Direct evidence of hydration and temperature effects on the structure, spectroscopy and dynamics of a novel Mn-based organic-inorganic metal halide material for high-performance LEDs

Low-dimensional organometallic lead-free hybrid halide materials have garnered considerable attention due to their exceptional optoelectronic properties. In this work, a novel 0D hybrid Mn-based metal halide material, (tert-Butylammonium)₃MnBr₅ (1), has been synthesized and characterized. The 1-as-synthesized sample exhibits a dual-band emission spectrum, that is temperature-dependent, and moisture sensitive, with a photoluminescence quantum yield of 34 %. At room temperature, its emission is attributed to free excitons (FE, green emission band originating from the ⁴T₁ → ⁶A¹ transition) and self-trapped excitons (STEs, red emission band) induced by the presence of solvent molecules. The STEs formation takes place in 17 μs leading to an equilibrated excited state with lifetime of ∼135 μs. At high temperatures, a new red-shifted emission band is observed and attributed to a structural transformation from 1 to a novel material with octahedral configuration of the Mn²⁺ centers. 1 has been integrated as phosphor layer of a down-converter LED with tunable CIE coordinates, and good stability of 86 % after 8 h of continuous operation. While these findings give new clues on how the solvent-induced lattice transitions affect the spectroscopy and dynamics of Mn-based luminescent perovskites, they also provide pivotal information to inspire further research on dual emissive lead-free perovskites for photonic applications, like photosensing and lighting.