Regulating Interfacial Recombination in Cd‐Free Sb 2 Se 3 Solar Cells via an Ultrathin Al 2 O 3 Layer

ABSTRACT Antimony selenide (Sb 2 Se 3 ) has emerged as a promising photovoltaic absorber owing to its suitable bandgap, high absorption coefficient, and low‐toxicity elemental composition. However, high‐efficiency Sb 2 Se 3 solar cells commonly rely on CdS electron‐transport layers, whereas Cd‐free alternatives often suffer from pronounced recombination losses at the Sb 2 Se 3 /buffer heterojunction. Herein, we introduce a sub‐nanometer atomic‐layer‐deposited aluminum oxide (Al 2 O 3 ) interlayer at the Sb 2 Se 3 /Zn x Sn 1‐x O heterojunction to improve its interfacial electronic quality. Spectroscopic and theoretical analyses indicate that the ultrathin Al 2 O 3 induces interfacial chemical interaction and charge redistribution, thereby modifying the local electronic structure and interfacial energetics. Complementary surface and device‐level measurements reveal a more homogeneous near‐surface electronic response, reduced field‐dependent carrier‐collection losses, and mitigated trap‐assisted recombination. Temperature‐dependent V OC measurements show an increase in the recombination activation energy from 0.910±0.018 to 1.045±0.021 eV, while intensity‐dependent V OC measurements reveal a decrease in the effective ideality factor from 1.37 to 1.21, consistently indicating a reduced contribution of trap‐assisted recombination, although interface‐related recombination remains an important loss pathway. Consequently, the optimized Sb 2 Se 3 /Al 2 O 3 /Zn x Sn 1‐x O device achieves a champion power conversion efficiency of 9.28% with an enhanced open‐circuit voltage. These results demonstrate that ultrathin Al 2 O 3 interfacial modification provides an effective strategy for mitigating recombination‐related losses in high‐performance Cd‐free Sb 2 Se 3 solar cells.