Cu-Doping Induced Phase Transformation in CsPbI3 Nanocrystals with Enhanced Structural Stability and Photoluminescence Quantum Yield

The synthesis of air-stable and cubic phases of all-inorganic halide perovskite nanocrystals (HPNCs) by a hot-injection approach is still challenging due to their rapid in situ phase transformations. Therefore, understanding and preventing this phase conversion by doping of cations is the key to improve the structural stability, environmental durability, and photoluminescence quantum yield. Here, the doping of divalent Cu ions at the Pb-site of cesium lead iodide (CsPbI3) is reported to achieve cubic-phase (alpha-phase) HPNCs with superior quantum yield and enhanced stability compared to undoped CsPbI3. Rietveld refined X-ray diffraction patterns and atomic-resolution transmission electron microscopy analyses reveal structural transformation from a mixed (gamma-orthorhombic and alpha-cubic) phase to a cubic (alpha-CsPbI3) one with a smaller lattice constant at an optimal (5.6%) Cu concentration. Computational density functional theory (DFT) analysis credits the resultant structural and environmental stability for the Cu-doped sample to the charge accumulation at the dopant site that leads to increased bond strength with consequent minimization of the energy of the system to give rise to the maximum stability of the HPNCs at this dopant ratio. With the increase of Cu-doping, a red shift is observed in the absorbance and photoluminescence spectra up to a critical doping level, making the system optically tuneable. The optimally doped (5.6% Cu) CsPbI3 HPNCs exhibit stronger light emission with higher carrier lifetime and higher quantum yield (>80%), which are absolutely essential requirements in air-stable optoelectronic devices.