Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons

Abstract

Understanding the origin of electron–phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead–halide–lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump–electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Size‐ and Temperature‐Dependent Lattice Anisotropy and Structural Distortion in CsPbBr3 Quantum Dots by Reciprocal Space X‐ray Total Scattering Analysis

“…Such a choice does not significantly affect the major results of the structural analysis (as briefly discussed in Section 2.5), despite that anisotropic and anharmonic vibrations of halides in the LHP are supported by theoretical models and experimental evidence at the bulk scale, and their role in the photoexcited ultrafast dynamic response of halide perovskites is well recognized. [ 63–66,104–112 ]…”

“…Such a choice does not significantly affect the major results of the structural analysis (as briefly discussed in Section 2.5), despite that anisotropic and anharmonic vibrations of halides in the LHP are supported by theoretical models and experimental evidence at the bulk scale, and their role in the photoexcited ultrafast dynamic response of halide perovskites is well recognized. [63][64][65][66][104][105][106][107][108][109][110][111][112] Figure 5b shows the 3D maps of GoF = ðχ 2 Þ 1 2 vs (x,y) for Br eq at z = 0.486 and at selected DW's, while maps for Br ax (at z = 0.75, symmetry-restrained) are displayed in Figure 5c. On one side, based on the grid exploration, Br's locations are pinpointed from the best match with the WAXTS data and Pb─Br distances and Pb─Br─Pb bond angles, derived therefrom, discussed vs the bulk angles and distances.…”

“…[62] On the side of structural disorder, the current debate on the existence of static disorder, possibly coexisting with a dynamic one, remains a non-trivial task to be solved in halide perovskites, both at bulk and nanoscale. [63][64][65][66] Despite all these aspects playing a central role in the understanding of photoexcited phenomena and their timescale, a comprehensive quantitative picture of size-driven lattice and structural distortions based on experimental evidence in the model CsPbBr 3 NC system is lacking, and the (static and/or dynamic) nature of disorder remains an open question. This circumstance may be attributed to two major factors.…”

“…[ 62 ] On the side of structural disorder, the current debate on the existence of static disorder, possibly coexisting with a dynamic one, remains a non‐trivial task to be solved in halide perovskites, both at bulk and nanoscale. [ 63–66 ]…”

Quantifying the Size‐Dependent Exciton‐Phonon Coupling Strength in Single Lead‐Halide Perovskite Quantum Dots

“…[7b,11a] Exciton-phonon coupling is particularly rich in lead-halide perovskite QDs due to the many vibrational degrees of freedom and the soft character of the multi-component host lattice. Exciton-phonon coupling in lead-halide perovskite QDs was previously investigated by means of Raman spectroscopy, [12] 2D electronic spectroscopy, [13] ab initio molecular-dynamics (AIMD) calculations, [1,14] and low-temperature single-QD spectroscopy. [15] The latter method is particularly powerful, as it i) directly probes the emission process, ii) distinguishes coupling to a broad range of phonon modes, and iii) is free of inhomogeneities inherent to many ensemble-based studies, as it probes intrinsic emission properties of single QDs.…”