Development of the hottest amorphous silicon solar

The development of amorphous silicon solar cells

introduction

in 1976, Carlson and leonski reported the birth of amorphous silicon (a-Si) thin film solar cells. At that time, the photoelectric conversion efficiency of small-area samples was 2.4 people. After more than 20 years, a-Si solar cells have now developed into one of the cheapest solar cells. Amorphous silicon technology has been transformed into a large-scale industry. The total component productivity in the world is more than 50 megawatts per year. The sales volume of components and related products is more than US $1 billion, and the application range is as small as watches and meters, and the power supply is as large as 10 watt independent power stations. It involves many kinds of consumer electronics, lighting and household power supply, clothing and animal husbandry, water pumping, broadcasting and communication station power supply and small and medium-sized power stations. A-Si solar cells have become a driving force of photovoltaic energy

and have played a huge role in promoting the development of clean and renewable photovoltaic energy. The growing maturity of amorphous silicon solar cell technology has increased people's confidence that photovoltaic energy, as a clean and renewable energy, can replace disposable conventional energy The main development process of amorphous silicon solar cells is vivid, complex and tortuous. Comprehensively summarizing the experience and lessons is of great significance to further promote the scientific and technological progress in the field of thin-film amorphous silicon solar cells and the development of related high-tech industries. Moreover, from the research of amorphous silicon materials and solar cells to the development of relevant emerging industries, it is a typical example of the transformation of science and technology into productivity. The regularity will also have beneficial implications for the development of other emerging science and technology fields and related industries. This paper will trace the origin and development of amorphous silicon solar cells. Briefly comment on the key points. Point out the direction of further development

I. The Birth of amorphous silicon solar cells

1. Social needs led to the birth of a-Si solar cells

solar cells in the mid-1970s, which should be regarded as an example of inventing scientists trying to adapt their scientific research work to the needs. In their report, they put forward two major goals for the invention of amorphous solar cells: to compete with the crystalline silicon j-day battery of onshi; Using amorphous silicon solar cells to generate electricity, it competes with conventional energy. There was a famous energy crisis in the 1970s; This background urges scientists to turn the general research on a-Si materials to the application technology innovation of low-cost solar cells, which is actually the third challenge from amorphous semiconductors to crystalline semiconductors. Solar cells were originally the application field of crystalline silicon. The Challenger said that although solar cells are high-grade optoelectronic devices, they do not have to be made of responsible crystalline semiconductor materials, and cheap amorphous silicon film materials can also be competent

2. Establishment of the theoretical and technical basis of amorphous silicon solar cells

amorphous materials first emerged in the field of optoelectronic devices in 1950. At that time, amorphous selenium (a-Se) and amorphous antimony trisulfide (a-sbs3) were found when people were looking for photoconductive materials suitable for TV camera tubes and copying equipment. At that time, there was no concept of amorphous materials and related fields, and the theoretical basis of crystalline semiconductors, energy band theory, had matured as early as the 1930s. The transistor has been invented. The photoelectric characteristics of crystal semiconductors and the development of devices are just hot spots. However, materials such as a-Se and a-sbs3 developed into a large industry with an output value of US $1 billion without basic theory. This first challenge of amorphous materials was very successful, and the scientific and technological research on amorphous materials was also launched. In 1957, spear successfully measured the drift mobility of a-Se materials. In 1958, Anderson of the United States first proposed in his paper that there is an electronic localization effect in the amorphous system. In 1960, in an article entitled "amorphous, amorphous and liquid electronic semiconductors", the former Soviet Union, Yoffie and Regel put forward an argument of great significance to the theory of amorphous semiconductors, that is, the main factor determining whether the basic electronic properties of solids belong to metals, semiconductors or insulators is the short-range structure of atoms constituting condensed states, that is, the coordination of nearest neighbors. Since 1960, people have been committed to preparing a-Si and age thin films. The sputtering method was mainly used earlier. At the same time, the optical properties of these films were systematically studied. In 1965, Stirling et al. Prepared hydrogenated amorphous silicon (a-si:h) films by glow discharge (GD) or plasma enhanced chemical vapor deposition (PECVD) for the first time. This method used RF electromagnetic field to excite low-pressure silane and other gases, glow discharge chemical decomposition, and formed a-Si films on the substrate. This is the main preparation method of a-Si material for solar cells later

in 1960, amorphous semiconductors faced the second challenge of crystalline semiconductors in the field of device applications. That was when American ovschinsky discovered that sulfur amorphous semiconductor materials had the function of electronic switch storage. Although this discovery is not successful in application, it is of breakthrough value in academia. Nobel Prize winner Mott said that this is more important than the invention of the transistor. It leads scientists' interest from traditional crystalline semiconductor materials to amorphous semiconductor materials. It has set off an upsurge of research on amorphous: semiconductor materials. It was in the late 1960s that China began to engage in research in this field. From 1966 to 1969, relevant scientists carried out in-depth basic theoretical research and solved the energy band theory of amorphous semiconductors. The electronic energy state separation is proposed; Cloth's mott-cf0 model and the idea of migrating edges

