Dynamic Disorder, Band Gap Widening, and Persistent Near-IR Photoluminescence up to At Least 523 K in ASnI₃ Perovskites (A = Cs⁺, CH₃NH₃⁺ and NH₂–CHNH₂⁺)

We report temperature-dependent photoluminescence (PL) in polycrystalline ASnI₃ perovskites (A = Cs⁺, CH₃NH₃⁺, and HC­(NH₂)₂⁺), demonstrating extremely robust emission up to very high temperatures (523 K for CsSnI₃). The PL peak energy (E_PL) monotonically blueshifts with increasing temperature, indicating band gap widening. Variable temperature synchrotron powder X-ray diffraction analysis confirms that these changes are associated with progressive emphanitic off-centering and dynamic fluctuations of the perovskite lattice. In CsSnI₃, three different temperature gradients of E_PL are defined (0.29 meV K^–1 below 200 K, 0.17 meV K^–1 from 200 to 400 K, and 0.48 meV K^–1 above 400 K), commensurate with the onset of dynamic structural disorder at 200 K and its saturation at 400 K as the Cs⁺ atoms rattle independently of the [SnI₃]⁻ perovskite lattice. These results explain how solution-processed perovskites with massive defect concentrations can yield high optoelectronic performance at elevated temperatures.