Ag‐Se Synergistic Diffusion for Highly Homogeneous Elemental Distribution and Significantly Improved ACZTSSe Solar Cells

ABSTRACT The performance of Cu 2 ZnSn(S x Se 1−x ) 4 (CZTSSe) solar cells is primarily limited by an inability to control the selenization process, where non‐uniform selenium distribution leads to profound Cu–Zn cation disorder and associated performance losses. Compositional engineering, such as in (Ag x Cu 1−x ) 2 ZnSn(S x Se 1−x ) 4 (ACZTSSe), can mitigate some defects; however, it fails to address the root cause: mastering the reaction kinetics to achieve elemental homogeneity. Herein, we introduce a self‐sacrificing Ag 2 Se layer on Mo substrates that enables a bottom‐up, cooperative Ag–Se diffusion pathway during grain growth. The rapid migration of Ag atoms modifies the local chemical environment, thereby acting as a kinetic driver that dramatically enhances the selenium diffusion flux. Molecular dynamics simulations confirm a 28.6% increase in the selenium diffusion coefficient ( D Se ), demonstrating that this kinetic enhancement activates the cooperative Ag–Se diffusion mechanism, thereby achieving a macroscopic improvement in material homogeneity. The resultant absorber exhibits an unprecedented, near‐linear elemental distribution, which effectively suppresses Cu‐related antisite defects (e.g., Cu Zn and Cu Sn ). This superior homogeneity minimizes bulk recombination and facilitates efficient carrier transport, thereby providing a fundamental basis for the enhancement of photoelectric properties. Consequently, the champion device treated with 65 nm Ag 2 Se (ASe 65) achieves a power conversion efficiency (PCE) of 14.36% (certified as 13.91%), primarily due to a remarkable enhancement in short‐circuit current density ( J sc ) and fill factor (FF). This work not only provides a foundational understanding of cooperative diffusion chemistry, but also establishes a novel paradigm for manipulating reaction kinetics in solid‐state energy materials.