Anomalous excitation wavelength dependent photoluminescence of Sb3+/Bi3+ co-doped double perovskites

Lead-free double perovskites have recently attracted lots of research interest due to their enhanced structural stability and decreased toxicity compared to lead-halide perovskites. However, origins of the dual emissions observed in photoluminescence (PL) are still controversial. Herein, PL properties of Sb/Bi co-doped Cs2NaInCl6 microcrystals are investigated. When the excitation wavelength is about 320–360 nm, the emission peak mainly locates at 450 nm. While excited at 250 nm, the emission peak reversely increases to ∼580 nm. According to Stokes shift, bandwidth, PL excitation spectra, PL lifetime, and temperature dependent PL, the 450 and 580 nm emissions are attributed to recombination of free exciton (FE) and self-trapped exciton (STE), respectively. Only when the excitation photon energy is large enough to generate lattice distortion and create excitons simultaneously does the STE form, exhibiting emission at 580 nm. As excitation wavelength increases, the photon energy becomes insufficient for the formation of the STE but is able to create FEs. Thus, the emission is dominated by FE recombination. Therefore, the excitation wavelength dependent PL peak position is observed. Moreover, we demonstrate an anti-counterfeiting pattern with excitation wavelength dependent color, which cannot be imitated by other luminescent materials, implying an improved security.