[Paper Review] Photoinduced Vibrations Drive Ultrafast Structural Distortion in Lead Halide Perovskite

Hong-Guang Duan, Vandana Tiwari+10 more|arXiv (Cornell University)|Apr 10, 2020

Perovskite Materials and Applications49 references4 citations

TL;DR

This study reveals that photoexcitation in methylammonium lead iodide perovskite impulsively excites librational vibrations in the organic MA cation and coherent lattice vibrations in the inorganic PbI₃ sublattice, with time-resolved 2D electronic spectroscopy and TDDFT calculations showing that these vibrations drive ultrafast structural distortion and polaron formation within ~300 fs, explaining long-lived charge carriers despite mid-gap states.

ABSTRACT

Organic-inorganic perovskites have shown great promise towards their application in optoelectronics. The success of this class of material is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation is hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which drive the structural distortion after photoexcitation. The vibrational analysis of the measured data allows us to directly monitor the time evolution of the librational motion of the MA cation along with the vibrational coherences of inorganic sublattice. Wavelet analysis of the observed vibrational coherences uncovers the interplay between these two types of phonons. It reveals the coherent generation of the librational motion of the MA cation within ~300 fs, which is complemented by the coherent evolution of the skeletal motion of the inorganic sublattice. We have employed time-dependent density functional theory (TDDFT) to study the atomic motion of the MA cation and the inorganic sublattice during the process of photoexcitation. The TDDFT calculations 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. Our calculations predict the transfer of the photoinduced vibrational coherence from the MA cation to the inorganic sublattice, which drives the skeleton motion to form a polaronic state leading to long lifetimes of the charge carriers. This work may lead to novel design principles for next generation of solar cell materials.

Motivation & Objective

• To understand the microscopic origin of long charge carrier lifetimes in lead halide perovskites despite mid-gap states.
• To investigate the role of organic cation (MA) and inorganic sublattice vibrations in photoinduced structural dynamics.
• To determine how coherent vibrational modes in the MA cation and PbI₃ lattice interplay to drive ultrafast structural distortion post-photoexcitation.
• To establish a link between vibrational coherence and polaron formation using combined ultrafast spectroscopy and TDDFT simulations.

Proposed method

• Ultrafast heterodyne-detected two-dimensional (2D) electronic spectroscopy with 1 fs time resolution to probe vibrational coherences and dynamics.
• Wavelet analysis of 2D spectra to extract time-evolving vibrational frequencies and amplitudes of librational and skeletal modes.
• Time-dependent density functional theory (TDDFT) simulations of excited-state molecular dynamics on a 2×2×2 supercell to model atomic motions after photoexcitation.
• Principal component analysis of MD trajectories to identify dominant structural motions and project them onto vibrational eigenvectors of the ground state.
• Fourier transform of velocity autocorrelation functions to extract vibrational spectra: $ I( au, au) = rac{1}{N_{at}}igackslashlangle extbf{v}_k( au) \cdot \textbf{v}_k(\tau+t) \exp(-\alpha t^2) \big angle $.
• Modeling charge localization via electron injection at $10^{19}~\text{cm}^{-3}$ to simulate excited-state behavior.

Experimental results

Research questions

• RQ1 How do photoexcitation-induced vibrations in the methylammonium cation and inorganic PbI₃ sublattice evolve on the ultrafast timescale?
• RQ2 What is the temporal sequence of coherent librational motion in the MA cation and skeletal motion in the inorganic lattice?
• RQ3 How do anharmonic interactions between the organic cation and inorganic sublattice drive structural distortion and polaron formation?
• RQ4 To what extent does vibrational coherence transfer from the MA cation to the inorganic lattice, and how does this affect charge carrier dynamics?

Key findings

• Coherent librational motion of the methylammonium cation is generated within ~300 fs after photoexcitation, as revealed by wavelet analysis of 2D spectra.
• The inorganic PbI₃ sublattice exhibits coherent skeletal vibrations with frequencies below 100 cm⁻¹, synchronized with MA cation librations.
• TDDFT simulations confirm the coherent generation of MA cation librations and predict a transfer of vibrational coherence to the inorganic lattice within 1 ps.
• The anharmonic coupling between the MA cation and PbI₃ lattice drives a structural distortion that forms a polaronic state, explaining long-lived charge carriers.
• Electron localization function analysis confirms strong charge localization on Pb atoms in the excited state, consistent with polaron formation.
• The observed vibrational coherences and structural dynamics are reproducible across different film batches and measurement spots, confirming robustness of the effect.