Enhanced Red-NIR Emission in Cs ₂ AgHoCl ₆ by Exciton Energy Delivery

Lanthanide ions offer sharp, stable 4f luminescence but suffer from parity-forbidden transitions and narrow absorption. Halide perovskites, with flexible lattices and low phonon vibrational energies, enable exciton formation that facilitates energy transfer to lanthanide ions. Here, we successfully synthesized the silver-based rare-earth (RE) halide double perovskite Cs₂AgHoCl₆. The incorporation of Bi³⁺ induces lattice distortion, generates self-trapped excitons (STEs), and enables ultrafast energy transfer from STEs to Ho³⁺. The material exhibits strong red and near-infrared (NIR) emission, with a total quantum yield of up to 33.1% and a maximum NIR QY of 29.1%. Moreover, Bi³⁺-introduced Cs₂AgHoCl₆ exhibited excellent stability under X-ray radiation exposure. The perovskite can be used for high-quality indirect imaging and an efficient NIR light-emitting diode. This work provides a new matrix for the development of novel RE-based halide optical materials and offers a new perspective for the spectral regulation of exciton- and lanthanide-ion-related luminescent materials.