Tuning the Se Content in Cu₂ZnSn(S, Se)₄ Absorber to Achieve 9.7% Solar Cell Efficiency from a Thiol/Amine-Based Solution Process

The Se content in a Cu 2 ZnSn(S, Se) 4 absorber layer has a significant impact on the electronic properties, but it is rather challenging to control the Se/(S + Se) ratio due to a complicated selenization process. Here, a low-toxicity thiol/amine-based solution process was developed to tune the Se content in a Cu 2 ZnSn(S, Se) 4 absorber layer to an optimal value by ingeniously controlling the SeO 2 in the precursor solution. We demonstrated that the crystal growth and the band gap of Cu 2 ZnSn(S, Se) 4 thin films are affected by the Se/(S + Se) ratio. By this approach, the open-circuit voltage deficit ( V oc,def ) of the device was effectively decreased, and the short-circuit density ( J sc ) and fill factor (FF) were remarkably improved; thus, the power conversion efficiency of the Cu 2 ZnSn(S, Se) 4 solar cells was successfully increased from 5.6% to 9.7% for the optimal band gap ( E g = 1.13 eV).