Charge localization and intrinsic self-trapped exciton photophysics in undoped Cs2NaScCl6 elpasolite single crystals

Abstract Even though the intrinsic photophysics of all-inorganic lead-free halide double perovskites remains poorly understood in the absence of dopants, they have emerged as robust platforms for exciton physics. This paper provides a comprehensive investigation of undoped Cs₂NaScCl₆ combining electrical, structural and steady-state and time-resolved spectroscopy characterization. With a rigid [ScCl₆]³⁻octahedral framework which is thermally stable up to ~ 760–870°C, a phase-pure elpasolite structure (\(\:Fm\stackrel{-}{3}m\)) is confirmed by both Raman spectroscopy and X-ray diffraction. Pointing to low electronic disorder, a small Urbach energy of 0.123 eV and a direct bandgap of 4.63 eV are revealed by UV–visible spectroscopy. With a mono-exponential lifetime of 1.087 µs, a broad blue photoluminescence band is indicated at ~ 453 nm by Cs₂NaScCl₆, which is fully assigned to self-trapped exciton (STE) emission. With no evidence of defect-mediated relaxation, sub-picosecond carrier cooling followed by 10–15 ps STE formation is shown through Femtosecond transient absorption measurements. Only weak linear photoconductivity and extremely low currents are revealed by I–V characteristics (265 nm illumination vs dark), which is in agreement with not only negligible free-carrier transport but also strongly-localized excitations. However, activation energy of 0.625 eV and single bulk relaxation with Arrhenius-type conductivity are yielded using impedance spectroscopy. Therefore, Cs₂NaScCl₆ is collectively established by such results as a model wide-bandgap elpasolite where deeply localized STEs are driven within a structurally and electronically pristine lattice through exciton–phonon coupling, which suggests not only a well-defined platform for fundamental studies of STE-mediated emission in lead-free halide perovskites but also a robust host for rare-earth doping.