Cation-Induced Lattice Modulation Effects in Efficient CZTSSe Solar Cells

Although Ag single-doping and Ag-Cd co-doping have been demonstrated to enhance the performance of CZTSSe solar cells, current understanding remains largely confined to defect-level modulation, lacking insight into the microscopic structural reconstruction induced by doping and its fundamental impact on carrier dynamics. By integrating experimental characterization with first-principles calculations, this study reveals a "lattice-modulation" mechanism driven by cation doping. The results show that introduced Ag+ preferentially occupy Cu sites within the Cu-Sn layers, inducing specific lattice expansion along the c-axis. This expansion saturates at an Ag/(Cu+Ag) ratio of ~11%, which aligns precisely with the optimal doping concentration for device efficiency. Such lattice modulation directly optimizes carrier dynamics: first-principles calculations indicate that the c-axis electron mobility exhibits an extremum near the optimal doping level, while the carrier lifetime is significantly extended. The synergistic improvement in mobility and lifetime increases the diffusion length, thereby suppressing bulk recombination and enhancing charge collection efficiency, ultimately leading to a notable increase in both open-circuit voltage and power conversion efficiency. Furthermore, Ag-Cd co-doping demonstrates a unique synergistic effect: Cd2+ occupying Zn sites further drives c-axis expansion and enhances c-axis electron mobility. This complementary site-occupancy mechanism enables cooperative optimization of both lattice structure and carrier transport, resulting in a champion efficiency of 13.25%. By anchoring the physical origin of performance improvement to observable and computable lattice parameters and carrier-dynamics parameters, this work provides a new perspective for the rational design of high-performance multi-cation solar cells.