Preparation And Characterization Of CdS Thin Films For CdS/CdTe Solar Cells

MENTOR:Professor Xiaoshan Wu、Professor Fengming ZhangCdTe/CdS is a new type of thin-film solar cell which is very popular these years. CdTe material is Ⅱ-Ⅵ compound semiconductor, it is the direct band gap material, with the band gap of1.45eV, which match the energy gap width of the solar spectrum. CdS material is an excellent window layer with the band gap of2.42eV. It can be formed with CdTe to be good heterojunction, thereby forming a heterojunction solar cell. Therefore, the quality of the CdS film directly affect the quality of the CdTe which is grown on the surface. It also has a significant impact on the ultimate performance of the solar cell.This paper studies the basic principles and experimental methods of the chemical bath deposition method, high vacuum thin-film evaporation method, and sol-gel method which can be used to prepare CdS thin film. By the change of the experimental conditions of the preparation method, we choose the best one. The structure, morphology, optical characteristics are analyzed to explore the impact of the growth process of CdS thin films. Eventually, About the chemical bath deposition method, the pH of the solution is higher and S27Cd2+is smaller, the rate of chemical reaction is lower. If the reaction rate is lower, the surface of the prepared CdS thin films is more uniform and the light-transmission is better, more than80%. About the high vacuum evaporation method, the better the degree of vacuum in the vacuum chamber, the better the crystallization of the prepared CdS. For the sol-gel method, the larger the coefficient of viscosity of the gol used, the CdS thin film is more uniform. The thickness can be obtained that h=kηβ/ωp, where η is the viscosity coefficient and ω is the rotating speed. The same time, we make use of XRD analysis to confirm that the smaller the particle size of the thin film, the bigger the bandgap of the film. We obtained the specific relationship is Eg∝(1/L)x, where in L is the size of the nano-grain. Using these three methods, we make the CdS thin films with thickness of100nm. We find that the chemical bath deposition method may prepare CdS thin film with a uniform surface, high vacuum evaporation method can prepare CdS thin film with a grate crystallization that, the sol-gel method can be used to make the thin films with bigger band gap, which can be2.52eV. Besides that it is applicable to the complex surface of a substrate requiring step coverage. For example the surface of CdTe with the periodic structure.