Systematic Efficiency Improvement for Cu₂ZnSn(S,Se)₄ Solar Cells By Double Cation Incorporation with Cd and Ge

Abstract The performance of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell is known to be severely limited by the nonradiative recombination near the heterojunction interface and within the bulk of the CZTSSe absorber resulting from abundant recombination centers and limited carrier collection efficiency. Herein, nonradiative recombination is simultaneously reduced by incorporating small amounts of Ge and Cd into the CZTSSe absorber. Incorporation of Ge effectively increases the p‐type doping, thus successfully improving the bulk conductance and reducing the recombination in the CZTSSe bulk via enhanced quasi‐Fermi level splitting, while the incorporation of Cd greatly reduces defects near the junction region, enabling larger depletion region width and better carrier collection efficiency. The combined effects of Cd and Ge incorporation give rise to systematic improvement in open‐circuit voltage (V_OC), short‐circuit current density (J_SC), and fill factor (FF), enabling a high conversion efficiency of 11.6%. This study highlights the multiple cation incorporation strategy for systematically manipulating the opto‐electronic properties of kesterite materials, which may also be applicable to other semiconductors.