Regulating the NaSex-assisted crystallization of Cu2ZnSn(S,Se)4 film by modifying the reaction between Na source and Se in the co-selenization process

Abstract: The NaSex-assisted crystallization plays an important role in determining the photo-electric quality of Cu2ZnSn(S,Se)4 (CZTSSe) films. However, the method to regulate such process is rarely reported. Herein, a co-selenization process is developed to regulate the NaSex-assisted crystallization in which different sodium sources are utilized together with Se. By changing the type of the sodium source, the reaction between the sodium source and Se can be altered, which results in NaSex flux with different properties. When Na2S is used as sodium source, NaSex with optimum properties can be generated. In this condition, the kinetic process for the growth of the CZTSSe grains can be modified, which helps to improve the defect formation of the film. Moreover, the NaSex flux could penetrate the grain-boundaries of the crystallized layer. This can enhance the bottom-to-top diffusion of the elements, thus eliminating the fine-grain layer. By using Na2S/Se co-selenization, both the defect properties and the morphology of the CZTSSe film are improved. The fabricated CZTSSe solar cells achieve the best efficiency of 12.4% (without anti-reflection coating). This paper clarifies the mechanism of the NaSex-assisted crystallization, which provides a new route for the crystallization optimization of CZTSSe film.