Cu-Doped CsPb₂Br₅ Nanocrystals for Soft Crystal Lattice-Induced Self-Trapped Excitonic Emission: Implications for Solid-State Lighting

Low-dimensional metal halides are favored for optoelectronic device applications due to their strong electron-phonon coupling capability, which produces exciton self-trapping and broadband emission optoelectronic properties. Conventional methods for enhancing electron-phonon coupling involve the introduction of permanent defects or lattice distortions that inhibit the intrinsic structure and properties. In this work, we report that Cu-doped CsPb₂Br₅ nanocrystals exhibit broadband red emission with a high Stokes shift, a long fluorescence lifetime (9.27 μs), and a high photoluminescence quantum yield (∼ 36%) in the visible to near-infrared spectral range (∼550 to 825 nm). At the same time, the fact that the emission spectrum is independent of the excitation wavelength and the excitation spectrum is independent of the emission wavelength, together with the similarity of the decay kinetics, suggests that the broad-band emission has intrinsic properties. The soft vibrational modes of Raman spectroscopy indicate the softness of the lattice and the highly anharmonic character of the structure on the global and local scales. These properties lead to short-range elastic lattice deformation of Cu/CsPb₂Br₅ nanocrystals upon photoexcitation by strong electron-phonon coupling, resulting in self-trapped exciton emission. This method of doping-induced local distortion and exciton self-capture provides a way to improve the properties of the optoelectronic materials. Meanwhile, the fluorescence change in the crystal structure from CsPbBr₃ to CsPb₂Br₅ can also be used for anticounterfeiting inks. Therefore, CsPb₂Br₅ crystals have promising applications in solid-state lighting and optoelectronic devices.