Solar cell is one of the most important modern battery products. With the help of the heat energy of the sun, solar cell provides power support for many devices. In order to enhance our understanding of solar cells, this paper will introduce monocrystalline silicon solar cells and polycrystalline silicon thin film solar cells. If you are interested in solar cells, read on.
1、 Monocrystalline silicon solar cells
Among the series of silicon 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 materials and related thermal processing technology. At present, the electrical grounding technology of monocrystalline silicon is nearly mature. In the production of battery, surface texturing, passivation of emission region, zoning doping and other technologies are generally used. The developed battery mainly includes planar monocrystalline silicon battery and grooved buried gate electrode monocrystalline silicon battery. The improvement of the conversion efficiency mainly depends on the surface microstructure treatment and doping technology. In this regard, Germany’s Fraunhofer Freiburg Solar System Research Institute maintains the world’s leading level. The Institute used photolithography technology to texture the surface of the battery and make it into inverted pyramid structure. And a 13 nm film was placed on the surface. The thick oxide passivation layer is combined with two antireflection coatings. Through the improved electroplating process, the ratio of gate width to gate height is increased: the conversion efficiency of the cell is more than 23%, and the maximum value is 23.3%. The conversion efficiency of large area (225cm2) single crystal solar cells prepared by Kyocera company is 19.44%. Beijing Solar Energy 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 single crystal silicon cells (2cmx2cm) is 19.79%, and the conversion efficiency of grooved buried gate electrode crystalline silicon cells (5cmx5cm) is 8.6%.
The conversion efficiency of monocrystalline silicon solar cells 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 materials and the corresponding cumbersome battery technology, the cost of monocrystalline silicon remains high, so it is very difficult to reduce its cost substantially. In order to save high-quality materials and find alternative products for 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、 Polysilicon thin film solar cells
Generally, the thickness of crystalline silicon solar cells is 350-450 μ This kind of silicon wafer is sawed from the lifting or casting ingot. So more silicon is actually consumed. In order to save materials, polysilicon thin films have been deposited on cheap substrates since the mid-1970s. However, due to the grain size of the grown silicon films, valuable solar cells have not been made. In order to obtain the films with large grain size, many methods have been proposed. 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 methods can also be used to prepare polycrystalline silicon thin film batteries.
Chemical vapor deposition (CVD) mainly uses SiH2Cl2, SiHCl3, SiCl4 or SiH4 as the reaction gas, reacts in a certain protective atmosphere to generate silicon atoms and deposits them on the heated substrate. The substrate materials are generally Si, SiO2, Si3N4, etc. However, it is found that it is difficult to form large grains on non silicon substrates, and it is easy to form voids between grains. The way to solve 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 layer. Therefore, recrystallization technology is undoubtedly a very important link. At present, the main technologies used are solid-state crystallization method and middle zone melting recrystallization method. In addition to the recrystallization process, the polycrystalline silicon thin film solar cells also use almost all the technologies to prepare monocrystalline silicon solar cells, so the conversion efficiency of the solar cells is significantly improved. The conversion efficiency of the polycrystalline silicon cell on fzsi substrate is 19%, which is prepared by Freiburg Solar Energy Research Institute in Germany. The efficiency of the cell prepared by Mitsubishi company in Japan is 16.42%.
The principle of liquid phase epitaxy (LPE) is to melt silicon into the matrix and precipitate silicon film at lower temperature. The efficiency of LPE battery made by American Astropower company is 12.2%. Chen Zheliang of China optoelectronic development technology center has grown silicon grains on metallurgical grade silicon wafers by liquid phase epitaxy, and designed a new type of solar cell similar to crystalline silicon thin film solar cell, which is called “silicon grain” solar cell, but there is no report about its performance.
The cost of polycrystalline silicon thin film cells is much lower than that of monocrystalline silicon cells, but the efficiency is higher than that of amorphous silicon thin film cells. Therefore, polycrystalline silicon thin film cells will soon occupy a dominant position in the solar power market.