Band Gap Tuning by Hydrostatic Pressure in (2-Thienyl)methylammonium Tin Iodide Halide Perovskite

Two-dimensional tin-based halide perovskites are promising lead-free materials for optoelectronics due to their lower toxicity, structural flexibility, and strong polaronic excitonic effects. Here, we investigate the structural and optical response of (2-thienyl)methylammonium tin iodide (TMA2SnI4) under hydrostatic pressure and variable temperature. Pressure-dependent photoluminescence reveals substantial band gap narrowing of the optical (excitonic) transition energy with a pressure coefficient of −187 ± 3 meV/GPa. Room-temperature X-ray diffraction confirms that TMA2SnI4 maintains a stable orthorhombic phase (Pbca) up to ∼1.6 GPa followed by a structural transition. Temperature-dependent PL and reflectance measurements identify a reversible phase transition near 180 K and highlight strong electron–phonon coupling and excitonic behavior. At cryogenic temperatures, the pressure induces additional emissive states, likely associated with polaronic excitonic complexes. These results characterize TMA2SnI4 as a structurally robust and optoelectronically responsive material with pronounced band gap tunability.