Coupled Electronic and Anharmonic Structural Dynamics for Carrier Self-Trapping in Photovoltaic Antimony Chalcogenides

V-VI antimony chalcogenide semiconductors have shown exciting potentials for thin film photovoltaic applications. However, their solar cell efficiencies are strongly hampered by anomalously large voltage loss (>0.6 V), whose origin remains controversial so far. Herein, by combining ultrafast pump-probe spectroscopy and density functional theory (DFT) calculation, the coupled electronic and structural dynamics leading to excited state self-trapping in antimony chalcogenides with atomic level characterizations is reported. The electronic dynamics in Sb₂ Se₃ indicates a ≈20 ps barrierless intrinsic self-trapping, with electron localization and accompanied lattice distortion given by DFT calculations. Furthermore, impulsive vibrational coherences unveil key SbSe vibrational modes and their real-time interplay that drive initial excited state relaxation and energy dissipation toward stabilized small polaron through electron-phonon and subsequent phonon-phonon coupling. This study's findings provide conclusive evidence of carrier self-trapping arising from intrinsic lattice anharmonicity and polaronic effect in antimony chalcogenides and a new understanding on the coupled electronic and structural dynamics for redefining excited state properties in soft semiconductor materials.