Enhancing the Green Luminescence Efficiency and Stability of Cs3Cu2Cl5 Nanocrystals by Na+ Substitutional Doping

Cesium halide copper compound semiconductors with a typical composition like Cs3Cu2X5 have the advantages of nontoxicity and low cost, which show great potential for photoelectronic devices and biomedical applications. However, due to the valence variability of Cu+ in Cs3Cu2X5 and the inherent structural defects on the surface of nanocrystals (NCs), it has the shortcomings of easy oxidation and poor stability. Such instability is the main bottleneck limiting its application in a real environment. In this paper, the substitutional doping strategy with typical alkaline metal sodium (Na) was developed to greatly improve the photoluminescence quantum yield (PLQY) of Cs3Cu2X5 NCs and the air stability of Cs3Cu2Cl5 NCs. Detailed microstructure characterizations using X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy indicated that Na+ doping into the NCs effectively replaced the Cs+ ion in the lattice, which can stabilize the Cu+ chemical state by increasing the Cu–Cl bond energy and prevent the oxidation from Cu+ to Cu2+. Substitutional Na doping also reduces defect-related nonradiative recombination and enhances the photocarrier recombination in the form of self-trapped excitons. The doped Cs3Cu2Cl5 NCs exhibited an PLQY as high as 96% at the optimized doping concentration, which was due to the enhanced exciton–phonon coupling and reduced electron–phonon coupling as proved by the detailed PL spectroscopy analysis. In addition, such doped lead-free perovskite NCs also show higher X-ray excited emission efficiency and a more sensitive X-ray response, which further indicate their potential applications in medical imaging and photodynamic therapy for deep tumors.