Spectral Instability of Layered Mixed Halide Perovskites Results from Anion Phase Redistribution and Selective Hole Injection

Abstract

Despite the ability to precisely tune their bandgap energies, mixed halide perovskites (MHPs) suffer from significant spectral instability, which obstructs their utilization for the rational design of light-emitting diodes. Here, we investigate the origin of the electroluminescence peak shifts in layered MHPs containing bromide and iodide. X-ray diffraction and steady-state absorption measurements prove effective integration of iodide into the cubic lattice and the spatially uniform distribution of halides in the ambient environment. However, the applied electric field during the device operation is found to drive the systematic halide migration. Quantum mechanical density functional theory calculations reveal that the different activation energies required for directional ion hopping lead to the redistribution of anions. In-depth analyses of the electroluminescence spectra indicate that the spectral shifting rate is dependent on the drift velocity of halides. Finally, it is suggested from our study that the dominant red emission is ascribed to the thermodynamically favorable selective hole injection. Our mechanistic study provides insights into the fundamental reason for the spectral instability of devices based on MHPs.

Energy-Funneling Process in Quasi-2D Perovskite Light-Emitting Diodes

“…Above all, to enhance the PLQYs of the mixed-halide perovskites, the quasi-2D perovskite structure was employed. As expected, the quasi-2D perovskites with the mixed halide not only inherit the feature that process the emission spanning the visible spectrum but also largely improve the PLQYs attributed to the energy-funneling process. − Specifically, by introducing an amount of typical large organic cation (e.g., BA + ), the resulting quasi-2D perovskite-based PeLED realized the peak EQEs of 2.4% and 6.2% at 465 and 487 nm, respectively (Figure a), whose EQEs were much higher than that of 3D perovskite-based PeLEDs in the same period (<1%). , By further optimizing the charge-injection pathway across the quasi-2D perovskite layer, Cao et al realized a precision modulation for the recombination zone and achieved further improved PeLEDs with an EQE of 5.7% at 480 nm . Zhong et al also used mixed ligands of PEABr and 3,3-diphenylpropylamine bromide to synergistically tailor quantum-well width distribution and achieved efficient blue PeLEDs with a high EQE of 8.8% at 473 nm .…”

Manipulating Color Emission in 2D Hybrid Perovskites by Fine Tuning Halide Segregation: A Transparent Green Emitter

“…[ 16,18–22 ] Br–Cl mixtures of 2DP materials have been recently studied in order to develop white‐light LEDs by exploiting the light emission from self‐trapped excitons formed in the distorted Br–Cl low‐dimensional phase: [ 20,21,23 ] in fact, due to the layered structure of these materials, the distortions induced by the different halide substitution are enhanced leading to the formation of broad light emission, as in the case of Br–Cl mixtures, or even phase separation. [ 20,21,24,25 ] For 2DPs comprised of I–Br halides mixture, working LEDs have been recently studied: [ 19,26 ] the high voltage necessary for their functioning favors halides migration, and subsequent formation of zones in which halides are inevitably segregated. Hence, in most cases, uncontrolled halide migration and separation in different phases are observed, which makes their manipulation and their direct implementation into working devices highly challenging.…”

Ion Migration in Metal Halide Perovskites

“…As the ionic radius of Br -(0.196 nm) is smaller than I -(0.220 nm), Brwill more easily migrate in the perovskite crystal despite the lattice size of the Brsubstituted perovskite in smaller than generic material. In contrast, other groups suggested that according to computational study, Brhas comparable energy activation to I - [30,31].…”