HCOO− Doping‐Induced Multiexciton Emissions in Cs3Cu2I5 Crystals for Efficient X‐Ray Scintillation

Abstract Self‐trapped exciton (STE) luminescence, typically associated with structural deformation of excited states, has attracted significant attention in metal halide materials recently. However, the mechanism of multiexciton STE emissions in certain metal halide crystals remains largely unexplored. This study investigates dual luminescence emissions in HCOO − doped Cs 3 Cu 2 I 5 single crystals using transient and steady‐state spectroscopy. The dual emissions are attributed to intrinsic STE luminescence originating from the host lattice and extrinsic STE luminescence induced by external dopants, respectively, each of which can be triggered independently at distinct energy levels. Theoretical calculations reveal that multiexciton emission originates from structural distortion of the host and dopant STEs within the 0D lattice in their respective excited states. By meticulously tuning the excitation wavelength and selectively exciting different STEs, the dynamic alteration of color change in Cs 3 Cu 2 I 5 :HCOO − crystals is demonstrated. Ultimately, owing to an extraordinarily high photoluminescence quantum yield (99.01%) and a diminished degree of self‐absorption in Cs 3 Cu 2 I 5 :HCOO − crystals, they exhibit remarkable X‐ray scintillation characteristics with light yield being improved by 5.4 times as compared to that of pristine Cs 3 Cu 2 I 5 crystals, opening up exciting avenues for achieving low‐dose X‐ray detection and imaging.