Although monocrystalline silicon solar cells have their advantages, their high price hinders the development of monocrystalline silicon solar cells in the low-cost market. The polycrystalline silicon solar cell is the first to reduce costs, followed by efficiency. Although polycrystalline silicon solar cells and monocrystalline silicon solar cells have different crystal structures, the photovoltaic principle is the same. There are three main ways to reduce the cost of polycrystalline silicon solar cells:
(1) The purification process does not completely remove impurities.
(2) Use a faster method to crystallize the silicon.
(3) Avoid waste caused by slicing.
For these three reasons, the manufacturing cost and time of polycrystalline silicon solar cells are lower and less than that of monocrystalline silicon solar cells, and thus the crystalline structure of polycrystalline silicon solar cells is also inferior. The main reasons for the poor crystal 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 crystal particles, the closer the efficiency is to that of single-crystal silicon solar cells. The smaller the crystal particles, the worse the efficiency. Because the bonding of silicon atoms at the crystal boundary is poor, it is easily damaged by ultraviolet rays and generates more suspending bonds. With the increase of time, the number of dangling bonds will also increase, and the photoelectric conversion efficiency will gradually decline. This is the main disadvantage of polycrystalline silicon solar cells, and the low cost is the main advantage.
At present, the conversion efficiency of polycrystalline silicon solar cells per 100cm² unit area is 15.8% (Sharp Corporation), the conversion efficiency of polycrystalline silicon solar cells per 4cm² unit area in the laboratory is 17.8% (UNSW), and the general conversion efficiency of polycrystalline silicon solar cells is 10 %~15%, and the conversion efficiency of polycrystalline silicon solar cell modules is 9%~12%.
Conventional crystalline silicon solar cells are fabricated on high-quality silicon wafers with a thickness of 350–450 μm, which are sawn from pulled or cast silicon ingots. Therefore, more silicon material is actually consumed. In order to save materials, people have been depositing polysilicon thin films on inexpensive substrates since the mid-1970s, but the grains of the grown silicon films were too small to make valuable solar cells. In order to obtain thin films with large-sized grains, research has not stopped and many methods have been proposed. At present, the preparation of polycrystalline silicon thin film solar cells 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 sputter deposition methods can also be used to fabricate polycrystalline silicon thin-film solar cells.
Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, SiCI4 or SiH4 as the reactive gas, and reacts under a certain protective gas (atmosphere) to generate silicon atoms and deposit them on the heated substrate. The substrate materials are generally Si, SiO2, Si3N4 Wait. However, studies have found that it is difficult to form larger grains on non-silicon substrates, and it is easy to form voids between grains. The solution to this problem is to first use LPCVD to deposit a thin layer of amorphous silicon on the substrate, then anneal this layer of amorphous silicon to obtain larger grains, and then layer this layer of seed crystals. To deposit a thick polysilicon film, recrystallization technology is undoubtedly a very important link. The currently used technologies mainly include solid phase crystallization method and mid-zone melting recrystallization method. In addition to the recrystallization process, the polycrystalline silicon thin-film solar cell adopts almost all the technologies for preparing monocrystalline silicon solar cells, and the conversion efficiency of the solar cells thus prepared is obviously improved. Typical characteristic parameters of industrially produced polycrystalline silicon solar cells are as follows:
Isc=2950mA; Voc=584mV; fill factor FF=0.72; conversion efficiency η=12.4% (test conditions: AM1.5, 1000W/m², 25℃).
Other characteristics of polycrystalline silicon solar cells are similar to those of monocrystalline silicon solar cells, such as temperature characteristics, changes in solar cell performance with incident light intensity, etc. In terms of production cost, it is cheaper than monocrystalline silicon solar cell materials, easy to manufacture, saves power consumption, and has a lower total production cost, so it has developed rapidly. In addition, the service life of polycrystalline silicon solar cells is also shorter than that of monocrystalline silicon solar cells. In terms of cost performance, monocrystalline silicon solar cells are still better than polycrystalline silicon solar cells.
In the utilization of solar photovoltaic, monocrystalline silicon and polycrystalline silicon solar cells play a huge role. At present, in order to make solar photovoltaic technology have a large 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 development process of international solar cells, it can be seen that its development trend is monocrystalline silicon, polycrystalline silicon, ribbon silicon, and thin film materials (including microcrystalline silicon-based thin films, compound-based thin films and dye thin films). From the perspective of industrialization development, the center of gravity has developed from single crystal to polycrystalline. The main reasons are:
(1) There are fewer and fewer head and tail materials available for making monocrystalline silicon solar cells.
(2) For solar cells, the square substrate is more cost-effective, and the polysilicon obtained by the casting method and the direct solidification method can directly obtain the square material.
(3) The production process of polysilicon has made continuous progress. The fully automatic casting furnace can produce silicon ingots of more than 200kg per production cycle (50h), and the size of the crystal grains reaches the centimeter level.
(4) Polycrystalline silicon thin film solar cells use much less silicon than monocrystalline silicon solar cells, so there is no problem of efficiency decline, and it is possible to prepare them on cheap substrate materials.
(5) The cost of polycrystalline silicon thin film solar cells is much lower than that of monocrystalline silicon solar cells, and the photoelectric conversion rate is nearly 12.4%, which is higher than that of amorphous silicon thin film solar cells.
Due to the rapid research and development of monocrystalline silicon technology in the past decade, its technology has also been applied to the production of polycrystalline silicon solar cells, such as selective etching of emitter junctions, back surface fields, etching texture, surface and bulk passivation, fine metal gate electrodes The use of 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 polysilicon can greatly shorten the process time, and the single-chip thermal process time can be within one minute. Completed, the conversion efficiency of solar cells made on 100cm² polycrystalline silicon wafers using this process exceeds 14%. It has been reported that the efficiency of solar cells fabricated on 50-60 μm polysilicon substrates currently exceeds 16%. The solar cell conversion efficiency of 100cm² polycrystalline silicon wafers using mechanical groove and screen printing technology exceeds 17%, and the efficiency of solar cells without mechanical grooves reaches 16% in the same area. The cell conversion efficiency reaches 15.8%.