Suppressing the Electron-Phonon Coupling in 2D Perovskite Cs₃Sb₂I₉ for Lead-Free Indoor Photovoltaics

Antimony-based perovskite-inspired materials (Sb-PIMs) are promising lead-free candidates for indoor photovoltaic application. Cs₃Sb₂I₉, in particular, with a ≈2.0 eV bandgap, is ideal for harvesting indoor white light. However, solution-processed Sb-PIMs preferentially crystallize into thermodynamically stable 0D structures, leading to strong self-trapped exciton (STE) formation, limiting device performance. Although chloride (Cl) doping can induce 2D structural transitions, it enhances Fröhlich electron-phonon coupling (EPC), creating an intrinsic trade-off. Here, we develop an anion-exchange strategy to fabricate phase-pure, Cl-free 2D Cs₃Sb₂I₉ films that suppress STE formation while enabling controlled dimensional reconstruction. This approach yields a reduced Huang-Rhys factor (from 30.7 to 21.5) and prolonged STE lifetime (8.60 to 9.19 ps). Density functional theory (DFT) calculations reveal a significant reduction in excited-state octahedral distortion (Δd = 0.898 × 10^-3 for Cs₃Sb₂I₉ vs. 5.752 × 10^-3 for Cs₃Sb₂I₆Cl₃), confirming intrinsically weaker EPC in Cl-free structures. The device achieves a power conversion efficiency (PCE) of 3.40% under AM 1.5G solar illumination and an 8.2% PCE under 1000 lux white LED conditions. alongside Long-term stability measurement confirms its environmental robustness. These results represent the highest indoor performance reported to date for Sb-based perovskite-inspired solar cells.