Multinary alloying for facilitated cation exchange and suppressed defect formation in kesterite solar cells with above 14% certified efficiency

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cells are highly promising low-cost thin-film photovoltaics. However, the efficiency of these solar cells is challenged by severe charge losses and complex defects. Here we reveal through a data-driven correlation analysis that the dominant deep defect in CZTSSe exhibits a donor character. We further propose that incomplete cation exchange in the multi-step crystallization reactions of CZTSSe is the kinetic mechanism responsible for the defect formation. To facilitate the cation exchange, we introduce a multi-elemental alloying approach aimed at weakening the metal–chalcogen bond strength and the stability of intermediate phases. This strategy leads to a significant reduction in charge losses within the CZTSSe absorber and to a total-area cell efficiency of 14.6% (certified at 14.2%). Overall, these results not only present a significant advancement for kesterite solar cells but could also help identify and regulate defects in photovoltaic materials. A detailed understanding of defects in kesterite solar cells is still lacking. Shi et al. identify a key defect and its formation mechanism and use alloying to suppress it, achieving solar cells with 14.2% certified efficiency.