide energy for orbiting satellites Today PV installations may be ground-mounted or built into the roof or walls of buildings, and are used for electric power in boats, cars, water pumps, radio stations, and more The majority of PV modules are used for grid connected power generation More than 100 countries use photovoltaics Solar power is pollution-free during use Due to the growing demand for renewable energy sources, the manufacturing of solar cells and PV arrays has advanced considerably in recent years

Solar cells and modules based on crystalline and polycrystalline silicon wafers, the representatives of the so-called first generation of solar cells, dominate the photovoltaic today and demonstrate high growth rates in the entire energy sector Nevertheless, despite the relatively high annual growth, the contribution of photovoltaics in the global energy system is small The reason for this lies in a large consumption of materials and energy, high labor intensiveness and, as a consequence,

a low productivity and high cost of modules with acceptable PV conversion efficiency for mass production Driven by advances in technology and increases in manufacturing scale, the cost of photovoltaics has declined steadily since the first solar cells were manufactured For decades, an intensive search for cheaper production technology of silicon solar cells is underway In many laboratories around the world, extensive research to improve the efficiency of solar cells and modules without increasing the cost of production are carried out A large variety of solar cells, which differ depending on the materials used, PV structure, design and even the principle of

PV conversion are designed to date Among the radical ways to reduce the cost of solar modules and to increase drastically the volume of their production is the transition to thin-film technology and the use of a cheap large-area substrate (glass, metal foil, plastic)

Amorphous silicon (a-Si) was the first material for commercial thin-film solar cells with all their attractiveness to reduce consumption of absorbing material, increase in

X Preface

area and downturn in price of modules Quite common in commercial solar cells are the multi-layer structures based on a-Si It seemed that the tandem structure, a representative of the third generation of solar cells, opened the prospect of developing efficient and low-cost solar cells Special place in the thin-film photovoltaics is the so-called micromorph solar cells, which are closely related to the a-Si However, the use

of a-Si and micromorph solar cells is limited preferably to areas, where low cost is more important than the efficiency of photoelectric conversion such as consumer electronics and building-integrated photovoltaics (BIPV)

Unquestionable leaders in thin-film technologies are solar cells on CuInxGa1-xSe2 (CIGS) and CdTe, the representatives of the so-called second generation photovoltaics For a long time, CIGS have been considered as promising material for high-performance thin-film solar cells and fabrication of monolithically interconnected modules intended for cost-effective power generation As a result of research, aimed to reducing the cost of CIGS solar modules, several companies developed the commercial CIGS solar modules and initiated their large-scale production In the early years of the 21st century, the technology and manufacturing of solar modules based on CdTe, which could compete with silicon counterparts, was also developed It should be emphasized that the growth rates of CdTe module production over the last decade are the highest in the entire solar energy sector

Dye-sensitized solar cells (DSSCs) are considered to be extremely promising because they are made of low-cost materials with simple inexpensive manufacturing procedures and can be engineered into flexible sheets Organic solar cells attract the attention also by the simplicity of technology leading to inexpensive, large-scale production for the future This type of cells as well as multi-junction structures based

on a-Si and micromorph silicon can be assigned to the so-called third generation solar photovoltaics GaAs based multi-junction devices were originally designed for special applications such as satellites and space exploration To date they are the most efficient solar cells

Four-volume edition under the joint name of "Solar cells" encompasses virtually all aspects of photovoltaics Research and development in the field of thin-film solar cells based on CIGS, CdTe, amorphous, micro- and polycrystalline silicon are presented in the first volume with the subtitle "Thin-film technology" The second volume subtitled

«Dye-Sensitized Devices» is devoted to the problems of developing high-efficiency solar modules using low-cost materials with simple inexpensive manufacturing processes The third volume subtitled « Silicon Wafer-Based Technologies» includes the chapters that present the results of research aimed ultimately to reduce consumption of materials, energy, labor and hence cost of silicon solar modules on wafer or ribbon silicon Chapters that present new scientific ideas and technical solutions of photovoltaics, new methods of research and testing of solar cells and modules have been collected in the forth volume subtitled «New Aspects and Solutions»

It is hoped that readers will find many interesting and useful material in all four volumes of «Solar Cells» covering highly topical issues of photovoltaics

From the above it follows that the first book of this four-volume edition is dedicated to one of the most promising areas of photovoltaics, which has already reached a large-scale production of the second-generation thin-film solar modules and has resulted in building the powerful solar plants in several countries around the world Thin-film technologies using direct-gap semiconductors such as CIGS and CdTe offer the lowest manufacturing costs and are becoming more prevalent in the industry allowing to improve manufacturability of the production at significantly larger scales than for wafer or ribbon Si modules It is only a matter of time before thin films like CIGS and CdTe would replace wafer-based silicon solar cells as the dominant photovoltaic technology Photoelectric efficiency of thin-film solar modules is still far from the theoretical limit The scientific and technological problems of increasing this key parameter of the solar cell are discussed in several chapters of this volume

The editor addresses special thanks to the contributors for their initiative and high quality work, and to the technical editors that conveyed the submitted chapters into a qualitative and pleasant presentation

Professor, Doctor of Sciences, Leonid A Kosyachenko

National University of Chernivtsi

Ukraine

According to the U.S Department of Energy, the world's generating capacity is now close to

