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

Abstract

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 (α-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 (γ-orthorhombic and α-cubic) phase to a cubic (α-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.

Unveiling Electron Dose-Induced Phase Decomposition and Energy Kinetics in Cu-Doped CsPbI3 Nanocrystals

“…Figure 1d shows the selected area electron diffraction (SAED) patterns of pristine CsPbI 3 NCs, where the diffraction spots associated with the (110) and (130) crystal planes along the [002] zone axis correspond to the orthorhombic phase of CsPbI 3 , consistent with the X-ray diffraction (XRD) and HRTEM results. 28 However, we also observed some additional diffraction spots marked by red circles, which are difficult to index, probably because of the orientation of some NCs in different directions or their overlap with each other. Figure 1e shows the HRTEM image of a single Cu-doped CsPbI 3 NC with an average size of 30 ± 5 nm, indicating increased NC size due to enhanced stability on doping.…”

“…In our previous work, we observed that doping Cu atoms in CsPbI 3 can enhance its stability, even under laboratory-ambient conditions . In this study, Cu-doped CsPbI 3 NCs with enhanced structural stability at room temperature have been used to investigate the effect of electron beam irradiation while probing single NCs at doses conventionally used in atomic-resolution TEM analysis.…”

“…In our previous work, we observed that doping Cu atoms in CsPbI 3 can enhance its stability, even under laboratoryambient conditions. 28 In this study, Cu-doped CsPbI 3 NCs with enhanced structural stability at room temperature have been used to investigate the effect of electron beam irradiation while probing single NCs at doses conventionally used in atomic-resolution TEM analysis. Precisely, we have been able to capture the point of prominent transformation of NCs from the stable cubic phase to the less stable orthorhombic phase, which is hitherto unreported in any all-inorganic LHP.…”

Ligand‐Mediated Revival of Degraded α‐CsPbI3 to Stable Highly Luminescent Perovskite

“…In the CsPbX 3 perovskite family, CsPbI 3 is especially prone to moisture so that it loses its structural integrity even at room temperature and under moderately humid conditions, causing undesired transformation to the orthorhombic γ-phase and finally degradation to the non-perovskite δphase. [14,15] In the thermally-annealed perovskite films, the α-CsPbI 3 is known to be stable at above 320 °C due to the strain generated at the film/substrate interface caused by thermal expansion mismatch. [16,17] Steele et al reported that the mechanical stress caused by the biaxial anisotropy near the substrate clamping led to a fast phase change (from α!β!γ) during the cooling process of the thin films.…”

The Compound Effect of Mg and Tris(2-aminoethyl)amine for Long-Term Phase Stability in the γ-CsPbI3 Perovskite Film

“…Another approach that has been identified to stabilize the photoactive phase of CsPbI 3 is by partial substitution of the B site occupied by Pb 2+ with some smaller radius elements such as Ca 2+ , Mn 2+ , Ge 2+ , Cu 2+ , Eu 2+ , Bi 3+ , , Sn 2+ , Zn 2+ , Mg 2+ , and Sr 2+ , or by partial substitution of the X site occupied by I – with some smaller radii halide like Br – − and Cl – In addition to tuning the tolerance factor, other effective ways of stabilizing the photoactive phase of CsPbI 3 is by reducing the crystallite size in the nanometer range and introducing strain. ,− …”