Versatile Sb2S3 Interlayer for Back-Interface Carrier Management in Narrow-Bandgap Sb2(Sx,Se1-x)3 Solar Cells.

Severe carrier recombination at the back interface, exacerbated by valence-band misalignment arising from the tunable S/Se composition in Sb2(Sx,Se1-x)3 (0 ≤ x < 1) absorbers, remains a critical bottleneck in these solar cells. Here, we address this issue by inserting an Sb2S3 interlayer between the narrow-bandgap Sb2(Sx,Se1-x)3 absorber and the PbS hole transport layer (HTL). KPFM and GIWAXS measurements reveal that this Sb2S3 interlayer reduces surface defect density and improves the crystalline quality of the film surface. Moreover, leveraging the complete mutual solubility between Sb2S3 and Sb2(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 Sb2(S,Se)3-derived devices, efficiency increases from 6.88% to 9.76%; when extended to the Sb2Se3-derived case, efficiency rises from 7.77% to 10.03%. These results set new benchmarks for all-inorganic Sb2(Sx,Se1-x)3 thin-film solar cells, especially for carbon-electrode devices, and establish a back-interface engineering strategy for Sb2(Sx,Se1-x)3 solar cells.