The effect of S/(S+Se) ratios on the formation of secondary phases in the band gap graded Cu2ZnSn(S,Se)4 thin film solar cells

Abstract

Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers with various S/(S + Se) ratios at the surface and in the bulk were synthesized to investigate the effect of the S/(S + Se) ratios on the formation of secondary phases in the CZTSSe absorber layer. The higher density ZnS and Cu2SnSe3 (CTSe) secondary phases were observed at the surface of the CZTSSe absorber layer when the S/(S + Se) ratios were 0.46 and 0.40 at the surface and in the bulk, respectively. The ZnS and CTSe secondary phases were considerably reduced at the surface of the absorber layers as the S/(S + Se) ratios decreased. The short circuit current density (Jsc) and fill factor were improved by the reduction of ZnS and CTSe secondary phases. Also, the open circuit voltage (Voc) increased even though the band gap energy decreased. However, the Jsc, Voc, and fill factor of the CZTSSe thin film solar cells with the lowest S/(S + Se) ratio of 0.31 at the surface were slightly degraded by a parasitic 960-nm-thick Mo(S,Se)2 layer. The best power conversion efficiency of 6.2%, Voc of 0.43 V, Jsc of 31.1 mA/cm2, and fill factor of 45.5% were achieved when the S/(S + Se) ratios at the surface and in bulk are 0.32 and 0.17, respectively.