Low-Temperature Annealing of CdS:In/Cu2ZnSn(S,Se)4 Heterojunction Boosting 14.5% Efficiency Kesterite Solar Cells

The persistent challenge in kesterite solar cells is the low open-circuit voltage (V_oc) and fill factor (FF) due to nonradiative recombination at the CdS/Cu₂ZnSn(S,Se)₄ (CZTSSe) interface. Here we demonstrate a convenient combination of low-temperature annealing and In doping within the buffer layer to establish an electrically benign high-quality CdS:In/CZTSSe heterojunction. The low-temperature annealing facilitates the migration of Cu, Zn, and Sn impurity elements from the buffer layer to the absorber side, improving lattice match and reducing detrimental defects and the conduction band offset (CBO) barrier involving large recombination losses. The In doping boosts the donor concentration and crystallinity of the buffer layer, thereby improving the electron transport and extraction processes. Consequently, the CdS:In device achieves the highest efficiency of 14.5% with the V_oc,deficit decreasing from 348 mV to 287 mV and the FF increasing from 66.6% to 70.3%, promising a significant efficiency leap for CZTSSe solar cells.