Suppressing the Electron–Phonon Coupling in 2D Perovskite Cs 3 Sb 2 I 9 for Lead‐Free Indoor Photovoltaics

Abstract Antimony‐based perovskite‐inspired materials (Sb‐PIMs) are promising lead‐free candidates for indoor photovoltaic application. Cs 3 Sb 2 I 9 , 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 Cs 3 Sb 2 I 9 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 Cs 3 Sb 2 I 9 vs. 5.752 × 10 −3 for Cs 3 Sb 2 I 6 Cl 3 ), 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.