Crystallization mechanism and defect passivation of Cu2ZnSn(S,Se)4 thin film solar cells via in situ potassium doping

Abstract

Alkali metal doping has achieved prominent results in Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film solar cells. In this paper, in situ K-doped CZTSSe thin film solar cells were prepared by magnetron sputtering with a K-containing CZTS target. The role and mechanism of K doping in the crystallization process of CZTSSe are investigated. It demonstrates that K doping can promote the formation of compact crystals with less pinholes on the surface of CZTSSe. The diffusion of Se from the surface towards the back contact is hindered during the selenization process, which gives CZTSSe films with smaller grains. However, electrical characterization indicated that the K-doped CZTSSe cells have better device performance and fewer defects in the depletion region and at the heterojunction interface. It is speculated that defects of absorbers are passivated by the improved electrical properties of the grain boundary and optimized heterojunction band alignments. The maximum efficiency of the K-doped CZTSSe solar cell reaches 12.6% (certified efficiency of 12.67%). This work provides a simple preparation method for alkali metal-doped CZTSSe absorbers, and proves that alkali metal doping has great potential in the development of CZTSSe solar cells.