Role of Electron–Phonon Coupling in the Thermal Evolution of Bulk Rashba-Like Spin-Split Lead Halide Perovskites Exhibiting Dual-Band Photoluminescence

Abstract

The optoelectronic properties of lead halide perovskites strongly depend on their underlying crystal symmetries and dynamics, sometimes exhibiting a dual photoluminescence (PL) emission via Rashba-like effects. Here we exploit spin-and temperature-dependent PL to study single crystal APbBr3 (A= Cs and methylammonium; CH3NH3) to evaluate peak energy, intensity and linewidth evolutions of the dual emissions. Both materials are identified to have two temperature regimesabove and below approximately 100 Kbeing governed by different carrier scattering and radiative recombination dynamics. With increasing temperature, high-energy optical phonons (>11 meV) are found to drive energy splitting of the dual bands and electron-longitudinal-optical-phonon coupling dominates the linewidth broadening, with a stronger coupling constant inferred in CsPbBr3 for the spin-split indirect bands (78 meV) compared to the direct one (54 meV). We find the unusual thermal evolution of all-inorganic CsPbBr3 and hybrid MAPbBr3 perovskites are comparablesuggesting A-site independence and dominance of dynamic spin-splitting effectsand are best understood within a framework which accounts for bulk Rashba-like effects. The interest for solution-processable lead halide perovskites within efficient solar cells 1,2 stems from their promising optoelectronic response to the solar photons and high tolerance to defects 3,4. This family of semiconductors are increasingly being considered as "soft" solid-state materials 5-7 , whereby the fate of photo-generated charges primarily rely on the fundamental carrier-lattice interaction dynamics. For instance, polaron formationvia carrier-longitudinaloptical-phonon (Fröhlich) interactionswithin the lattice has been linked to several favourable qualities, like long carrier lifetimes and diffusion lengths 8-10. Recent indications of spin splitting and indirect tail state formation in lead halide perovskites 11-16 due to Rashba-like effects 17 motivate a reconsideration of how electron-phonon coupling can exist within its perturbed electronic band structure. Universally, for the application of any polar metal halide perovskite, the properties of the free charge carriers and phonon scattering mechanisms are central to its optoelectronic performance at room temperature (RT).

Photophysics of Methylammonium Lead Tribromide Perovskite: Free Carriers, Excitons, and Sub‐Bandgap States

“…Significant discrepancies are found in literature about the position of the PL peak with values between 500 and 574 nm, and dual PL emission has also been observed . The variation in PL position and the dual PL emission have been attributed to different origins such as reabsorption, reabsorption combined with carrier diffusion, distinct emission from bulk and surface states (due to different bandgaps or different perovskite phases), different band energies of ordered and disordered domains, and finally to a Rashba‐like spin‐split . These phenomena are discussed below in more detail.…”

“…Finally, an additional electronic phenomenon that can contribute to the dual emission observed in MAPbBr 3 is the strong Rashba-like spin-split present in this perovskite. [91,[99][100][101] The combination of two phenomena leads to this effect: First, the strong SOC that is induced by the heavy Pb atom and second, a static or dynamic breaking of the inversion symmetry in the crystal that leads to the appearance of an effective magnetic field on the electrons by SOC, lifting spin degeneracy and splitting the band electronic structure. [91] How the Rashba-like effect leads to dual emission is explained by Steele et al, [91] supported by temperature dependent PL measurements, and is schematically shown in Figure 3f.…”

“…[91,[99][100][101] The combination of two phenomena leads to this effect: First, the strong SOC that is induced by the heavy Pb atom and second, a static or dynamic breaking of the inversion symmetry in the crystal that leads to the appearance of an effective magnetic field on the electrons by SOC, lifting spin degeneracy and splitting the band electronic structure. [91] How the Rashba-like effect leads to dual emission is explained by Steele et al, [91] supported by temperature dependent PL measurements, and is schematically shown in Figure 3f. The effect causes the shift of the valence band maxima (VBM) [102] and conduction band minima (CBM) away from the high symmetry points in the Brillouin zone, giving rise to LE indirect tail states.…”

Halide Perovskite Nanocrystal Emitters

“…[ 49–53 ] However, this emission feature can be also correlated to the Rashba effect induced by the centrosymmetry breaking by the perovskite's organic cations. [ 54,55 ] In addition, the grain size of conventional CH 3 NH 3 PbBr 3 bulk crystals can also determine which band structure dominates the overall PL spectra in between different structural phases (e.g., tetragonal‐to‐orthorhombic phase transition) at low temperature (e.g., 40–50 K), generating a dual‐color emission when the same weight in both bands is achieved upon an optimum grain size of about 100 nm. [ 56 ] Recently, Wu et al clarified that dual‐color emission is even a common character of all lead halide perovskites, but the relative intensity of the low‐energy peak (i.e., at longer wavelength) strongly depends on the surrounding conditions, such as the environmental temperature, and can lead to a nondistinguishable second peak in some cases.…”

Nature of the different emissive states and strong exciton–phonon couplings in quasi-two-dimensional perovskites derived from phase-modulated two-photon micro-photoluminescence spectroscopy

“…12,13 By suppressing nonradiative recombination and balancing carrier injection, the luminescence efficiency of perovskite-based devices has been significantly improved. [14][15][16][17] Despite a significant increase in efficiency, the development of 3D-perovskite-based LEDs is plagued by their poor stability. 18,19 Firstly, because of their soft structure, they can easily decompose at high-temperature or high-humidity.…”