Delafossite CuAlO2 Modification‐Induced Synergistic Passivation Effects for Heterojunction Interface and Absorber toward Efficient Kesterite Solar Cells

Achieving a high‐quality heterojunction interface (HEI) and absorber is crucial for the efficient Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. Herein, an alternative and feasible passivation strategy aiming to harness synergistic passivation effects (SPE) by introducing CuAlO 2 (CAO) into HEI is developed. During the selenization process, interdiffusion of HEI elements occurs between absorber and CAO, resulting in the anticipated inversion of electrical properties from p‐ to n‐type in the shallow bulk of absorber and enabling the construction of a homogeneous field passivation. Moreover, the Cu‐poor characteristic of CAO promotes the formation of a thicker Cu‐deficient shallow bulk, thereby facilitating the increased Na diffusion from soda lime glass substrate to both absorber and HEI. This behavior contributes to passivating dangling bonds within absorber grain boundaries and HEI, as well as promoting grain growth and reducing local potential fluctuations. Consequently, carrier recombination at absorber and HEI is depressed simultaneously, leading to a more than 10% efficiency device with a remarkably low open‐circuit voltage deficit of ≈0.3 V. The impact of SPE and mechanism underpinning device performance improvement are comprehensively investigated. The findings provide valuable insights for achieving high‐quality HEI and absorbers with effectively suppressed recombination, thus enabling efficient CZTSSe solar cells.