Improvement of the Open Circuit Voltage of CZTSe Thin-Film Solar Cells by Surface Sulfurization Using SnS

The objective of this study is to find an effective method to improve V_oc without J_sc loss for Cu₂ZnSnSe₄ (CZTSe) thin film solar cells, which have been fabricated by the one step co-evaporation technique. Surface sulfurization of CZTSe thin films is carried out by using one technique that does not utilize toxic H₂S gas; a sequential evaporation of SnS after CZTSe deposition and the annealing of CZTSe thin films in selenium vapor. A Cu₂ZnSn(S,Se)₄ (CZTSSe) thin layer is grown on the surface of the CZTSe thin film after the annealing. The conversion efficiency of the completed device is improved due to the enhancement of V_oc, which could be attributed to the formation of a hole-recombination barrier at the surface or the passivation of the surface and grain boundary by S incorporation.