Efficiency improvement of Cu2ZnSn(S,Se)4 submodule with graded bandgap and reduced backside ZnS segregation

A new record conversion efficiency of 11.0% was achieved on a Cu₂ZnSn(S,Se)₄ (CZTS) submodule by introducing a bandgap grading. The S/(S+Se) compositional ratio was graded from the bottom (higher S/(S+Se)) to the top (lower S/(S+Se)) of the CZTS absorber layer to modify the conduction band minimum. The submodule with the S-graded CZTS layer exhibited V_oc and J_sc improvements, and V_oc × J_sc was increased from 16.8 mW/cm² in our previous best submodule (η = 10.8%) to 17.6 mW/cm² (η = 11.0%). However, in most cases, the S grading was found to be accompanied with a ZnS segregation at the backside of the absorber. The backside ZnS layer increases the series resistance, and this is the reason that our new record submodule has higher V_oc × J_sc but lower fill factor. To decrease the series resistance, we have tried to reduce the ZnS segregation, with maintaining the S grading, by adjusting the precursor and selenization/sulfurization conditions. Finally, an S-graded CZTS absorber layer with less ZnS segregation has been successfully acquired, and the recovery of the series resistance has been confirmed. Further improvement of the efficiency will be achieved by optimizing and applying these techniques to current record submodule.