Early-stage local chemistry regulation enabling open-circuit voltage of 847 mV in wide-bandgap Cu2ZnSnS4 solar cells

Wide-bandgap kesterite Cu 2 ZnSnS 4 (CZTS) is a promising top-cell candidate for tandem photovoltaics, yet its progress is limited by persistent open-circuit voltage ( V OC ) deficits. Previous reports of wide-bandgap CZTS solar cells have exhibited substantial V OC losses relative to the radiative limit, largely due to a mismatch between the designed global composition and the local chemistry formed during early crystallization, where elemental diffusion induces secondary phases and detrimental defects. Here we demonstrate a defect regulation strategy that stabilizes the local chemical environment and suppresses diffusion-induced secondary phases, therefore maintaining the desired crystallization environment and pathway. This approach promotes benign defects over detrimental ones, thereby reducing non-radiative recombination losses. Consequently, we achieve a certified champion V OC of 847 mV (65.3% of the radiative limit) and an efficiency of 12.4% in Cd-free wide-bandgap CZTS solar cells. This defect regulation strategy provides a generalized insight into the synthesis of multinary compound semiconductors.