Optoelectronic Dichotomy of Mixed Halide CH₃NH₃Pb(Br_1–xCl_x)₃ Single Crystals: Surface versus Bulk Photoluminescence

Abstract

In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CHNHPb(BrCl ) (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite ( x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at lower percentages of halide substitution ( x ≤ 11%). The observed opposing trends, based on 1PE versus 2PE, highlight a dichotomy between extrinsic (surface) and intrinsic (bulk) effects of chloride substitution on the carrier dynamics in lead bromide perovskites. We discuss the physical relation between halide exchange and bulk carrier lifetimes in CHNHPbBr in terms of the Rashba effect. We propose that the latter is suppressed at the surface due to disorder in the alignment of the MA and that it increases in the bulk with Cl concentration because of the reduction in lattice parameters, which compresses the space available for the MA orientational degrees of freedom.

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time‐Resolved Microspectroscopy

“…Such dichotomy between extrinsic (surface) and intrinsic (bulk) effects of halide substitution in lead bromide perovskites was also reported in another kind of mixed halide single crystal (MAPb(Br x Cl 1−x ) 3 ) observed by the contrast between one-and two-photon excitation. [129] More recently, Wang and coworkers showed that photo-induced phase segregation in CsPb(Br x I 1−x ) 3 microplatelets can be significantly mitigated by simple surface passivation of covering a thin layer of PMMA, as shown in Figure 9D. [9] Lai et al studied anion interdiffusion in CsPbCl 3 -CsPbBr 3 single-crystalline perovskite nanowire heterojunctions fabricated via anion exchange chemistry, [130] as shown in Figure 9E, which directly visualize the intrinsic anionic inter-diffusion between Cl/Br rich ends of nanowire at elevated temperature, and reveal a mechanism of halide vacancies assisted ion migration via PL lifetime mapping and molecular simulation.…”

“…Such dichotomy between extrinsic (surface) and intrinsic (bulk) effects of halide substitution in lead bromide perovskites was also reported in another kind of mixed halide single crystal (MAPb(Br x Cl 1− x ) 3 ) observed by the contrast between one‐ and two‐photon excitation. [ 129 ] More recently, Wang and coworkers showed that photo‐induced phase segregation in CsPb(Br x I 1− x ) 3 microplatelets can be significantly mitigated by simple surface passivation of covering a thin layer of PMMA, as shown in Figure 9D. [ 9 ] Lai et al.…”

Probing the Spatial Heterogeneity of Carrier Relaxation Dynamics in CH3NH3PbI3 Perovskite Thin Films with Femtosecond Time‐Resolved Nonlinear Optical Microscopy

“…In our experiments, the probe wavelength was set to 745 nm, which is within the strong transient gain peak near the band edge as determined by previous fs-TA spectroscopy [35] . For this probe wavelength, our experimental setup restricted the pump to a longer wavelength, here 817 nm, which is below the band gap of these perovskite samples (see Figure S2 for representative linear absorption and emission spectra), but excitation of photo-generated carriers is still possible via multi-photon absorption processes [36][37][38][39] (see the supplementary information for details of the excitation conditions). Previous results demonstrate that the resulting TA spectra under two-photon excitation are similar to those under single-photon excitation [40] .…”

Charge Transport in Halide Perovskite Single Crystals: Experimental and Theoretical Perspectives

“…Furthermore, introducing new concepts might be necessary to overcome the unique research challenges encountered in the study of the HaP materials. As suggested by Zhu et al.,, HaPs could be treated as “crystalline liquid” as they belong to the phonon glass electron crystals family due to their binary structure formed by a PbX 3 ‐sublattice (which forms a cage) and a weakly bound A + cation sublattice. The particular nature of these materials leads to the stark contrast in the charge carrier and phonon transport properties.…”