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 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 socalled 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).

Preface XI

It is hoped that readers will find many interesting and useful material in all four

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 largescale 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

volumes of «Solar Cells» covering highly topical issues of photovoltaics.

parameter of the solar cell are discussed in several chapters of this volume.

qualitative and pleasant presentation.

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

**Professor, Doctor of Sciences, Leonid A. Kosyachenko** 

National University of Chernivtsi

Ukraine

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 highperformance 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.

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 socalled 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

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 highperformance 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

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

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

electronics and building-integrated photovoltaics (BIPV).

the highest in the entire solar energy sector.

solar cells.

Solutions».

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 largescale 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

**1** 

*Ukraine* 

**Thin-Film Photovoltaics** 

Leonid A. Kosyachenko *Chernivtsi National University* 

 **as a Mainstream of Solar Power Engineering** 

Provision of energy is one of the most pressing problems facing humanity in the 21st century. Without energy, it is impossible to overcome the critical issues of our time.

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

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,

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

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

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

Industrial world suggests continuous growth in energy consumption in the future.

source of energy in an environmentally friendly way.

**1. Introduction** 

**2. Silicon solar cells** 

dominant in photovoltaics (Fig. 1).
