Probing femtosecond lattice displacement upon photo-carrier generation in lead halide perovskite

Abstract

Electronic properties and lattice vibrations are expected to be strongly correlated in metal-halide perovskites, due to the soft fluctuating nature of their crystal lattice. Thus, unveiling electron–phonon coupling dynamics upon ultrafast photoexcitation is necessary for understanding the optoelectronic behavior of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal vibrational modes of methylammonium lead-bromide perovskite under electronically resonant and non-resonant conditions. We identify two excited state coherent phonons at 89 and 106 cm−1, whose phases reveal a shift of the potential energy minimum upon ultrafast photocarrier generation. This indicates the transition to a new geometry, reached after approximately 90 fs, and fully equilibrated within the phonons lifetime of about 1 ps. Our results unambiguously prove that these modes drive the crystalline distortion occurring upon photo-excitation, demonstrating the presence of polaronic effects.

White light emission in low-dimensional perovskites

“…82 In MAPbX 3 , lattice displacement after photoinduced carrier generation was probed through impulsive vibrational spectroscopy, providing further experimental evidence of the polaronic nature of the photoexcitation. 83,84 In low-dimensional systems, the coulombic and dielectric confinement sustain tightly bound excitons which interact with the highly polar lead halide framework. Recently, strong photoinduced lattice deformations have been identified in several 2D perovskites, where excitonic polarons must be considered to account for their characteristic luminescence and absorption line-shapes.…”

Defect Activity in Lead Halide Perovskites

“…Upon disclosure of the first reports on efficient solid state perovskite solar cells, one of the main surprises was the high open circuit voltage of such devices despite the absorber materials being solution processed. The reported long carrier lifetimes and diffusion lengths have directed a considerable amount of research toward investigating the nature of the carriers, and the role of electronic structure in their transport and dynamics. To complete the picture, researchers require an accurate understanding of the role played by defects in the photophysics of these materials, which by extension requires an understanding of the chemical nature and activity of those defects.…”

Two-Dimensional Impulsively Stimulated Resonant Raman Spectroscopy of Molecular Excited States

“…Furthermore, Raman spectroscopy benefits from resonance enhancements of specific chromophore signatures and thereby provides a route to comprehensively investigate vibrational energy flow during reactive transformations by selectively probing specific environments [25]. In particular, time-domain impulsive stimulated Raman scattering (ISRS) [26][27][28][29][30][31][32][33][34][35][36] offers several advantages over its frequency-domain analogues for the detection of vibronic features, especially for low-frequency modes, by efficiently removing elastic scattering contributions and background noise [37][38][39][40]. Its multidimensional extension, 2D-ISRS, has been theoretically proposed initially [41] and realized in both nonresonant [42,43] and resonant [44,45] implementations to study ground-state intramolecular vibrational anharmonicities, nonlinear corrections to the molecular polarizability, product-reactant correlations, and solvation dynamics, up to the recent realization of single-pulse 2D spectroscopy by means of appropriately shaped light pulses [46], with possible applications theoretically suggested for the x-ray domain [47].…”