Influence of Defects on Excited-State Dynamics in Lead Halide

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Perspective Cite This: J. Phys. Chem. Lett. 2019, 10, 3788−3804

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Influence of Defects on Excited-State Dynamics in Lead Halide Perovskites: Time-Domain ab Initio Studies Wei Li,*,† Run Long,*,‡ Jianfeng Tang,*,† and Oleg V. Prezhdo*,§ †

College of Science, Hunan Agricultural University, Changsha 410128, People’s Republic of China College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing 100875, People’s Republic of China § Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States Downloaded via 94.231.217.48 on July 22, 2019 at 11:32:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: This Perspective summarizes recent research into the excitedstate dynamics in lead halide perovskites that are of paramount importance for photovoltaic and photocatalytic applications. Nonadiabatic molecular dynamics combined with time-domain ab initio density functional theory allows one to mimic time-resolved spectroscopy experiments at the atomistic level of detail. The focus is placed on realistic aspects of perovskite materials, including point defects, surfaces, grain boundaries, mixed stoichiometries, dopants, and interfaces. The atomistic description of the quantum dynamics of electron and hole trapping and recombination, provided by the time-domain ab initio simulations, generates important insights into the mechanisms of charge and energy losses and guides the development of high-performance perovskite solar cell devices.

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result in high carrier concentration are needed to achieve efficient solar cell devices with high voltage and large current. Defects are abundant in classical semiconductors, such as crystalline silicon, and have relatively high concentration. Whereas nonradiative charge recombination involving defect states is significant in the traditional semiconductors,8,9 the reported charge carrier lifetimes are exceptionally long in perovskites, ranging from a few into hundreds of nanoseconds10 and indicating that defect states in perovskite materials are benign electrically.

ybrid organic−inorganic perovskite solar cells (PSCs) have drawn intense interest in recent years because of the rapid evolution of the power conversion efficiency. Perovskite materials feature the chemical formula of ABX3, where A is a monovalent cation, e.g., Cs+, methylammonium (MA+), formamidinium (FA+) or guanidinium (GA+); B is a divalent cation, e.g., Pb2+ or Sn2+; and X is a halide anion such as Cl−, Br−, or I− (Figure 1). The record efficiency of PSCs based on the mixed (FAPbI3)0.95(MAPbBr3)0.05 perovskite reached 23.2% in 2018. Hybrid perovskites have many advantages over other materials, including low exciton binding energy, large carrier diffusion length, optimal band gap, high absorption coefficient, long charge carrier lifetime, abundant component elements, and low cost of fabrication. These superior properties motivate perovskite applications in photovoltaics,1,2 photocatalysis,3 thermoelectrics,4 light-emitting diodes,5,6 etc. Despite the significant progress in PSCs, the record efficiency of 23.2% is still below the detailed balance limit of 30% established for p−n junction solar energy devices with the energy gap at 1.1 eV.7 A thorough understanding of the mechanisms for charge and energy losses is essential for further progress. Defects are inevitable in solution-processed single-crystalline and polycrystalline films, and they are frequently present both in the bulk and at surfaces. Defects in solid materials exist in the form of point defects, such as vacancies and interstitials, and extended defects, including grain boundaries (GBs) and interfaces with other materials. Defects usually introduce subgap states, which are expected to trap charge carriers, facilitating nonradiative electron−hole recombination and energy losses. Generally, long charge carrier lifetimes that © 2019 American Chemical Society

Nonradiative charge recombination limits further improvement of perovskite solar cells. The questions of whether and how defects decrease charge carrier lifetimes are still under debate. We provide theoretical insights into the influence of defects on charge recombination, in direct connection with time-resolved experiments.

