In order to find alternatives to monocrystalline silicon cells, in addition to polycrystalline silicon and amorphous silicon thin-film solar cells, people continue to develop solar cells of other materials. Among them are mainly arsenic compounds like III-V, cadmium sulfide, cadmium sulfide and copper-indium-selenium thin-film batteries.
Multi-element compound solar cells refer to solar cells that are not made of a single element semiconductor material. Among compound semiconductor solar cells, CaAs, InP, CuInSe2 and CdTe solar cells are currently researched and applied. Because compound semiconductors are more or less toxic and easily cause environmental pollution, they have a small output and are often used in some special occasions. The multi-compound thin-film solar cell materials are inorganic salts, which mainly include arsenic compounds, cadmium sulfide and copper-indium-selenium thin-film batteries.
1. Gallium arsenide solar cell
The conversion efficiency of gallium arsenide (GaAs) III-V compound battery can reach 28%. GaAs compound materials have a very ideal optical band gap, high absorption efficiency, strong resistance to radiation, and are not sensitive to heat, making them suitable for manufacturing high-efficiency single junction cells. However, the high price of GaAs materials limits the popularity of GaAs batteries to a large extent. Currently, gallium arsenide solar cells are mostly prepared by liquid phase epitaxy or metal organic chemical vapor deposition (MOCVD) technology. Therefore, the cost is high and the output is limited. Cost reduction and production efficiency improvement have become the research focus.
At present, silicon single wafer preparation technology is mature and low cost, so it is a very promising way to reduce the cost of GaAs solar cells by using silicon wafers as substrates and using heteroepitaxial methods based on MOCVD technology to manufacture GaAs solar cells. At present, the efficiency of this battery has reached more than 20%. However, the lattice constants of GaAs and Si crystals are relatively related. When performing heteroepitaxial growth, the epitaxial layer lattice mismatch is serious, and it is difficult to obtain a high-quality epitaxial layer. For this reason, a layer of Ge crystal whose lattice constant is less different from that of GaAs is often grown on the Si substrate as a transition layer, and then the GaAs epitaxial layer is grown. This kind of heteroepitaxial battery with Si/Ge/GaAs structure is under continuous development. By controlling the thickness of each layer and changing the structure appropriately, the photon energy of various wavelengths in sunlight can be effectively used. At present, the efficiency of multi-layer solar cells based on GaAs is close to 40%.
2. Indium Phosphide Solar Cell
Indium phosphide solar cells have particularly good anti-radiation performance, so they are paid attention to in aerospace applications. At present, the efficiency of this kind of battery has reached 17% to 19%.
3. Nanocrystalline chemical solar cells
Since Professor Gratzel of Switzerland successfully developed nano-TiO2 chemical solar cells, research in this area is also ongoing in China.
Nanocrystalline chemical solar cells (NPC cells for short) are formed by a kind of band gap semiconductor material modified and assembled on another kind of high band gap semiconductor material. The narrow band gap semiconductor material uses organic compound dyes such as transition metal Ru and Os, and the high band gap semiconductor material uses nano-polycrystalline TiO2 to make electrodes. In addition, nanocrystalline chemical batteries also use appropriate redox electrolytes. The photoelectric conversion efficiency of nanocrystalline TiO2 solar cells is more than 10%, the production cost is 1/5~1/10 that of silicon solar cells, and the life span can reach more than 20 years. However, since the research and development of this type of battery has just started, it is estimated that it will gradually enter the market soon.
4. Polymer multilayer modified electrode type solar cell
Substituting polymers for inorganic materials in solar cells is a research direction of solar cells that has just begun. The principle is to use the different redox electromotive force of different redox polymers to carry out multi-layer composite on the surface of the conductive material (electrode) to make a unidirectional conductive device similar to an inorganic PN junction. The inner layer of one of the electrodes is modified by a polymer with a lower reduction potential, and the outer layer polymer has a higher reduction potential, and the direction of electron transfer can only be transferred from the inner layer to the outer layer; the modification of the other electrode is just the opposite, and the reduction potential of the two polymers on the first electrode is higher than the reduction potential of the latter two polymers. When two modified electrodes are placed in an electrolyte containing a photosensitizer, the electrons generated after the photosensitizer absorbs light are transferred to the electrode with a lower reduction potential, and the electrons accumulated on the electrode with a lower reduction potential cannot be transferred to the outer polymer, and can only generate photocurrent through the external circuit.
The raw materials of polymer multilayer modified electrode solar cells are organic materials, which are flexible, easy to manufacture, widely sourced, and low cost. This research has just started, and neither the service life nor the battery efficiency can be compared with inorganic materials, especially silicon batteries. Whether it can be developed into a product of practical significance remains to be further studied and explored.