Suppressing surface and bulk effect enables high efficiency solution-processed kesterite solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising photovoltaic material due to its environmentally friendly composition and excellent photovoltaic performance. Unfortunately, open-circuit voltage (Voc) deficit and insufficient fill factor (FF) due to the interface and bulk carrier recombination have become major obstacle to further improve the device performance. A new effective strategy is proposed to simultaneously improve the front interface and suppress the CZTSSe absorber defects by introducing a thin ammonium sulfide layer on the absorber surface by a pre-soaking and spin-coating-annealing method. Through this strategy, the absorber surface is optimized by adding S and removing the secondary phase. At the same time, the diffusion of S into the bulk transforms the dominant deep VSe defect into shallower CuZn defect, which effectively reduces carrier recombination in the CZTSSe bulk and at the CZTSSe/CdS interface, prolongs minority carrier lifetime, facilitates carrier collection, and reduces charge loss. The optimized sulfurization treatment significantly improves VOC and FF to the point that device efficiency increases from 11.26% to 13.19%. These findings provide new insights into achieving higher efficiency in CZTSSe-based thin film solar cells by simultaneously optimizing the interface and bulk.