LAUSR

Cd‐free high‐bandgap Cu2ZnSnS4 (CZTS) solar cells are expected to offer green and low‐cost solutions to single‐junction and tandem photovoltaic markets. Despite attracting considerable attention in past years, reported certified efficiency has yet to exceed the 10% threshold. Sn‐related defects, especially those originating from Sn2+ oxidation states, lead to severe recombination loss and limit device performance. Here, the formation of these detrimental defects is suppressed by creating a more benign chemical environment during sulfurization annealing. With dominant Sn4+ states in the source material and the precursor, the oxidation state of Sn remains primarily in its native Sn4+ state during annealing and in the final film. Engineering the Sn‐related defects leads to shallower tail states in bulk CZTS and fewer interfacial defects. As a result, both radiative and non‐radiative recombination losses are alleviated, contributing to a certified 10.7% efficiency of the Cd‐free high‐bandgap CZTS solar cell. This strategy may advance various kesterite materials and other technologies with multivalent constituents.