Boosting triplet self-trapped exciton emission in Te(IV)-doped Cs₂SnCl₆ perovskite variants

Perovskite variants have attracted wide interest because of the lead-free nature and strong self-trapped exciton (STE) emission. Divalent Sn(II) in CsSnX₃ perovskites is easily oxidized to tetravalent Sn(IV), and the resulted Cs₂SnCl₆ vacancy-ordered perovskite variant exhibits poor photoluminescence property although it has a direct band gap. Controllable doping is an effective strategy to regulate the optical properties of Cs₂SnX₆. Herein, combining the first principles calculation and spectral analysis, we attempted to understand the luminescence mechanism of Te⁴⁺-doped Cs₂SnCl₆ lead-free perovskite variants. The chemical potential and defect formation energy are calculated to confirm theoretically the feasible substitutability of tetravalent Te⁴⁺ ions in Cs₂SnCl₆ lattices for the Sn-site. Through analysis of the absorption, emission/excitation, and time-resolved photoluminescence (PL) spectroscopy, the intense green-yellow emission in Te⁴⁺:Cs₂SnCl₆ was considered to originate from the triplet Te(IV) ion ³P₁→¹S₀ STE recombination. Temperature-dependent PL spectra demonstrated the strong electron-phonon coupling that inducing an evident lattice distortion to produce STEs. We further calculated the electronic band structure and molecular orbital levels to reveal the underlying photophysical process. These results will shed light on the doping modulated luminescence properties in stable lead-free Cs₂MX₆ vacancy-ordered perovskite variants and be helpful to understand the optical properties and physical processes of doped perovskite variants.