Versatile Sb₂S₃ Interlayer for Back-Interface Carrier Management in Narrow-Bandgap Sb₂(S_x,Se_1-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(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 Sb₂S₃ interlayer between the narrow-bandgap Sb₂(S_x,Se_1-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(S_x,Se_1-x)₃ thin-film solar cells, especially for carbon-electrode devices, and establish a back-interface engineering strategy for Sb₂(S_x,Se_1-x)₃ solar cells.