The production of solar cells is mainly based on semiconductor materials, and its working principle is to use photoelectric materials to absorb light energy and generate a photoelectric conversion effect. Depending on the materials used, solar cells are classified as shown in Figure 1. According to the crystalline state, solar cells can be divided into two categories: crystalline thin film type and amorphous thin film type, and the former is further divided into single crystal type and polycrystalline type. According to the material, it can be divided into silicon film type, compound semiconductor film type and organic film type, and compound semiconductor film type is divided into amorphous type (a-Si: H, a-Si: H: F, a-SixGel-x: H, etc.), III, V group (GaAs, InP, etc.), II, VI group (Cds system) and zinc phosphide (Zn3P2) and so on.
Silicon solar cells are divided into three types: monocrystalline silicon solar cells, polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells. Monocrystalline silicon solar cells have the highest conversion efficiency, the highest conversion efficiency in the laboratory is 23%, and the efficiency in mass production is 15%. Silicon solar cell technology is relatively mature, the forbidden band of semiconductor materials is not too wide, the photoelectric conversion rate is high, and the material itself does not cause pollution, so silicon is currently the most ideal solar cell material. It occupies a dominant position in large-scale applications and industrial production, but due to the high cost of monocrystalline silicon, it is difficult to greatly reduce its cost. In order to save silicon materials, polycrystalline silicon thin films and amorphous silicon thin films have been developed as monocrystalline silicon solar cells. replacement product.
Compared with monocrystalline silicon solar cells, polycrystalline silicon thin film solar cells have low cost and higher efficiency than amorphous silicon thin film solar cells. The highest conversion efficiency in the laboratory is 18%, and the conversion efficiency in industrial scale production is 10%. Therefore, polycrystalline silicon thin film solar cells will soon dominate the solar cell market.
Monocrystalline silicon and polycrystalline silicon solar cells are made of P-type (or N-type) silicon substrates through phosphorus (or boron) diffusion to form PN junctions. Monocrystalline silicon solar cells are limited to the size of a single crystal, and it is difficult to make a single cell with a large area. At present, the larger diameter of the wafer is 10~20cm. Polycrystalline silicon solar cells are made from cast polycrystalline silicon ingot slices, and the cost is lower than that of monocrystalline silicon solar cells. , the efficiency is lower than that of monocrystalline silicon solar cells. At present, the research work of monocrystalline silicon and polycrystalline silicon solar cells mainly focuses on the following aspects:
(1) Use buried layer electrodes, surface passivation, and dense grid technology to optimize the back electric field and contact electrodes to reduce the recombination loss of photogenerated carriers, improve the collection efficiency of carriers, and thus improve the efficiency of solar cells. Using these methods, the Green Laboratory at the University of South Wales in Australia has developed the highest efficiency currently recognized by the silicon solar cell industry at AM1.5 conditions of 24%.
(2) Reduce the reflection and transmission loss of light by optimizing the anti-reflection film, concave-convex surface, high reverse back electrode, etc. to improve the efficiency of solar cells.
(3) Use the cast polycrystalline silicon ingot grown by the directional solidification method to replace the single crystal silicon, optimize the screen printing process of the silver paste and aluminum paste of the front and back electrodes, and improve the cutting, grinding, polishing and other processes of the silicon wafer, so as to improve the efficiency of solar cells. efficient.
Calculations show that if a large-area high-quality polysilicon film with a thickness of 30~50μm can be prepared on a metal, ceramic, glass and other substrate at a low cost, the solar cell manufacturing process can be further simplified and the cost can be greatly reduced. Therefore, polysilicon thin film solar energy Batteries are becoming a research hotspot.