Solar energy is an inexhaustible source of renewable energy. It is also a clean energy source that does not produce any environmental pollution. Among the effective use of solar energy, solar energy utilization is one of the fastest-growing and most dynamic research areas in recent years. It is one of the most highly anticipated projects. For this reason, people have developed and developed solar cells. The production of solar cells is based on semi-conductor materials. The principle of operation is to use photoelectric materials to absorb light energy and generate photoelectric conversion reactions. According to the different materials used, solar cells can be divided into: 1. Solar cells; 2. Inorganic Salts such as gallium arsenide III-V compounds, cadmium sulfide, copper indium selenium and other compounds as a battery material; 3, the functional polymer material preparation of solar cells; 4, nanocrystalline solar cells. No matter what kind of material is used to make batteries, the general requirements for solar cell materials are: 1. The bandgap of semiconductor materials cannot be too wide; 2. The photoelectric conversion efficiency should be high: 3. The material itself does not cause pollution to the environment; 4, the material is easy to industrial production and material properties and stability. Based on the above considerations, silicon is the most ideal solar cell material. This is also the main reason why solar cells are dominated by silicon materials.
However, with the continuous development of new materials and the development of related technologies, solar cells based on other villages are increasingly showing attractive prospects. This article briefly reviews the types of solar cells and their research status, and discusses the development and trends of solar cells. 1 Silicon-based solar cells 1.1 Monocrystalline silicon solar cells Silicon-series solar cells have the highest conversion efficiency and the most mature technologies. High-performance single-crystal silicon cells are based on high-quality monocrystalline silicon materials and related heat-generating processing processes. Nowadays, the electrical ground technology of single-crystal silicon has been almost mature. In the production of batteries, surface texturing, passivation of emission regions, and zone doping techniques are generally used. The batteries developed mainly include planar monocrystalline silicon cells and etched trenches. Gate electrode monocrystalline silicon battery. The improvement of conversion efficiency is mainly based on the monocrystalline silicon surface microstructure processing and zone doping process. In this respect, the Fleurieu Institute of Solar Energy Systems in Flandrehof, Germany, maintains a world-leading level. The Institute used photolithographic techniques to texture the surface of the cell to create inverted pyramid structures. And put a 13nm on the surface. A thick oxide passivation layer is combined with two antireflection coatings. The ratio of the width and height of the grid is increased by the improved plating process: The conversion efficiency of the battery obtained by the above is over 23, which is a large value of up to 23.3%. Kyocera's large-area (225cm2) single crystal solar cell has a conversion efficiency of 19.44. The Beijing Solar Energy Research Institute is also actively conducting the research and development of high-efficiency crystalline silicon solar cells and developing a planar high-efficiency single crystal silicon cell (2cmX2cm). The conversion efficiency reached 19.79, and the conversion efficiency of the trench buried gate electrode crystalline silicon cell (5cmX5cm) was 8.6.
The conversion efficiency of monocrystalline silicon solar cells is undoubtedly the highest. It still occupies a dominant position in large-scale applications and industrial production. However, due to the price of monocrystalline silicon materials and the corresponding cumbersome battery process, the cost of monocrystalline silicon is high. No less, it is very difficult to drastically reduce its cost. In order to save high quality materials and find alternatives to monocrystalline silicon cells, thin-film solar cells are now being developed, with polysilicon thin-film solar cells and amorphous silicon thin-film solar cells being typical examples. 1.2 Polysilicon thin-film solar cells A typical crystalline silicon solar cell is fabricated on a high-quality silicon wafer with a thickness of 350 to 450 μm. This silicon wafer is sawn from a pulled or cast silicon ingot. Therefore, more silicon material is actually consumed. In order to save materials, people began to deposit polysilicon films on inexpensive substrates since the mid-1970s, but due to the grain size of the grown silicon films, valuable solar cells could not be made. In order to obtain thin films of large size, people have not stopped researching and proposed many methods. At present, polycrystalline silicon thin-film batteries are mostly prepared by chemical vapor deposition methods, including low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) processes. In addition, liquid phase epitaxy (LPPE) and sputter deposition methods can also be used to prepare polysilicon thin film batteries.
Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, SiCl4, or SiH4 as reaction gases, reacts under certain protective atmosphere to generate silicon atoms and deposits on heated substrates. Substrate materials generally use Si, SiO2, Si3N4, and the like. However, it has been found that it is difficult to form large grains on non-silicon substrates and easily form gaps between crystal grains. The solution to this problem is to use LPCVD to deposit a thin layer of amorphous silicon on the substrate and then anneal this layer of amorphous silicon to get larger grains and then on the seed. Deposition of thick polysilicon film, therefore, recrystallization technology is undoubtedly a very important part of the current technology used mainly solid-phase crystallization and the middle zone remelting recrystallization. In addition to the recrystallization process, polysilicon thin-film batteries also employ almost all techniques for the preparation of monocrystalline silicon solar cells, which significantly improves the conversion efficiency of solar cells. The conversion efficiency of polycrystalline silicon cells fabricated on FZSi substrates by the Institute of Solar Energy Research, Freiburg, Germany, was 19%. Japanese Mitsubishi Corporation used the method to prepare batteries with an efficiency of 16.42. The principle of the liquid phase epitaxy (LPE) method is to melt the silicon in the matrix and lower the temperature to precipitate the silicon film. US Astropower Corporation uses LPE to produce a battery efficiency of 12.2%. Chen Zheliang of the China Optoelectronics Development and Technology Center used silicon-phase epitaxy to grow silicon grains on a metallurgical-grade silicon wafer and designed a new type of solar cell similar to a crystalline silicon thin-film solar cell called "Silicon Grain" solar energy. Battery, but reports about performance have not been seen.
Polycrystalline silicon thin-film batteries use less silicon than single-crystal silicon, and have no efficiency degradation issues. They may be fabricated on inexpensive substrate materials. The cost is much lower than that of single-crystal silicon cells, and the efficiency is higher than that of amorphous silicon films. Batteries, therefore, polysilicon thin-film batteries will soon dominate the solar-electricity market. 1.3 Amorphous Silicon Thin Film Solar Cells Two key issues in the development of solar cells are: improving conversion efficiency and reducing costs. Due to the low cost of amorphous silicon thin-film solar cells and their ease of large-scale production, they are generally valued and rapidly developed. In fact, as early as the early 1970s, Carlson et al. have already started to develop amorphous silicon cells. In recent years, its development work has been rapidly developed. At present, there are many companies in the world that are producing this kind of battery product.
Amorphous silicon as a solar energy material is a good battery material, but because of its optical band gap of 1.7 eV, the material itself is insensitive to the long-wave region of the solar radiation spectrum, thus limiting the amorphous silicon solar cells. The conversion efficiency. In addition, its photoelectric efficiency will be attenuated with the continuation of the illumination time, namely the so-called photo-induced recession S-W effect, making the battery performance unstable. The solution to these problems is to prepare tandem solar cells, which are made by depositing one or more Pin subcells on the prepared p-, i-, and n-layer single junction solar cells. The key problems for tandem solar cells to improve conversion efficiency and solve the instability of single-junction cells are: 1 It groups together different materials with different band gaps to increase the spectral response range; the i-layer of 2 top cells is thin The intensity of the electric field generated by light does not change much, ensuring that the photogenerated carriers in the i-layer are extracted; 3 the carrier generated by the bottom cell is about half of the single cell, and the photoinduced recession effect is reduced; 4 each of the tandem solar cells The batteries are connected in series. Amorphous silicon thin-film solar cells can be prepared in many ways, including reactive sputtering, PECVD, LPCVD, etc. The reaction source gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel sheet, and the amorphous silicon is made. The thin film can be made into single-junction cells and tandem solar cells through different cell processes.
At present, the research of amorphous silicon solar cells has made two major advances: the conversion efficiency of the first and third stacked amorphous silicon solar cells has reached 13%, setting a new record; secondly, the annual production capacity of three stacked solar cells has reached 5MW. The maximum conversion efficiency of a single-junction solar cell produced by VSSC is 9.3, and the maximum conversion efficiency of a triple-bandgap triple-stacked battery is 13%. The above highest conversion efficiency is in a small area (0.25cm2) battery Made on. There have been reports in the literature that the conversion efficiency of single-junction amorphous silicon solar cells exceeds 12.5%, and Academia Sinica adopted a series of new measures, resulting in a conversion efficiency of 13.2% for amorphous silicon cells. Domestic research on amorphous silicon thin-film batteries, especially tandem solar cells, is limited. Nanyang University's Xinhua and others used industrial materials to prepare an a-back electrode with an area of ​​20X20cm2 and a conversion efficiency of 8.28%. Si/a-Si tandem solar cells. Amorphous silicon solar cells have great potential due to their high conversion efficiency, low cost and light weight. But at the same time, due to its low stability, it directly affects its practical application. If you can further solve the stability problem and increase the conversion rate, then, amorphous silicon solar energy battery is undoubtedly one of the major development products of solar cells.