Gradient bandgaps in sulfide kesterite solar cells enable over 13% certified efficiency

Sulfide kesterite Cu2ZnSnS4 (CZTS)—a non-toxic and low-cost photovoltaic material—has always faced severe charge recombination and poor carrier transport, resulting in its cell efficiency record stagnating at around 11% for years. The implementation of gradient bandgaps is a promising approach to relieving these issues, but it has not been effectively realized in kesterite solar cells due to challenges around controlling the elemental distribution. Here, based on Cd-alloyed CZTS, we propose a pre-crystallization strategy to reduce the intense vertical mass transport and Cd rapid diffusion in the film growth process, thereby realizing a Cd-gradient CZTS absorber. This absorber, exhibiting a downward-bent conduction band structure, effectively enhances the bulk carrier transport and additionally improves the interface properties of the CZTS/CdS heterojunction. These benefits significantly enhance the photoelectric conversion performance of the cell and help in achieving a certified total-area cell efficiency of about 13.2% with obviously reduced voltage loss, realizing a substantial step forward for the pure-sulfide kesterite solar cell.