Energy Band Alignment and Defect Synergistic Regulation Enable Air‐Solution‐Processed Kesterite Solar Cells With the Lowest VOC Deficit

Abstract The major challenge in preparing high‐performance Cu 2 ZnSn(S,Se) 4 solar cells is the large open circuit voltage deficit ( V OC ‐def). A new strategy utilizing the synergistic substitution of Ag and In dual cations has been proposed to simultaneously address the problems of undesirable interface band alignment and high‐density detrimental bulk defects, obtaining decreased carrier recombination rate and increased minority carrier lifetime. The shorter In−S/Se bonds move the CBM higher by generating stronger repulsive force than the Sn−S/Se bonds, thus adjusting the interface band alignment. Ag substitution can effectively suppress Cu─Zn disorder, while Ag, In dual substitution can further passivate Sn‐related defects and solve the issue of low carrier concentration in Ag single‐substituted samples. Besides, the superior carrier property of In‐Se materials significantly enhanced the device carrier lifetime and minority carrier diffusion length. The state‐of‐the‐art air‐solution‐processed CZTSSe device without any addition treatment with 14.33% efficiency and 580 mV V OC is obtained, possessing the lowest V OC ‐def value currently available in the CZTSSe field ( V OC / V OC SQ = 64.7%). This work emphasizes the synergistic modulation of band alignment, defect level, grain growth, and carrier transportation by dual cation substitution, which paves a convenient and effective way to realize high‐performance solar cells and photovoltaic devices.