Surface Pb-Dimer Passivated by Molecule Oxygen Notably Suppresses Charge Recombination in CsPbBr3 Perovskites: Time-Domain Ab Initio Analysis

Significant Impact of Defect Fluctuation on Charge Dynamics in CsPbI3: A Study Combining Machine Learning with Quantum Dynamics

In this study, we developed a machine-learned force field for CsPbI3 using a neural network potential, enabling molecular dynamics simulations (MD) with ab initio accuracy over nanoseconds. This approach, combined with ab initio MD and nonadiabatic MD, was used to study the charge trapping and recombination dynamics in both pristine and defective CsPbI3. Our simulations revealed key transitions affecting carrier lifetimes, especially in systems with iodine vacancy and interstitial iodine defects. An iodine trimer, formed when iodine replaces cesium, exhibits a high-frequency phonon mode. This mode enhances nonadiabatic coupling, accelerating charge recombination in defective systems compared to pristine ones. In the iodine vacancy system, recombination times varied significantly due to differences in NA coupling and energy gaps. The interplay between nonadiabatic coupling and pure dephasing time is crucial in determining recombination times for interstitial iodine defects. Our findings highlight the role of defect evolution in perovskites, offering insights for enhancing perovskite performance.

Moisture-Dependent Blinking of Individual CsPbBr3 Nanocrystals Revealed by Single-Particle Spectroscopy

All-inorganic metal halide perovskite nanocrystals (NCs) have been exceptional candidates for high-performance solution-processed optoelectronic and photonic devices compared with organometal halide perovskite NCs due to their superior stability. However, the interactions between all-inorganic perovskite NCs and moisture, which is an acknowledged detrimental factor, are still under debate, and detailed investigations to uncover such fundamentals remain to be performed. Herein, with wide-field fluorescence microscopy, the burst photoluminescence blinking responses of CsPbBr₃ NCs were observed in ambient air, and moisture rather than oxygen was verified to be the key factor that leads to the enhanced PL intensity and reduced OFF duration. This behavior is rationalized through an effective passivation effect of the adsorbed water molecules on the surface halide vacancies on CsPbBr₃ NCs. This work validates that ∼40% humidity atmospheres are helpful for better utilizing the all-inorganic perovskites, which is evidence of their promising prospect for application.

Elimination of Charge Recombination Centers in Metal Halide Perovskites by Strain

Metal halide perovskites exhibit enhanced photoluminescence and long-lived carriers in experiments under strain. Using ab initio nonadiabatic molecular dynamics, we demonstrate that compressive and tensile strain can eliminate charge recombination centers created by defect states, by shifting traps from bandgap into bands. A compressive strain enhances coupling of Pb-s and I-p orbitals, pushes the valence band (VB) up in energy, and moves the trap state due to iodine interstitial (I_i) into the VB. The strain distorts the system and breaks the I-dimer responsible for the I_i trap. A tensile strain increases Pb-Pb distance, weakens overlap of Pb-p orbitals, and pushes the conduction band (CB) down in energy. The trap state created by replacement of iodine with methylammonium (MA_I) is moved into the CB. Application of strain to the defective systems not only eliminates midgap traps but also creates moderate disorder that reduces overlap of electron and hole wave functions, activates phonon modes accelerating coherence loss within the electronic subsystem, and extends carrier lifetimes even beyond those in pristine MAPbI₃. Our investigation rationalizes the high performance of perovskites solar cells under strain and reveals how strain passivates I_i and MA_I defects in MAPbI₃, providing a new nonchemical strategy for defect control and engineering.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Preventing Superoxide Generation on Molecule-Protected CH3NH3PbI3 Perovskite: A Time-Domain Ab Initio Study

