Impact of Cation Substitution in (Ag_xCu_1−x)₂ZnSnSe₄ Absorber-Based Solar Cells toward 10% Efficiency: Experimental and Theoretical Analyses

Solar cells based on kesterite Cu₂ZnSnSe₄ (CZTSe) compounds with earth-abundant elements are highly desirable for the low-cost and high-efficiency production of renewable energy. However, the occurrence of intrinsic defects substantially impairs the photovoltaic properties of CZTSe. Herein, a cation substitution method to control and passivate the defect states in bandgap of kesterite CZTSe by incorporating Ag ions is introduced. Intensity-dependent low-temperature photoluminescence measurements show that Ag incorporation can reduce the density and depth of intrinsic defects in CZTSe. The results reveal that 10% Ag-alloyed CZTSe provides the shallowest defect states and less nonradiative recombination. It is also confirmed by first-principles calculations that Ag incorporation enables the formation and suppresses the beneficial and detrimental defects, respectively. Based on the theoretical results, the observed subband photoluminescence peaks can be assigned to the intrinsic point and cluster defects. The best power conversion efficiency of 10.2% is achieved for the 10% Ag-alloyed CZTSe cell, along with an enhanced open-circuit voltage. These results open up a new avenue for further improving the performances of CZTSe-based device via defect engineering.