Creating Extrinsic Self-Trapped Excitons by Isoelectronic Doping for Lead-Free High-Efficiency Light Emission and High-Performance X-ray Scintillators.

Metal-ion doping has been widely employed to enhance light emission from self-trapped excitons (STEs) in metal halide perovskites. However, the literature often fails to clearly differentiate between intrinsic and extrinsic STEs. In this work, we demonstrate an exemplary extrinsic STE and identify its characteristics through experimental signatures and first-principles calculations. By substituting isoelectronic Cu for Ag in one-dimensional perovskite CsAgI, we transform it from a nonemissive ion to an efficient STE emitter with near-unity quantum efficiency at room temperature. Density functional theory calculations reveal that Cu induces a local lattice distortion in the [CuI] complex, which subsequently creates localized states at the top of the valence band. This complex traps the photoexcited hole, resulting in a bound exciton and a concomitantly bound STE. These desirable properties make extrinsic STEs ideal for engineering perovskites, advancing fundamental studies, and enabling diverse device applications.