Versatile Sb 2 S 3 Interlayer for Back‐Interface Carrier Management in Narrow‐Bandgap Sb 2 (S x ,Se 1−x ) 3 Solar Cells

ABSTRACT Severe carrier recombination at the back interface, exacerbated by valence‐band misalignment arising from the tunable S/Se composition in Sb 2 (S x ,Se 1−x ) 3 (0 ≤ x < 1) absorbers, remains a critical bottleneck in these solar cells. Here, we address this issue by inserting an Sb 2 S 3 interlayer between the narrow‐bandgap Sb 2 (S x ,Se 1−x ) 3 absorber and the PbS hole transport layer (HTL). KPFM and GIWAXS measurements reveal that this Sb 2 S 3 interlayer reduces surface defect density and improves the crystalline quality of the film surface. Moreover, leveraging the complete mutual solubility between Sb 2 S 3 and Sb 2 (S,Se) 3 , TOF‐SIMS confirms the formation of a compositionally graded heterojunction from the Se‐rich bulk to the S‐rich back surface during annealing, creating a continuous energy staircase. Consequently, the hole‐extraction barrier is eliminated, the built‐in electric field is enhanced, and the depletion region is broadened, accelerating hole collection. This strategy delivers the following performance: for Sb 2 (S,Se) 3 ‐derived devices, efficiency increases from 6.88% to 9.76%; when extended to the Sb 2 Se 3 ‐derived case, efficiency rises from 7.77% to 10.03%. These results set new benchmarks for all‐inorganic Sb 2 (S x ,Se 1−x ) 3 thin‐film solar cells, especially for carbon‐electrode devices, and establish a back‐interface engineering strategy for Sb 2 (S x ,Se 1−x ) 3 solar cells.