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

Abstract

The NaSe_x-assisted crystallization may play important roles in determining the properties of Cu₂ZnSn(S,Se)₄ (CZTSSe) films. However, the mechanism and method to regulate such process are rarely reported. Herein, it is found the NaSe_x flux mainly results from the reaction between Se and sodium contained matter (e.g., the soda-lime glass substrate) in the selenization process. Accordingly, co-selenization processes are proposed to regulate NaSe_x-assisted crystallization in which different sodium sources (NaF, Na₂S, etc.) are utilized together with Se. By changing the type of sodium source, the reaction between the sodium source and Se can be altered, which results in NaSe_x flux with different properties. Consequently, the NaSe_x-assisted crystallization of CZTSSe film can be regulated by modifying the properties of NaSe_x flux. When Na₂S is used as sodium source, NaSe_x with optimum properties can be generated. In this condition, the kinetic process for the growth of CZTSSe grains may be modified, which improves the defect formation of the film. Moreover, the NaSe_x flux could penetrate through 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 uniformity and defect properties of the CZTSSe film are improved. The fabricated CZTSSe solar cells achieve the best efficiency of 12.4 % (without anti-reflection coating). This work clarifies the mechanism of the NaSe_x-assisted crystallization, which provides a new route for the crystallization optimization of CZTSSe film.