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 24%, and the conversion efficiency during mass production is 15%. Silicon solar cell technology is relatively mature, the band gap of semiconductor materials is not too wide, the photoelectric conversion efficiency is high, and the material itself does not cause pollution, so silicon is currently the most ideal solar cell material. Monocrystalline silicon and polycrystalline silicon batteries are a kind of silicon ore that uses sintering, crystal pulling, and chip making processes, and then cuts them into appropriate small pieces according to the relevant process requirements, and connects them together by welding wires to form a group of pieces. Because its substrate is very thin, low-power solar cells need to be installed on an insulating substrate for use, while high-power solar cells are laminated on the insulating substrate with strengthened glass, and finally an aluminum alloy frame is added for protection to make a flat solar cell. The difference between monocrystalline silicon cells and polycrystalline silicon cells is that the surface of polycrystalline silicon has a large area of ice flower-like patterns, while monocrystalline silicon cells are fine particles, and their surfaces are coated with a blue or purple anti-reflective film. Occupy a leading position in large-scale applications and industrial production. However, due to the high cost of monocrystalline silicon and the cumbersome manufacturing process, the cost of monocrystalline silicon remains high, which has become a major obstacle to the development of monocrystalline silicon solar cells, and it is difficult to significantly reduce its cost. In order to save silicon materials, polycrystalline silicon thin films and amorphous silicon thin films have been developed as alternative products for monocrystalline silicon solar cells.
Compared with monocrystalline silicon solar cells, polycrystalline silicon thin-film solar cells have lower cost and higher efficiency than amorphous silicon thin-film cells. The maximum conversion efficiency of the laboratory is 18%, and the conversion efficiency of industrial-scale production is 10%. Therefore, polycrystalline silicon thin film batteries 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 a PN junction. Monocrystalline silicon solar cells are limited to the size of single crystals, and it is difficult to make monolithic cells have a large area. At present, relatively large wafers with a diameter of 10-20 cm are available. Polycrystalline silicon cells are made of cast polycrystalline silicon ingot slices, and the cost is lower than that of monocrystalline silicon cells. Monolithic cells can also be made relatively large (for example, 30cm×30cm square slices), but due to factors such as grain boundary recombination, the efficiency is lower than that of monocrystalline silicon cells. At present, the research work of monocrystalline silicon and polycrystalline silicon cells mainly focuses on the following aspects:
(1) Use buried electrode, surface passivation, close grid technology to optimize the back electric field and contact electrode to reduce the recombination loss of photo-generated carriers, improve the collection efficiency of carriers, and improve the efficiency of solar cells. The Green Laboratory of the University of South Wales in Australia has adopted these methods and has developed the highest efficiency of 24% under AM1.5 conditions recognized by the silicon solar cell industry.
(2) Reduce light reflection and transmission loss with optimized anti-reflection film, concave-convex surface, high-reverse back electrode, etc., to improve solar cell efficiency.
(3) Use cast polycrystalline silicon ingots grown by the directional solidification method to replace monocrystalline silicon, optimize the screen printing process of silver paste and aluminum paste of the front and back electrodes, and improve the cutting, grinding, polishing and other processes of silicon wafers, to improve the efficiency of solar cells.
Calculations show that if a large-area high-quality polysilicon film with a thickness of 30-50um can be prepared on metal, ceramic, glass and other substrates at low cost, the solar cell manufacturing process can be further simplified and the cost can be greatly reduced. Therefore, polycrystalline silicon thin film solar cells are becoming a research hotspot.
Both polycrystalline silicon and monocrystalline silicon are a form of elemental silicon. When molten elemental silicon solidifies under supercooling conditions, silicon atoms are arranged in the form of a diamond lattice into many crystal nuclei. These crystal nuclei grow into crystal grains with different crystal plane orientations, and these crystal grains combine to crystallize into polysilicon. Polycrystalline silicon can be used as a raw material for pulling monocrystalline silicon. The essential difference between polycrystalline silicon and monocrystalline silicon is that there are grain boundaries in polycrystalline silicon. A large number of studies have shown that the size and shape of crystal grains in polysilicon, the nature of grain boundaries, and the content and distribution of impurities in different crystal grains are the main factors affecting the performance of polysilicon solar cells. The difference between polycrystalline silicon and monocrystalline silicon is mainly manifested in physical properties. For example, in terms of mechanical properties, optical properties, and thermal properties, it is far less obvious than single crystal silicon; in terms of electrical properties, the conductivity of polycrystalline silicon crystals is also far less significant than single crystal silicon. However, from the perspective of industrialization development, the center of gravity has developed from single crystal to polycrystalline. The main reasons are as follows:
(1) There are fewer and fewer head and tail materials available for the production of semiconductor devices for monocrystalline silicon solar cells.
(2) For solar cells, square substrates are more cost-effective, and square materials can be directly obtained from polysilicon obtained by casting and direct solidification methods.
(3) Continuous progress has been made in the production process of polysilicon. 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 batteries use much less silicon than single crystal silicon batteries, so there is no problem of efficiency degradation, and it is possible to prepare them on cheap substrate materials.
(5) Due to the rapid research and development of monocrystalline silicon technology in the past ten years, its technology has also been applied to the production of polycrystalline silicon cells. The conversion efficiency of polycrystalline silicon cells has been greatly improved, and the photoelectric conversion efficiency is nearly 12.4%, which is higher than that of amorphous silicon thin film cells.