Multiple Synergistic Effects Enabling Efficient Kesterite Solar Cells via Introducing a Cu 2 O‐Bi 2 O 3 Sacrificial Interlayer

Synergistic optimization of unwanted bulk and interface defects has been one of the effective strategies for high‐efficiency kesterite Cu 2 ZnSn(S, Se) 4 solar cells. Here, a novel multiple synergistic optimization strategy is developed by introducing a Cu 2 O‐Bi 2 O 3 sacrificial interlayer. This interlayer can inhibit the decomposition reaction of the back interface. Then Cu + can supplement Cu‐loss at the back interface and help achieve uniform elemental distributions. Another part of Cu + can also diffuse into the MoSe 2 layer and affect its properties. Meanwhile, Bi 3+ can bidirectionally diffuse into the bulk Cu 2 ZnSn(S, Se) 4 absorbers and the MoSe 2 layers. A portion of Bi 3+ ions in the bulk enables the formation of Cu 2 Zn(Sn, Bi)(S, Se) 4 by substituting Sn 4+ with Bi 3+ , improving its properties such as crystallinity, bulk defects, hole carrier concentration, and band gap. The other portion can diffuse into the MoSe 2 layer, where, together with Cu + , they enhance its charge transport ability and adjust its band structure. These processes collectively enlarge the band offset of the Cu 2 Zn(Sn, Bi)(S, Se) 4 /MoSe 2 :(Cu, Bi) interface, facilitate carrier separation, suppress interface recombination, and thus enable the DMSO‐processed kesterite devices to achieve a high efficiency of 13.53%. Our findings provide new strategies for optimizing the performance of kesterite solar cells, which can be applicable to enhance the performance of other devices.