(1) Gallium arsenide (GaAs) thin film solar cells.
Gallium arsenide (GaAs) is a typical III-V compound semiconductor material, an electronic material and photoelectric material second only to silicon, and an important thin-film solar cell material. It is stable in air and water vapor, insoluble in hydrochloric acid, but soluble in nitric acid and aqua regia. It can be oxidized above 600C, and chemical dissociation can occur above 800C. There are mainly covalent bonds between Ga and As atoms, but also some ionic bonds. The characteristics of gallium arsenide thin-film solar cells are: ①The theoretical efficiency of photoelectric conversion is higher than that of silicon cells, and the absorption rate of light is high. As long as the thickness is 3μm, it can absorb 95% of the solar energy, saving materials; ②High temperature resistance and resistance The radiation performance is better than that of silicon solar cells; ③In the same doping, the electrical internal resistance is small, and the energy consumption is small; ④The PN junction has high self-built electric field, high open circuit voltage, good spectral response characteristics and long service life. However, the preparation of gallium arsenide has high energy consumption, complex production equipment, long production cycle, and high cost. It was originally only used in aerospace engineering, but it is gradually being used in other fields. If two or more layers of gallium arsenide are superimposed together, a double-junction or multi-junction GaAs thin-film solar cell can be made, which can receive more solar energy spectrum and improve the photoelectric conversion efficiency. The photoelectric conversion efficiency can theoretically reach 40% , But the cost is too high. Currently researching high-efficiency and low-cost multi-junction GaAs thin-film solar cells.
(2) Cadmium telluride (CdTe) and cadmium sulfide (Cds) thin-film solar cells.
CdTe is a Ⅱ-VI compound semiconductor material. Its polycrystalline film has a band gap of 1.45 eV and a theoretical photoelectric conversion efficiency of about 29% (currently more than 16% in the laboratory). The preparation process is simple and it is a highly efficient, stable, Low-cost thin-film solar cell materials are easy to achieve large-scale production. CdTe is stable and non-toxic at room temperature, but the decomposed Cd and Te are toxic, waste gas and wastewater are difficult to treat, and Te is a rare metal that is more expensive and costly. Nevertheless, due to the high efficiency and low cost of CdTe, if it can solve the existing problems, it is still attractive in application. Cds is a direct band gap optoelectronic material, the gap energy band is about 2.4eV, and the light absorption coefficient is higher than 104~105cm-1. Cds is mainly used as an N-type semiconductor window material for thin-film solar cells, and can form a heterojunction with good performance with thin-film materials such as CdTe and CulnSea.
(3) CulnSe2 (CulnS2).
The ternary compound CulnSe2 (CIS) film material has a large light absorption coefficient, reaching 105cm-1, the forbidden bandwidth is 1.04eV, and the theoretical light-to-electric conversion efficiency reaches 25%~30%, which can be absorbed by only 1~2μm thick film 99% of sunlight can greatly reduce the cost of solar photovoltaic materials, and the development prospects are very good. The CulnSe2 in the solar cell has a chalcopyrite structure and is stable from room temperature to 800C. The rare element In in CulnSe2 is replaced by 1%~30% Ga to become CulnxGa1-xSe2 (CIGS). As the concentration of Ga increases, its band gap increases linearly from 1.02 to 1.68 eV. Using CIGS as an absorption layer can reduce costs and increase the efficiency of solar cells. Therefore, most of the current CulnSe2 solar cells are doped with Ga.