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 Cu2ZnSn(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 Cu2ZnSn(S, Se)4 absorber layer to an optimal value by ingeniously controlling the SeO2 in the precursor solution. We demonstrated that the crystal growth and the band gap of Cu2ZnSn(S, Se)4 thin films are affected by the Se/(S + Se) ratio. By this approach, the open-circuit voltage deficit (Voc,def) of the device was effectively decreased, and the short-circuit density (Jsc) and fill factor (FF) were remarkably improved; thus, the power conversion efficiency of the Cu2ZnSn(S, Se)4 solar cells was successfully increased from 5.6% to 9.7% for the optimal band gap (Eg = 1.13 eV).