Self‐Trapped Exciton to Dopant Energy Transfer in Rare Earth Doped Lead‐Free Double Perovskite

Abstract Low dimensional halide perovskites with self‐trapped excitons (STEs) emission have emerged as promising white light phosphors because of their ultrabroadband emission covering the entire visible spectrum from 400 to 800 nm. Such a broad emission from a single material can overcome emission color change and self‐absorption problems within multiple phosphors. However, the color rendering index (CRI) and correlated color temperature (CCT) as two essential parameters of white light quality can hardly be modulated in these perovskite materials. Here, rare earth ion Ho3+ is introduced into Cs2(Na,Ag)InCl6 for the first time, utilizing the hydrothermal method. Besides the strong warm white STEs emission, the as‐synthesized materials exhibit effective characteristic emission of Ho3+ in the visible region. Further, the mechanism of associated emission is explored and the existence of energy transfer from STEs to rare earth is first confirmed. A white light‐emitting diode (LED) prototype is also fabricated by employing the Ho3+ doped Cs2(Na,Ag)InCl6 as the color conversion material on a commercial 365 nm GaN LED chip, achieving an improved CRI from 70.3 to 75.4 compared to the pure Cs2(Na,Ag)InCl6. This result suggests a promising way to achieve high quality single phase all‐inorganic white phosphors and this mechanism has enormous potentials in other optoelectronic applications.