Insights into the Efficiency Improvement for CZTSSe Solar Cells with over 12% Efficiency via Ga Incorporation

Suppressing the band tailing and nonradiative recombination caused by massive defects and defect clusters is crucial for mitigating open-circuit voltage (V-oc) deficit and improving the device performance of CZTSSe thin film solar cells. Cation substitution is one of the most commonly used strategies to address the above issues. The latest world record efficiency of 13.0% is obtained through this strategy (Ag substitution for Cu). Nevertheless, the importance of the approach to implementing metallic ion doping is easily overlooked by researchers. Here, different approaches are adopted to realize Ga doping and the differences in the efficacy and mechanism are thoroughly investigated. It is found that the secondary phase easily emerged when the physical-based method is employed, and thus challenging to regulate the doping effect. In the case of the chemical-based method, Ga doping can enlarge the depletion region widthand lower the defect activation energyand Urbach energy. Furthermore, Ga-Sn defects located at grain boundaries can expand the energy band bending between GBs and grain interiors (GIs), thereby suppressing the deep defect states and nonradiative recombination. Consequently, power conversion efficiency as high as 12.12% with a V-oc of 522 mV is achieved at 2% Ga doping.