Buffer-Layer Cation Substitution Engineering for Lattice Strain Relief-Induced Carrier Dynamics Modulation in...

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.