Solar cells are a type of photoelectric element that can convert energy. Their basic structure is formed by combining P-type and N-type semiconductors. The most basic material of semiconductors is "silicon", which is non-conductive. However, if different impurities are added to semiconductors, P-type and N-type semiconductors can be made. Then, the potential difference between the P-type semiconductor with a hole (the P-type semiconductor lacks a negatively charged electron, which can be regarded as an additional positive charge) and the N-type semiconductor with an additional free electron is used to generate current. Therefore, when sunlight shines, the light energy excites the electrons in the silicon atoms, and produces convection of electrons and holes. These electrons and holes are affected by the built-in potential and are attracted by the N-type and P-type semiconductors respectively, and gather at both ends. At this time, if the outside is connected with electrodes to form a circuit, this is the principle of solar cell power generation.
Solar cells can be divided into two categories according to their crystal state: crystalline thin film type and non-crystalline thin film type (hereinafter referred to as a-), and the former is further divided into single crystal type and polycrystalline type.
According to the material, they can be divided into silicon thin film type, compound semiconductor thin film type and organic film type, and compound semiconductor thin film type is further divided into non-crystalline type (a-Si:H, a-Si:H:F, a-SixGel-x:H, etc.), IIIV group (GaAs, InP, etc.), IIVI group (Cds series) and zinc phosphide (Zn3p2), etc.
According to the different materials used, solar cells can also be divided into: silicon solar cells, multi-compound thin film solar cells, polymer multilayer modified electrode solar cells, nanocrystalline solar cells, organic solar cells, plastic solar cells, among which silicon solar cells are the most mature and dominate in applications.
1. Silicon solar cells
Silicon solar cells are divided into three types: single crystal silicon solar cells, polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells.
(1) Single crystal silicon solar cells have the highest conversion efficiency and the most mature technology. The highest conversion efficiency in the laboratory is 24.7%, and the efficiency in large-scale production is 15% (as of 2011, it is 18%). It still occupies a dominant position in large-scale applications and industrial production, but due to the high cost of single-crystalline silicon, it is difficult to significantly reduce its cost. In order to save silicon materials, polycrystalline silicon thin film and amorphous silicon thin film have been developed as alternatives to single-crystalline silicon solar cells.
(2) Compared with single-crystalline silicon, polycrystalline silicon thin film solar cells are cheaper and more efficient than amorphous silicon thin film cells. Its highest laboratory conversion efficiency is 18%, and the conversion efficiency of industrial-scale production is 10% (as of 2011, it is 17%). Therefore, polycrystalline silicon thin film cells will soon occupy a dominant position in the solar cell market.
(3) Amorphous silicon thin film solar cells are low in cost and light in weight, with high conversion efficiency, easy to mass produce, and have great potential. However, due to the photoelectric efficiency decay effect caused by its material, its stability is not high, which directly affects its practical application. If the stability problem can be further solved and the conversion rate problem can be improved, then amorphous silicon solar cells will undoubtedly be one of the main development products of solar cells.
2. Crystalline thin film solar cells
Polycrystalline thin film cells Cadmium sulfide and cadmium telluride polycrystalline thin film cells are more efficient than amorphous silicon thin film solar cells, cheaper than monocrystalline silicon cells, and easy to mass produce. However, cadmium is highly toxic and will cause serious environmental pollution. Therefore, it is not the most ideal alternative to crystalline silicon solar cells.
The conversion efficiency of gallium arsenide (GaAs) III-V compound cells can reach 28%. GaAs compound materials have a very ideal optical band gap and high absorption efficiency, strong radiation resistance, and are insensitive to heat. They are suitable for manufacturing high-efficiency single-junction cells. However, the price of GaAs materials is high, which greatly limits the popularity of GaAs cells.
Copper indium selenide thin film cells (CIS for short) are suitable for photoelectric conversion, do not have the problem of light-induced degradation, and have the same conversion efficiency as polycrystalline silicon. With the advantages of low price, good performance and simple process, it will become an important direction for the development of solar cells in the future. The only problem is the source of the material. Since indium and selenium are relatively rare elements, the development of this type of battery is inevitably limited.
3. Organic polymer solar cells
Replacing inorganic materials with organic polymers is a newly developed research direction for solar cell manufacturing. Due to the advantages of good flexibility, easy production, wide material sources and low cost of organic materials, it is of great significance to the large-scale use of solar energy and the provision of cheap electricity. However, the research on the preparation of solar cells with organic materials has just begun. Whether it can be developed into a product with practical significance remains to be further studied and explored.
4. Nanocrystalline solar cells
Nanocrystalline solar cells are newly developed. Their advantages are their low cost, simple process and stable performance. Their photoelectric efficiency is stable at more than 10%, and the production cost is only 1/5 to 1/10 of that of silicon solar cells. The life span can reach more than 20 years. The research and development of such batteries has just started, and they will gradually enter the market in the near future.