3. Basic structure of a-Si solar cell

the work of doping a-Si film to control its conductivity type and quantity was first realized by lecomber and spear in 1975. At the same time, the fabrication of a-Si PN junction is realized. In fact, due to the characteristics of multiple defects in a-Si, doping often further increases the defect density. The basic structure of a-Si solar cells is not PN junction but PIN junction. Boron doping forms p region, phosphorus doping forms n region, and I is an intrinsic layer without impurities or lightly doped with B (because undoped a-Si is a weak n type). Heavily doped P and N regions form built-in potentials inside the battery to collect charges. At the same time, the two can form ohmic contact with the conductive electrode to provide external electric power. Zone I is a photosensitive zone. The photoconductivity/dark conductivity ratio is 105-106. Photogenerated electron holes in this area are the source of photovoltaic power. The long-range disorder of amorphous silicon structure destroys the selection rule of photoelectron transition in crystalline silicon. It changes from indirect band gap material to direct band gap material. The absorption coefficient of photons is very high, and the light in the sensitive spectral domain is completely absorbed. Therefore, the thickness of a-Si battery with p/i/n structure is about 5000, while the thickness of P and N layers as dead light absorption zone is limited to the order of 100

in a word, amorphous silicon solar cells are not only the product of application needs, but also the result of amorphous semiconductor technology exploration and basic theoretical research. The combination of scientific and technological innovation and social needs produces great value. Nowadays, every scientific and technological innovation includes technological exploration and basic theoretical research, one of which cannot be neglected. Of course, different topics or different development stages of a topic will have different focuses. In the history of the development of science and technology, the basic theories formed in some fields have not yielded fruitful results until the technology is mature. There are also some fields that first produced applied technology. Technological development promoted basic theoretical research and produced theoretical results. The establishment of theory also guides the application technology to mature

Second, the initial development of amorphous silicon solar cells

1. Initial technological progress and prosperity

semiconductor giant electronic devices - solar cells can be made with cheap amorphous silicon materials and processes, which has inspired researchers and research institutions to invest in the research in this field, and has also attracted the attention of the business community and the attention and attention of many governments. This has brought about the great development of amorphous silicon solar cells. Amorphous silicon solar cells soon walked out of the laboratory and into pilot test lines and large-scale production lines. In terms of technology, the progress of amorphous silicon solar cells at this stage is mainly reflected in: (1) from simple it0/P/I/N (a-Si)/Al to sn02 (f)/p-a-sic/i-a-si/n-a-si/AI, which is a more complex and practical structure. Sn0: transparent conductive film is more stable than it0, lower cost, easy to realize texture, thus increasing the absorption of light by solar cells. Using asic:h as the p-type window layer, the band gap is wider, which reduces the light absorption loss of the p-layer and makes better use of the incident solar energy. (2) Laser marking and segmentation of a-Si layer and two electrode thin layers are realized respectively, and the production of integrated components is realized. (3) There are two ways to produce amorphous silicon films: batch production with single air and flow production with multiple chambers. In production, there are also two production methods of battery components with transparent conductive glass as the substrate and flexible materials (such as stainless steel) as the substrate. There are many enterprises or branches of enterprises with a-s1 solar cells as their main products in the world. For example, chr0nar, s0larex and ECD in the United States, and Sanyo, Fuji and YOUPU in Japan. Chr0nar is a pioneer in the development of ASL solar cell industry. It not only has its own production line, but also exports excess MW production lines to other countries. American and Japanese companies have also installed test power stations for outdoor power generation with their own products. The largest has a capacity of 100 kW. In the mid-1980s, amorphous silicon accounted for 40% of the total sales of solar cells in the world. Amorphous silicon, polycrystalline silicon and monocrystalline silicon are three pillars

2. Advantages of a-Si solar cells

the rapid transformation of technology to productivity shows that amorphous silicon solar cells have unique advantages. These advantages are mainly reflected in the following aspects: (1) low cost of materials and manufacturing processes. This is because the substrate materials, such as glass, stainless steel, plastic, etc., are cheap. The thickness of silicon film is only thousands of angstroms, and the amount of pure silicon material used is very small. The production process is a low-temperature process (100-300 ℃), with low power consumption/short energy recovery time. (2) It is easy to form large-scale production capacity. This is because the core-core process is suitable for making a-Si alloy films with large areas without structural defects; PIN junction and corresponding laminated structure can be realized by changing gas phase composition or gas flow rate; The whole production process can be automated. (3) There are many varieties and wide uses. Thin film a-Si solar cells are easy to realize integration. The power, output voltage and output current of the device can be designed and manufactured freely, which can easily produce a variety of products suitable for different needs. Due to its high light absorption coefficient, dark conductivity and good low, it is suitable for making low-power power power supplies for indoor use, such as watch batteries, calculator batteries, etc. Because the silicon structure of a-Si film has strong mechanical properties. It is suitable for making light-weight large "batteries on flexible substrates. Flexible and diverse manufacturing methods can produce building integrated batteries, which are suitable for the installation of household rooftop power stations.

3. The development momentum is frustrated.

amorphous silicon solar cells, despite the above many advantages, have obvious disadvantages. Mainly the initial photoelectric conversion