Metal halide perovskites are promising materials for photovoltaics and optoelectronics. However, transfer of an electron from perovskite to oxygen leads to the formation of superoxide that significantly decreases the stability and charge carrier lifetime of perovskites, which constitutes major issues for real applications. Using nonadiabatic (NA) molecule dynamics simulations, we demonstrate that the introduction of a perylene diimide (PDI) molecule into the CH₃NH₃PbI₃ system adsorbed with an oxygen molecule creates a midgap state above the trap state generated by the oxygen molecule, and thus the PDI midgap state can rapidly capture the photogenerated electron of perovskite at about 100 ps prior to the O₂-related trap state, which takes about double the time. The route simultaneously avoids the formation of superoxide and enhances the stability of perovskites. The fast electron trapping originates from the strong NA coupling and small energy gap between the PDI midgap state and the CH₃NH₃PbI₃ conduction band minimum. Our simulations suggest that a rational choice an electron-accepting molecule can improve the stability and performance of perovskite solar cells and photoelectric devices.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective

Perovskite solar cells have attracted intense attention over the past decade because of their low cost, abundant raw materials, and rapidly growing power conversion efficiency (PCE). However, nonradiative charge carrier losses still constitute a major factor limiting the PCE to well below the Shockley-Queisser limit. This Perspective summarizes recent atomistic quantum dynamics studies on the photoinduced excited-state processes in metal halide perovskites (MHPs), including both hybrid organic-inorganic and all-inorganic MHPs and three- and two-dimensional MHPs. The simulations, performed using a combination of time-domain ab initio density functional theory and nonadiabatic molecular dynamics, allow emphasis on various intrinsic and extrinsic features, such as components, structure, dimensionality and interface engineering, control and exposure to various environmental factors, defects, surfaces, and their passivation. The detailed atomistic simulations advance our understanding of electron-vibrational dynamics in MHPs and provide valuable guidelines for enhancing the performance of perovskite solar cells.

Ferroelastic domains drive charge separation and suppress electron-hole recombination in all-inorganic halide perovskites: time-domain ab initio analysis

All-inorganic perovskites have great potential in photovoltaic applications and their performance is subject to phonon-assisted charge recombination dynamics. Local microstructures, such as ferroelastic domains, are considered to have a significant influence on the charge carrier lifetime in the CsPbBr₃ perovskite. Employing a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics simulations, we demonstrate that the formation of ferroelastic domains weakens the NA coupling and suppresses the non-radiative electron-hole recombination. This effect originates from the ferroelastic domains separating electron and hole wave functions spatially and decreasing the NA coupling by a factor of 2.4 compared to pristine CsPbBr₃, delaying electron-hole recombination by a factor of 4.2. We also show that symmetry breaking enhances electronic-vibrational interactions, activating more phonon modes and accelerating quantum decoherence by 1 fs or so, which further slows recombination. Both factors compete successfully with the slightly reduced bandgap of about 0.2 eV and prolong the charge carrier lifetime to several nanoseconds. Our study advances the understanding of the atomistic mechanism for inhibited recombination in the CsPbBr₃ perovskite in the presence of ferroelastic domains, providing an effective route to design high-performance all-inorganic halide perovskites.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA_0.75MA_0.25PbI₃. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

Covalent Functionalized Black Phosphorus Greatly Inhibits Nonradiative Charge Recombination: A Time Domain Ab Initio Study

Mono- or few-layer black phosphorus (BP) has emerged as a promising photovoltaic and optoelectronic material with realistic applications subjected to instability and short charge carrier lifetime. Experiments show that covalent functionalization can improve the stability, but the underlying mechanism for the prolonged lifetime remains elusive. By performing spin-polarized time domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we demonstrate that BP passivated with both phenyl and nitrophenyl can suppress the nonradiative electron-hole recombination by a factor of 2 and 3, respectively, relative to the pristine system. The slow recombination is due to the interplay between energy gap, NA coupling, and decoherence time, which happens either through a hole-trap-assisted process or in a direct way between a free electron and hole in the spin-up channel. The observations hold in the spin-down channel. The study suggests that the passivating strategy should work for BP and other two-dimensional materials.