As the materials of solar cells, III-V compounds are prepared from rare elements. Although the conversion efficiency of solar cells made from this is very high, from the perspective of material sources, this type of solar cells will not be able to dominate in the future. The other two types of solar cells, nanocrystalline solar cells and polymer-modified electrode solar cells have just started research, the technology is not very mature, and the conversion efficiency is still relatively low. These two types of batteries are still in the exploratory stage, and it is impossible to replace silicon solar cells in a short period of time. Therefore, from the perspective of conversion efficiency and material sources, the focus of future development is still silicon solar cells, especially polycrystalline silicon and amorphous silicon thin film cells. Because of the high conversion efficiency and relatively low cost of multi-product silicon and amorphous silicon thin-film batteries, they will eventually replace monocrystalline silicon batteries and become the leading products in the market.
Increasing the conversion efficiency and reducing the cost are the two main factors considered in the preparation of solar cells. For the current silicon-based solar cells, it is more difficult to further improve the conversion efficiency. Therefore, in addition to continuing to develop new solar cell materials, the focus of future research should be on how to reduce costs; the existing solar cells with high conversion efficiency are made on high-quality silicon wafers, which is the most expensive part of manufacturing silicon solar cells. Therefore, it is particularly important to reduce the cost of the substrate when the conversion efficiency is still high, and this is also an urgent problem to be solved in the development of solar cells in the future. Recently, foreign countries have used certain technologies to produce silicon strips as the substrate of polycrystalline silicon thin-film solar cells to achieve the purpose of reducing costs, and the effect is still relatively ideal. 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.
(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, improve the cutting, grinding, polishing and other processes of silicon wafers to reduce costs, and improve the efficiency of solar cells.
(4) Thin-film polysilicon cells are still under vigorous research and development. 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.
The simplest definition of solar photoelectric conversion efficiency is: the proportion of photons hitting the solar cell that can be converted into usable current. In fact, the calculation of efficiency is more complicated, and it is not enough to just absorb light and generate free carriers. In order to generate usable energy, electron and hole carriers must reach the electrodes of the solar cell. If the electron-hole pairs formed by incident photons recombine too quickly in the process of moving toward the electrode, then they will not be able to bring any help to the photocurrent.
Many problems may reduce the mobility of carriers (or increase the possibility of recombination). For single-junction cells, bulk crystalline silicon solar cells have the least problems but are highly efficient. This is because the process of flowing carriers from the junction surface they generate to the battery electrode that provides the load is relatively simple: the electron-hole pairs are separated separately and pass through the commensurate N-type or P-type material. Polycrystalline silicon solar cells work in the same way, but due to grain boundary problems, the recombination phenomenon of electron-hole pairs increases, and the mobility and conversion efficiency are also reduced to levels close to those of thin-film cells using CIGS (15%~18%).