Enhancing the Photovoltaic Performance of Cu2ZnSn(S,Se)4 Solar Cells with Ba Trace Doping: Large Chemical Mismatch Cation Incorporation

Being absent from larger mismatch of ionic radius and chemical coordination between the cations in Cu 2 ZnSn(S,Se) 4 (CZTSSe) can aggravate the formation of detrimental cation antisite defects and clusters with low formation energy, which results in a serious open‐circuit voltage deficit and a challenge for higher power conversion efficiency of CZTSSe solar cells. External cation doping or alloying is a more effective strategy to solve this issue. Herein, samples of trace Ba doping into CZTSSe are fabricated by the sol–gel method and the mechanism of such trace Ba doping are studied combined by X‐ray diffraction (XRD), Raman, and photoluminescence (PL) measurements. The performance of CZTSSe solar cells is increased by 25.7% as 1% Ba is doped into CZTSSe to substitute Zn. The results highlight the roles of the trace Ba doping on the performance of CZTSSe solar cells, which is beneficial to the reduced formation of the detrimental defects and associated clusters. As a consequence, the bandgap fluctuations are reduced, which results in the improvement of V OC and thus the performance of the solar cell. The cation substitution engineering with larger chemical mismatched alkaline‐earth metal cations provides insights for further improvement of CZTSSe solar cells based on earth abundant elements.