Photoinduced Small Hole Polarons Formation and Recombination in All-Inorganic Perovskite from Quantum Dynamics Simulation

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

Electron- versus Spin-Phonon Coupling Governs the Temperature-Dependent Carrier Dynamics in the Topological Insulator Bi2Te3

Ultrafast charge and spin dynamics have immense effects on the applications of topological insulators (TIs). By performing spin-adiabatic nonadiabatic molecular dynamics simulations in the presence of electron-phonon (e-ph) and spin-phonon couplings, we investigate temperature-dependent intra- and interband charge and spin relaxation dynamics via the bulk and surface paths in the three-dimensional TI Bi2Te3. The e-ph coupling dominates charge relaxation in the bulk path, and the relaxation rate is positively correlated with temperature due to the large energy gaps and weak spin polarization. Conversely, the relaxation dynamics exhibits an opposite temperature dependence in the surface path because of electron re-excitation and spin mismatching induced by spin-phonon coupling, which arises from small energy gaps and strong spin polarization. The two mechanisms rationalize the charge carriers being long-lived in the bulk and surface phases at low and room temperature, respectively. Additionally, strong thermal fluctuations of the topological states' magnetic moments destroy the spin-momentum locking and trigger backscattering at room temperature.

Critical Roles of Octahedron Bilayer Surface/Interior Bromide Defects in Photodynamics of Multi-Quantum-Well-Structured Cesium Bismuth Bromide

We investigate theoretically the roles of the intrinsic point defects in the photophysics of wide-bandgap multi-quantum-well-structured Cs₃Bi₂Br₉ based on the Shockley-Read-Hall statistics and multiphonon recombination theory. The GW plus Bethe-Salpeter equation calculation reveals that there is a prominent exciton peak below the interband absorption edge, and it clarifies the experimental debate. The most energetically favorable native defects possess deep thermodynamic transition levels. The bromide self-interstitials within the octahedron bilayers exhibit as efficient carrier trapping centers through the non-radiative multiphonon recombination, with a lifetime of 184 ns being on the same order of magnitude as the experimental value. The octahedron bilayer surface bromide self-interstitials account for the experimentally observed dominant blue luminescence in Cs₃Bi₂Br₉. These results reveal that the intrinsic point defects at different sites of the multi-quantum-well-like octahedron bilayers play different roles in the photodynamics of such unique layer-structured semiconductors.

Superior Limit of Light-Absorption Improvement in Two-Dimensional Haeckelite GaN-ZnO by Nonadiabatic Molecular Dynamics Simulation

A weak internal electrostatic field is usually required to improve optical performance; however, this is not the case in two-dimensional haeckelite (8|4) GaN-ZnO that has physical properties that are better than those of their binary counterparts. By performing nonadiabatic molecular dynamics simulations, we ascribe the superior limit of improvement of light absorption to the convergence of the electron-hole recombination time when the thickness of the 8|4 phase exceeds a critical value, which arises from the competition between nonadiabatic coupling and quantum decoherence. We show that nonadiabatic coupling continuously becomes weaker because of the reduced nucleus velocity with an increase in thickness. We further demonstrate that the quantum decoherence is first accelerated and then decelerated because of the thickness-dependent electron-phonon coupling controlled by the peculiar in-plane A' and A″ phonon modes. Our study clarifies the issue with regard to light absorption, which provides useful guidance for improving our understanding of the optical properties in two-dimensional polar semiconductors.

Charge Recombination Dynamics in a Metal Halide Perovskite Simulated by Nonadiabatic Molecular Dynamics Combined with Machine Learning

Nonadiabatic coupling (NAC) plays a central role in driving nonadiabatic dynamics in various photophysical and photochemical processes. However, the high computational cost of NAC limits the time scale and system size of quantum dynamics simulation. By developing a machine learning (ML) framework and applying it to a traditional CH₃N₃PbI₃ perovskite, we demonstrate that the various ML algorithms (XGBoost, LightGBM, and random forest) combined with three descriptors (sine matrix, MBTR, and SOAP) can predict accurate NACs that all agree well with the direct calculations, particularly for the combination of LightGBM and sine matrix descriptor showing the best performance with a high correlation coefficient of ≤0.87. The simulated nonradiative electron-hole recombination time scales agree well with each other between the NACs obtained from direct calculations and ML prediction. The study shows the advantage in accelerating quantum dynamics simulations using ML algorithms.

Tuning the Nonradiative Electron-Hole Recombination with Defects in Monolayer Black Phosphorus

We use nonadiabatic (NA) molecular dynamics to demonstrate that the nonradiative electron-hole recombination is delayed and accelerated by the Stone-Wales (SWs) and phosphorus divacancy (DV-(5|7)) defects in monolayer black phosphorus (BP). Both types of defects increase the bandgap by 0.1 eV without creating midgap states. Driven by P-P stretching vibrations, the recombination proceeds within 1 ns in the SW and within 100 ps in the DV-(5|7), respectively, which occurs within 332 ps in BP. The SW defect slows down recombination because the notably reduced NA coupling combined with a large bandgap competes to the long-lived coherence. In contrast, the DV defect accelerates recombination since long-lived coherence is superior to the slightly decreased NA coupling correlated with a tiny increased bandgap. The diverse time scales rationalize the broad range of charge carrier lifetimes reported experimentally. The study provides a strategy to engineer excited-state dynamics for improving the BP-based optoelectronics.