Buffer‐Layer Cation Substitution Engineering for Lattice Strain Relief‐Induced Carrier Dynamics Modulation in High‐Efficiency Antimony Selenide Solar Cells

Antimony selenide (Sb 2 Se 3 ) solar cells have attracted considerable attention owing to their excellent optoelectronic properties. However, their efficiency remains severely limited by non‐radiative recombination. To address this issue, previous studies have mainly focused on crystal orientation, defect passivation, and interfacial energy‐level alignment. Nevertheless, the role of strain in regulating thin‐film growth and carrier dynamics has long been overlooked. Here, a heterojunction engineering strategy based on Co 2+ ‐doped CdS buffer layers is proposed to regulate lattice strain through tailoring the CdS lattice parameters. Co 2+ is found to partially substitute for Cd 2+ sites in the CdS lattice. Owing to its smaller ionic radius, Co 2+ incorporation induces CdS lattice contraction, thereby reducing the lattice mismatch at the CdS/Sb 2 Se 3 interface, markedly promoting strain relaxation, and improving the growth quality of Sb 2 Se 3 films. Meanwhile, the partial back‐diffusion of Co 2+ into the Sb 2 Se 3 absorber modulates its crystallization process, promotes preferred orientation, and suppresses defect formation. Moreover, density functional theory calculations combined with experimental characterizations reveal that Co 2+ incorporation strengthens interfacial electronic coupling and facilitates charge transfer across the heterojunction. Benefiting from the synergistic effects, the optimized device delivers a champion power conversion efficiency of 9.44%, corresponding to a 16.4% enhancement over the control device.