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

Antimony selenide (Sb2Se3) 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 Co2+-doped CdS buffer layers is proposed to regulate lattice strain through tailoring the CdS lattice parameters. Co2+ is found to partially substitute for Cd2+ sites in the CdS lattice. Owing to its smaller ionic radius, Co2+ incorporation induces CdS lattice contraction, thereby reducing the lattice mismatch at the CdS/Sb2Se3 interface, markedly promoting strain relaxation, and improving the growth quality of Sb2Se3 films. Meanwhile, the partial back-diffusion of Co2+ into the Sb2Se3 absorber modulates its crystallization process, promotes preferred orientation, and suppresses defect formation. Moreover, density functional theory calculations combined with experimental characterizations reveal that Co2+ 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.