Photoinduced Vibrations Drive Ultrafast Structural Distortion in Lead Halide Perovskite

Alexey Akimov,Zheng Li,Henry J. Snaith,Michael Thorwart,R. J. Dwayne Miller,Oriol Vendrell,Vandana Tiwari,Golibjon R. Berdiyorov,Ajay Jha,Mohamed E. Madjet,Pabitra K. Nayak,Hong-Guang Duan

doi:10.1021/jacs.0c03970

Abstract

The success of organic–inorganic perovskites in optoelectronics is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation are hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use ultrafast heterodyne-detected two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which drive the structural distortion after photoexcitation. Vibrational analysis of the measured data allows us to monitor the time-evolved librational motion of the MA cation along with the vibrational coherences of the inorganic sublattice. Wavelet analysis of the observed vibrational coherences reveals the coherent generation of the librational motion of the MA cation within ∼300 fs complemented with the coherent evolution of the inorganic skeletal motion. To rationalize this observation, we employed the configuration interaction singles (CIS), which support our experimental observations of the coherent generation of librational motions in the MA cation and highlight the importance of the anharmonic interaction between the MA cation and the inorganic sublattice. Moreover, our advanced theoretical calculations predict the transfer of the photoinduced vibrational coherence from the MA cation to the inorganic sublattice, leading to reorganization of the lattice to form a polaronic state with a long lifetime. Our study uncovers the interplay of the organic cation and inorganic sublattice during formation of the polaron, which may lead to novel design principles for the next generation of perovskite solar cell materials.

Highlights

On the quest for a low-cost and processable photoactive material, organic−inorganic lead halide perovskites (LHPs) have emerged as a class of material with tremendous potential
Photovoltaics based on LHPs have displayed a remarkable increase in power conversion efficiencies (PCE) as compared to other technologies in the past decade advancing to 24.2% based on a solid polycrystalline perovskite.[1−6] Largely due to their easy solution processability, large carrier diffusion length, and high photoluminescence quantum yield,[7−9] perovskites have found their application in other areas such as photodetectors and lasing.[3,10−14] Despite the success of the material to reach macroscopic performance scales such as the highly efficient PCE, a comprehensive understanding of the underlying microscopic phenomena of charge generation following photoexcitation is still far from being understood
To unravel the microscopic mechanisms of the success of perovskite in photoinduced charge generation, numerous experimental and theoretical works have been reported focusing on understanding the elementary photophysical processes in LHPs.[15−23] One of the important unresolved questions pertaining to perovskite photophysics is the origin of long charge carrier lifetimes using processing methods that generally lead to midgap states

Summary

Introduction

On the quest for a low-cost and processable photoactive material, organic−inorganic lead halide perovskites (LHPs) have emerged as a class of material with tremendous potential. To unravel the microscopic mechanisms of the success of perovskite in photoinduced charge generation, numerous experimental and theoretical works have been reported focusing on understanding the elementary photophysical processes in LHPs.[15−23] One of the important unresolved questions pertaining to perovskite photophysics is the origin of long charge carrier lifetimes using processing methods that generally lead to midgap states. Those act as efficient recombination centers, the charge carrier mobilities in LHPs have been reported to be modest, i.e., only ∼50−100 cm[2] V−1 s−1.8,24,25 This puzzling observation has attracted enormous attention by various research groups.

Methods

Discussion

Conclusion