From binary chalcogenide Sb2S3 to mixed-cation chalcohalide (Sb,Bi)SI thin films with tunable bandgap and photoconductivity

Pnictogen chalcogenides and chalcohalides represent an emerging class of solid-state materials whose electronic structure and transport properties can be tuned through compositional engineering. In this work, Sb2S3 and mixed-cation (Sb,Bi)SI thin films are synthesized via a scalable, one-step molecular-ink spin-coating route, enabling direct comparison between binary and chalcohalide systems prepared under identical conditions. Structural and chemical analyses confirm the formation of orthorhombic Sb2S3 and SbSI-type chalcohalide phases, with Bi/I incorporation preserving Sb(III) coordination while inducing controlled lattice distortion and enhanced dielectric screening. Optical spectroscopy reveals indirect bandgaps of ∼1.63 eV for Sb2S3 and ∼1.47 eV for (Sb,Bi)SI, demonstrating effective bandgap tuning. Reflection electron energy-loss spectroscopy identifies near-edge and sub-gap states associated with chalcohalide formation. Ultraviolet photoelectron spectroscopy further reveals reduced surface band bending in (Sb,Bi)SI compared to Sb2S3. Hall measurements confirm p-type conductivity and show nearly two orders of magnitude carrier activation upon removal of surface depletion effects. These modifications manifest as multichannel photoluminescence and an order-of-magnitude enhancement in visible-light photoconductivity. Collectively, the results establish a direct link between compositional engineering, surface electronic structure, and macroscopic optoelectronic performance in solution-processed pnictogen chalcogenide and chalcohalide thin films.