Although monocrystalline silicon solar cells have their advantages, their high price hinders the development of monocrystalline silicon solar cells in the low-price market. The problem that polycrystalline silicon solar cells need to solve is first to reduce costs, and secondly to efficiency. Although the crystalline structure of polycrystalline silicon solar cells is different from that of monocrystalline silicon solar cells, the photovoltaic principle is the same. There are three main ways to reduce the cost of polycrystalline silicon solar cells:
(1) Impurities are not completely removed during the purification process.
(2) Use a faster way to crystallize silicon.
(3) Avoid waste caused by slicing.
Because of these three reasons, the manufacturing cost and time of polycrystalline silicon solar cells are less than that of monocrystalline silicon solar cells, but this also makes the crystalline structure of polycrystalline silicon solar cells inferior. The main reasons for the poor crystalline structure of polycrystalline silicon solar cells are:
(1) It contains impurities.
(2) Silicon crystallizes faster, and silicon atoms do not have enough time to form a single crystal lattice and form many crystal particles. The larger the crystalline particles are, the closer the efficiency is to that of monocrystalline silicon solar cells. The smaller the crystalline particles, the worse the efficiency. Moreover, the bonding of silicon atoms at the crystal boundary is poorer, which is susceptible to UV damage and produces more suspended bonds. As the use time increases, the number of floating bonds will also increase, and the photoelectric conversion efficiency will gradually decline. This is the main disadvantage of polycrystalline silicon solar cells, and low cost is its main advantage.
Currently, polycrystalline silicon solar cells can achieve a unit conversion efficiency of 15.8% per 100 cm2 (Sharp Corporation). In the laboratory, the unit conversion efficiency per 4cm2 of area is 17.8% (UNSW). The general conversion efficiency of polycrystalline silicon solar cells is about 10%~15%, and the conversion efficiency of modularization is about 9%~12%.
Conventional crystalline silicon solar cells are made on high-quality silicon wafers with a thickness of 350-450μm. This silicon wafer is sawn from a silicon ingot that is lifted or cast, and therefore consumes more silicon material. In order to save materials, people have been incandescent polysilicon films on cheap substrates since the mid-1970s, but the grown silicon film grains are too small to make valuable solar cells. In order to obtain a thin film with large-scale grains, people have never stopped researching, and many methods have been proposed. At present, the preparation of polycrystalline silicon thin film batteries mostly adopts 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 sputtering deposition methods can also be used to prepare polycrystalline silicon thin film batteries.
Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, SiC4 or SiH4 as the reaction gas, reacts under a certain protective gas (atmosphere) to generate silicon atoms and deposits on the heated substrate. The substrate material is generally Si, SiO2, Si3N4, etc. However, studies have found that it is difficult to form larger crystal grains on non-silicon substrates, and it is easy to form voids between the crystal grains. The solution to this problem is to first use LPCVD to deposit a thinner amorphous silicon layer on the substrate, and then anneal this amorphous silicon layer to obtain larger crystal grains. and then deposit a thick polysilicon film on the seed crystal. Therefore, the recrystallization technology is undoubtedly a very important link. The current technologies mainly include solid phase crystallization and mid-zone melting recrystallization. In addition to the recrystallization process used in the polycrystalline silicon thin film battery, almost all the techniques for preparing monocrystalline silicon solar cells are also used, so that the conversion efficiency of the solar cells prepared in this way is significantly improved. Polycrystalline silicon thin-film batteries use far less silicon than monocrystalline silicon and have no efficiency degradation problem, and can be prepared on cheap substrate materials. The cost is much lower than that of monocrystalline silicon cells, and the efficiency is higher than that of amorphous silicon thin-film cells. Therefore, polycrystalline silicon thin film batteries will soon dominate the solar cell market. The typical characteristic parameters of industrially produced polycrystalline silicon solar cells are: Isc=2950mA; Voc=584mV; fill factor FF=0.72; conversion efficiency n=12.4% (test conditions: AM1.5, 1000W/m2, 25°C).
Other characteristics of polycrystalline silicon solar cells are similar to those of monocrystalline silicon solar cells, such as temperature characteristics and changes in solar cell performance with incident light intensity. In terms of manufacturing cost, it is easier to manufacture than monocrystalline silicon solar cell materials, saves power consumption, and has a lower total production cost, so it has been developed and promoted. In addition, the service life of polycrystalline silicon solar cells is shorter than that of monocrystalline silicon solar cells. In terms of cost performance, monocrystalline silicon solar cells are still superior to polycrystalline silicon solar cells.
In the utilization of solar photovoltaic, monocrystalline silicon and polycrystalline silicon solar cells play a huge role. Although at present, in order to make solar photovoltaic systems have a larger market and be accepted by the majority of consumers, it is necessary to improve the photoelectric conversion efficiency of solar cells and reduce production costs. From the current international solar cell development process, it can be seen that its development trend is monocrystalline silicon, polycrystalline silicon, ribbon silicon, and thin film materials (including microcrystalline silicon-based films, compound-based films and dye films).
Due to the rapid research and development of monocrystalline silicon technology in the past ten years, the technology has also been applied to the production of polycrystalline silicon cells, such as selective corrosion of the emission junction, back surface field, corrosion texture, surface and volume passivation, and fine metal gate electrode . The screen printing technology can reduce the width of the gate electrode to 50μm and the height to more than 15μm. The rapid thermal annealing technology used in the production of polycrystalline silicon can greatly shorten the process time. The single-chip thermal process time can be completed within one minute. The conversion efficiency of solar cells produced on a 100cm2 polycrystalline silicon wafer using this process exceeds 14%. According to reports, the current efficiency of polycrystalline silicon solar cells made on 50-60μm polycrystalline silicon substrates exceeds 16%. The conversion efficiency of polycrystalline silicon solar cells made on a 100cm2 polycrystalline silicon wafer using mechanical grooves and screen printing technology exceeds 17%, and the efficiency of the same area without mechanical grooves reaches 16%. The conversion efficiency of the polycrystalline silicon solar cell produced on a 130cm2 polycrystalline silicon wafer with a buried gate structure and mechanically grooved is 15.8%.