18 TW The main source of energy even in highly developed countries is fossil fuel, i.e coal, oil and natural gas However, resources of fossil fuel are limited, and its production and consumption irreversibly affect the environmental conditions with the threat of catastrophic

climate change on Earth Other energy sources, particularly nuclear energy, are also used that would fully meet in principle the energy needs of mankind Capacity of existing nuclear

reactors (nearly 450 in the world) is  370 GW However, increasing their capacity up to  18 TW

or about 50 times (!), is quite problematic (to provide humanity with electric energy, the capacity

of nuclear power should be increased about 10 times) Resources of hydroelectric, geothermal, wind energy, energy from biofuels are also limited At the same time, the power of solar radiation of the Earth's surface exceeds the world's generating capacity by more than 1000 times It remains only to master this accessible, inexhaustible, gratuitous and nonhazardous source of energy in an environmentally friendly way

Solar energy can be converted into heat and electricity Different ways of converting sunlight into electricity have found practical application The power plants, in which water

is heated by sunlight concentrating devices resulting in a high-temperature steam and operation of an electric generator, are widespread However, solar cells are much more

attractive due to the direct conversion of solar radiation into electricity This is the so-called

photovoltaics Under the conditions of the growing problems of global warming,

photovoltaics is the most likely candidate to replace fossil fuels and nuclear reactors

2 Silicon solar cells

Over the decades, solar modules (panels) based on single-crystalline (mono-crystalline, c-Si), polycrystalline (multi-crystalline, mc-Si), ribbon (ribbon-Si) and amorphous (a-Si) silicon are dominant in photovoltaics (Fig 1)

In recent years, photovoltaics demonstrates high growth rates in the entire energy sector According to the European Photovoltaic Industry Association , despite the global financial and economic crisis, the capacity of installed solar modules in the world grew by 16.6 GW in

Solar Cells – Thin-Film Technologies

2

Other semiconductors

7 % a-Si

Fig 1 Distribution of capacity of photovoltaic energy in the world

2010, and their total capacity reached ~ 40 GW, that is almost 8 times more than in 2005 The growth rate of the photovoltaic energy for the next 4-5 years is expected to be quite high In

2014, capacity of installed modules will be about 14 GW and 30 GW according to the pessimistic and optimistic forecast, respectively Nevertheless, despite the relatively high annual growth, the contribution of semiconductor solar cells in the global energy system is

small (less than 0.3%!), and the prospects for desired rapid development of photovoltaics are

not reassuring (Fig 2) The contribution of Si photovoltaic solar power plants in generation capacity in the world will reach ~ 1% only in the years 2018-2020, and may exceed 10% in the years 2045-2050 (EPIA, 2011; EUR 24344 PV, 2010; Jager-Waldau, 2010) Analysts do not accept the development of the PV scenario shown by the dashed line in Fig 2 Thus, solving the energy problems by developing Si photovoltaics seems too lengthy.1

The reason for the slow power growth of traditional silicon solar modules lies in a large consumption of materials and energy, high labor intensiveness and, as a consequence, a low productivity and high cost of modules with acceptable photovoltaic conversion efficiency for mass production (16-17 and 13-15% in the case of single-crystalline and polycrystalline material, respectively) (Szlufcik et al., 2003; Ferrazza, 2003).2 The problem is fundamental and

lies in the fact that silicon is an indirect semiconductor and therefore the total absorption

needs its significant thickness (up to 0.5 mm and more) As a result, to collect the charge photogenerated in a thick absorbing layer, considerable diffusion length of minority carriers

(long lifetime and high mobility) and, therefore, high quality material with high carrier

diffusion length of hundreds of micrometers are required

Estimating the required thickness of the semiconductor in solar cells, one is often guided by

an effective penetration depth of radiation into the material –1, where  is the absorption coefficient in the region of electronic interband transitions However, the value of  varies

Thin-Film Photovoltaics as a Mainstream of Solar Power Engineering 3

Fig 2 Evolution of world cumulative installed PV capacity until 2050: historical data,

, , forecasts (EPIA, 2011; EUR 24344 PV, 2010; Hegedus & Luque, 2011)

rather widely, especially in the indirect semiconductor, and solar radiation is distributed

over the spectrum in a complicated manner (Fig 3(a)) Therefore, the absorptive capacity

(absorptivity) of the material, used in solar cell, can be described by a certain integral

characteristic, which takes into account the absorption spectrum of the material and the

spectral distribution of solar radiation For a structure with flat surfaces, the integral

absorption ability of the radiation, which has penetrated into the material (certain part of

radiation is reflected from the front surface), can be represented as

i i

d

A d

exp( )

where Φі is the spectral power density of solar radiation at the wavelength і under

standard solar irradiation AM1.5 shown in Fig 3(a), і is the spacing between neighboring

wavelengths in Table 9845-1 of the International Organization for Standardization (Standard

ISO, 1992), and і is the absorption coefficient at wavelength і The summation in Eq (1) is

made from   300 nm to  = g = hc/Eg, since at wavelengths  shorter than 300 nm,

terrestrial radiation of the Sun is virtually absent, and when   g, radiation is not absorbed

in the material with the generation of electron-hole pairs

Fig 3(b) shows the dependences of absorptivity of solar radiation A of single-crystalline

silicon on the thickness of the absorber layer d calculated by Eq (1)

As seen in Fig 3(b), in crystalline silicon, the total absorption of solar radiation in the

fundamental absorption region (hv  Eg) occurs when d is close to 1 cm (!), and 95% of the

radiation is absorbed at a thickness of about 300 m More absorption can be achieved by using

the reflection of light from the rear surface of the solar cell, which is usually completely