(1) silicon-based solar cell
1, monocrystalline silicon solar cell
Among silicon-based solar cells, monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. High-performance monocrystalline silicon battery is based on high-quality monocrystalline silicon material and related heating processing technology. At present, the electrical grounding technology of monocrystalline silicon is close to maturity. In battery manufacturing, technologies such as surface texturing, emitter passivation, and partition doping are used. Be widely adopted. The developed batteries mainly include planar monocrystalline silicon batteries and trench buried gate electrode monocrystalline silicon batteries. Improving the conversion efficiency mainly depends on the surface microstructure treatment and zoning doping process of monocrystalline silicon.
In this respect, Flawn Hof Solar Energy System Research Institute maintains a world-leading level. In this study, the surface of the battery is textured by lithography and photography to make an inverted pyramid structure. And put a 13nm on the surface. The thick oxidation passivation layer combined with two anti-reflective coatings improves the aspect ratio of grid by improved electroplating process: the conversion efficiency of the battery made by the above method exceeds 23%, but the maximum value can reach 23.3%. The conversion efficiency of large-area (225cm2) monocrystalline silicon solar cells prepared by Kyocera Corporation is 19.44%, and China Beijing Solar Research Institute is also actively engaged in the research and development of high-efficiency crystalline silicon solar cells. The conversion efficiency of planar high-efficiency monocrystalline silicon battery (2cmX2cm) is 19.79%, and that of trench buried gate electrode crystalline silicon battery (5cmX5cm) is 8.6%. The conversion efficiency of monocrystalline silicon solar cell is undoubtedly the highest, and it still occupies a dominant position in large-scale application and industrial production. However, due to the influence of the price of monocrystalline silicon material and the corresponding complicated battery technology, the cost of monocrystalline silicon remains high, and it is very difficult to greatly reduce its cost. In order to save high-quality materials and find alternative products of monocrystalline silicon cells, thin-film solar cells have been developed, among which polycrystalline silicon thin-film solar cells and amorphous silicon thin-film solar cells are typical representatives.
2. Polycrystalline silicon thin film solar cells
The usual crystalline silicon solar cells are made on high-quality silicon wafers with a thickness of 350 ~ 450 microns, which are sawn from drawn or cast silicon ingots. More silicon material is actually consumed. In order to save materials, people began to deposit polycrystalline silicon thin films on cheap substrates in the mid-1970s, but due to the grain size of the grown silicon films, they failed to make valuable solar cells. In order to obtain films with large grain size, people have never stopped studying and put forward many methods. At present, chemical vapor deposition (CVD) is widely used to prepare polycrystalline silicon thin film batteries, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). In addition, liquid phase epitaxy (LPPE) and sputtering deposition can also be used to prepare polycrystalline silicon thin film batteries. Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, Sicl4 or SiH4 as reaction gases, which react in a certain protective atmosphere to generate silicon atoms and deposit them on heated substrates. The substrate material is usually silicon, silicon dioxide, silicon nitride, etc.
However, it is found that it is difficult to form large grains on non-silicon substrates, and it is easy to form gaps between grains. The solution to this problem is to deposit a thin amorphous silicon layer on the substrate by LPCVD, then anneal the amorphous silicon layer to obtain larger grains, and then deposit a thick polysilicon film on the seed crystal. Therefore, recrystallization technology is undoubtedly a very important link. At present, the main technologies used are solid-state crystallization and zone melting recrystallization. In addition to recrystallization process, polycrystalline silicon thin film battery adopts almost all the processes for preparing monocrystalline silicon solar cells, which obviously improves the conversion efficiency of the prepared solar cells. The conversion efficiency of polycrystalline silicon cells prepared by Freiburg Solar Energy Research Institute in Germany on FZSi substrate is 19%, and that of Mitsubishi Corporation in Japan is 16.42%. The principle of liquid phase epitaxy (LPE) is to melt the silicon in the matrix and lower the temperature to precipitate the silicon film. The efficiency of the battery prepared by LPE of Astropower Company in the United States reached 12.2%.
Chen Zheliang of China Photoelectric Development Technology Center used liquid phase epitaxy to grow silicon grains on metallurgical grade silicon wafers, and designed a new type of solar cell similar to crystalline silicon thin-film solar cell, called "silicon grain" solar cell, but no reports about its performance have been seen. The amount of silicon used in polycrystalline silicon thin film battery is far less than that of monocrystalline silicon, so there is no problem of efficiency decline, and it can be prepared on cheap substrate materials. Its cost is much lower than that of monocrystalline silicon battery, but its efficiency is higher than that of amorphous silicon thin film battery. Therefore, polycrystalline silicon thin film batteries will soon occupy a dominant position in the solar energy market.
3. Amorphous silicon thin film solar cells
Two key issues in developing solar cells are: improving conversion efficiency and reducing cost. Amorphous silicon thin film solar cells have attracted people's attention and developed rapidly because of their low cost and convenience for mass production. In fact, as early as the early 1970s, Carlson and others had started the research and development of amorphous silicon batteries, and in recent years, their research and development work has developed rapidly. At present, many companies in the world are producing this battery product. Although amorphous silicon is a good solar cell material, its optical band gap is 1.7eV, which makes the material itself insensitive to the long-wave region of solar radiation spectrum, thus limiting the conversion efficiency of amorphous silicon solar cells. In addition, its photoelectric efficiency will decrease with the extension of illumination time, which is the so-called photo-induced attenuation S-W effect, which makes the battery performance unstable. The solution to these problems is to prepare laminated solar cells, which are made by depositing one or more P-i-n daughter cells on the prepared P, I and N single-junction solar cells.
The key problems to improve the conversion efficiency and solve the instability of single-junction solar cells are as follows: ① it combines materials with different band gaps to improve the spectral response range; (2) The I layer of the top battery is thin, and the intensity of the electric field generated by illumination changes little, which ensures the extraction of photogenerated carriers in the I layer; (3) The carrier generated by the bottom battery is about half that of the single battery, and the photo-induced fading effect is reduced; (4) Each sub-cell of the laminated solar cell is connected in series. There are many methods to prepare amorphous silicon thin film solar cells, including reactive sputtering, PECVD, LPCVD and so on. The reactant gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel sheets. Amorphous silicon thin films can be made into single junction cells and laminated solar cells by different cell processes.
At present, two major advances have been made in the research of amorphous silicon solar cells: the conversion efficiency of the first and third stacked amorphous silicon solar cells reached 13%, setting a new record; The annual production capacity of the second and third-tier solar cells reaches 5MW. The maximum conversion efficiency of single-junction solar cells manufactured by United Solar Corporation (VSSC) is 9.3%, and the maximum conversion efficiency of three-band-gap three-layer solar cells is 13%, as shown in Table 1. The maximum conversion efficiency is achieved on a small area (0.25cm2) battery. It is reported that the conversion efficiency of single-junction amorphous silicon solar cells exceeds 65,438+02.5%. Academia Sinica of Japan has adopted a series of new measures, and the conversion efficiency of amorphous silicon solar cells is 13.2%. There is little research on amorphous silicon thin film batteries, especially laminated solar cells in China. Geng Xinhua of Nankai University and others have prepared A-Si/A-Si laminated solar cell with an area of 20X20cm2, conversion efficiency of 8.28% and aluminum back electrode by using industrial materials. Amorphous silicon solar cells have great potential because of their high conversion efficiency, low cost and light weight. But at the same time, its low stability directly affects its practical application. If we can further solve the stability problem and improve the conversion rate, then amorphous silicon solar cells will undoubtedly be one of the main development products of solar cells.