Photovoltaic cell are classified according to materials, which can be divided into two categories: silicon photovoltaic cell and compound photovoltaic cell (see figure). This chapter introduces the types of silicon photovoltaic cell.
There are two types of silicon photovoltaic cell, namely crystalline silicon photovoltaic cells and amorphous silicon photovoltaic cell.
① Crystalline silicon photovoltaic cells. It can be further divided into monocrystalline silicon photovoltaic cells and polycrystalline silicon photovoltaic cells.
a. Monocrystalline silicon photovoltaic cells. It is a photovoltaic cell developed earlier, with the highest conversion efficiency and larger output. At present, the conversion efficiency of monocrystalline silicon photovoltaic cells in my country has reached an average of 16.5%, and the highest conversion efficiency recorded in the laboratory exceeds 24.7%. This photovoltaic cell generally uses high-purity monocrystalline silicon rods as raw materials, with a purity requirement of 99.9999%. In order to reduce production costs, solar-grade monocrystalline silicon rods are now used in photovoltaic cells for ground applications, and the material performance indicators have been relaxed. Some can also use the head and tail materials and waste monocrystalline silicon materials processed by semiconductor devices, which can be made into monocrystalline silicon rods for photovoltaic cells after re-furnace drawing. Cut the monocrystalline silicon rod into silicon wafers, the thickness of the wafers is generally 180~220um. After the silicon wafer has undergone processes such as testing, cleaning, and texturing, the surface layer is doped and diffused with trace elements such as boron, phosphorus, brocade, etc., to form a PN junction, which has the basic characteristics of a battery. In order to prevent a large number of photons from being reflected off the smooth silicon wafer surface, it is necessary to coat a silicon nitride anti-reflection film on the silicon wafer surface by PECVD method, etc., which also plays a protective role. Then, after dephosphorization silicon glass and plasma etching, screen printing is used to print the prepared silver paste on the silicon wafer to make grid lines, and at the same time make the back electrode, and then go through the sintering process to make a single product Silicon photovoltaic cells.
b. Polycrystalline silicon photovoltaic cells. It is a photovoltaic cell based on multi-product silicon material. Since polysilicon materials mostly use casting to replace the drawing process of single crystal silicon, the production time is shortened and the manufacturing cost is greatly reduced. In addition, the monocrystalline silicon rods are cylindrical, and the photovoltaic cells made with this are also wafers, so the plane utilization rate of photovoltaic modules is low. Compared with monocrystalline silicon photovoltaic cells, polycrystalline silicon photovoltaic cells appear to have certain competitive advantages. However, during the growth of polysilicon materials, due to the effect of thermal stress, a large number of dislocations will be generated in the crystal grains. In addition, the accumulation of metal impurities and oxygen-carbon impurities on the dislocations will cause recombination centers and uneven electrical properties, which will greatly reduce the minority carrier life and affect the conversion efficiency of photovoltaic cells. The manufacturing process of polycrystalline silicon photovoltaic cells is not much different from that of monocrystalline silicon photovoltaic cells, and the equipment used is basically the same, except that the recombination loss of photo-generated carriers at the grain boundaries of the polycrystalline silicon photovoltaic cells should be minimized when manufacturing polycrystalline silicon photovoltaic cells. In recent years, the research and development of polycrystalline silicon cells have been changing with each passing day. Process measures such as phosphorus and aluminum gettering, hydrogen passivation, and the establishment of interface fields have been adopted to greatly improve the conversion efficiency of photovoltaic cells. At present, the conversion efficiency of industrialized multi-product silicon cells has reached 14% to 16%.
With the advancement of science and technology, market demand and the guidance of industrial policies from countries around the world, in recent years, polycrystalline silicon photovoltaic cells have developed rapidly, and a variety of new manufacturing processes have emerged at the same time. Therefore, according to the manufacturing process, polycrystalline silicon photovoltaic cells can be divided into cast polycrystalline silicon photovoltaic cells, ribbon polycrystalline silicon photovoltaic cells and thin film polycrystalline silicon photovoltaic cells.
②Amorphous silicon photovoltaic cells. It is a new type of thin film battery made from amorphous silicon.
Amorphous silicon is a semiconductor with an amorphous crystal structure. Photovoltaic cells made with it are only 1μm thick, which is equivalent to 1/300 of that of monocrystalline silicon photovoltaic cells. Compared with monocrystalline silicon and polycrystalline silicon, its manufacturing process is greatly simplified, the consumption of silicon material is less, and the unit power consumption is also reduced a lot. In addition, it has the advantage of low-light power generation, so it is widely used in electronic computers, electronic clocks and copiers. The PECVD method is generally used to produce amorphous silicon cells, and the main processes include glass cleaning, vapor deposition, laser scribing and magnetron sputtering. In order to solve the performance deficiencies of amorphous silicon photovoltaic cells, people began to study a laminated photovoltaic cell. It combines materials with different band gaps to increase the response range of the spectrum, reduce attenuation and increase conversion efficiency. At present, the highest conversion efficiency of single-junction photovoltaic cells made by American companies is 9.3%, and the highest conversion efficiency of triple-layer cells is 13%. Because amorphous silicon photovoltaic cells have the characteristics of simple process, low silicon material consumption, low cost, light weight, low light power generation, and strong adaptability, they will become the most promising photovoltaic power generation material
