Unveiling Mechanism of Temperature-Dependent Self-Trapped Exciton Emission in 1D Hybrid Organic–Inorganic Tin Halide for Advanced Thermography

Lead-free hybrid metal halide phosphors/crystals showing self-trapped exciton (STE) emission have been recently explored for thermography due to the strong temperature dependence of their photoluminescence (PL) lifetime (τ). However, realizing high-spatial-resolution thermography using polycrystalline powders or crystals presents a challenge. Moreover, the underlying mechanism of temperature-dependent STE remains elusive. Herein, a homogeneous 1D ODASn₂I₆ (ODA, 1,8-octanediamine) nm-scale thin film exhibiting efficient STE emission is investigated. The PL decay shows a strong temperature dependence from 275 K (τ ≈ 1.31 µs) to 350 K (τ ≈ 0.65 µs) yielding a thermal sensitivity of 0.014 K⁻¹. By employing temperature-dependent transient absorption spectroscopy, detailed information is obtained about the relaxation processes prior to the STE formation. Simultaneous analyses of steady-state and time-resolved spectroscopies lead to a self-consistent model where the thermally activated phonon-assisted nonradiative pathway explains the temperature dependence of the PL lifetime via a conical intersection between the ground state and STE potential energy surfaces. Finally, a discernible 50 ns variation in PL lifetimes across different heated regimes over a distance of 1.15 mm is successfully demonstrated with fluorescence lifetime imaging microscopy, underscoring the substantial potential of ODASn₂I₆ thin film for high-spatial-resolution thermography.