Received: March 5, 2019 Accepted: June 17, 2019 Published: June 17, 2019 3788

DOI: 10.1021/acs.jpclett.9b00641 J. Phys. Chem. Lett. 2019, 10, 3788−3804

The Journal of Physical Chemistry Letters

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Figure 1. (a) Crystal structure of ABX3 halide perovskites. A is a monovalent organic or inorganic cation, B a divalent metal cation, and X a halide anion. Schematics of (b) perfect lattice and various defects considered in this work: (c) iodine interstitial; (d) iodine vacancy; (e) Pb vacancy; (f) MAPbI3 with PbI surface termination; (g) MAPbI3 with MAI surface termination; (h) Σ5 (012) grain boundary (GB) in MAPbI3; (i) chlorinedoped Σ5 (012) GB; (j) MAPbI3 with some MA replaced by FA or GA; and (k) mixed halide CsPbBr1.5I1.5.

Nonradiative excited-state decay depends on the energy gap, NA coupling (NAC), and pure-dephasing/decoherence time. NAC between adiabatic orbitals j and k is defined by dR −iℏ⟨ϕj|∇R |ϕk ⟩· dt . The first term characterizes how adiabatic orbitals depend on a nuclear coordinate R, and the second term is the corresponding nuclear velocity. Note that R is a vector. In order for the NA coupling to be large, the orbitals j and k have to be localized in the same place; that is, they should overlap, the orbitals should depend significantly on nuclear coordinates, and the nuclear velocities for these coordinates should be large, e.g. when atoms are light and phonons have high frequencies. The coherence time is long if phonon-induced fluctuation of the corresponding energy gap is small and if the fluctuation maintains long correlation.42 According to Fermi’s golden rule, the nonradiative decay rate is proportional to the coupling squared and inversely proportional to the energy gap. Quantum dynamics is faster if coherence time is large. Rapid decoherence leads to the quantum Zeno effect, according to which quantum dynamics stops.43 In this Perspective, we summarize the recent progress that has been made to understand the excited-state dynamics in halide perovskites in the presence of defects using the simulation tool PYXAID,44,45 which implements state-of-theart NA-MD in the framework of time-domain density functional theory (TD-DFT) developed by the Prezhdo group.46 The discussions focus on the influences of several types of defects, including point defects, surfaces, and boundaries, as well as dopants and mixed stoichiometries, on the charge carrier trapping and relaxation processes. Exemplifying these practical aspects in halide perovskites, the combined NA-MD/TD-DFT methodology demonstrates how hole trapping by the iodine interstitial defect can extend charge carrier lifetimes; how charge recombination can be controlled by modulating oxidation states of the iodine vacancy; that a lead vacancy can suppress nonradiative electron−hole recombination; why PbI 2-rich perovskites show better performance than MAI-rich systems; how passivation of perovskite surfaces with Lewis bases can extend charge carrier lifetime by different mechanisms; that GBs accelerate charge recombination, whereas doping can repair the detrimental

Many experimental and theoretical works are devoted to understanding the unique defect physics in PSCs.11−24 It has been shown from the experimental viewpoint that defects in perovskites are correlated with such critical issues as structural instability,25 ion migration,25 current−voltage hysteresis,26 giant photovoltaic effect switchable by electric field,27 and formation of n−i−p structures.28 An ad hoc model based on bistable amphoteric native defects has been used to account for these uncommon phenomena.29 Most theoretical works use first-principles calculations based on density functional theory (DFT) to investigate the defect properties, including formation energies and defect energy levels. Investigation of such properties requires big supercells, and therefore, bare DFT functionals are employed for this purpose. Although such functionals predict correct energy gaps because of error cancellation, a more reliable description of perovskite electronic structure requires hybrid DFT functionals with spin−orbital coupling effects, or GW calculations, which are very time-consuming.16,30,31 Several groups used first-principles calculations to study energy levels and formation energies of native point defects in MAPbI3, demonstrating that most defects are shallow and have low formation energies and that defects with deep levels are hard to form.16,19,32,33 The concept of “defect tolerance” was used to explain such particular properties of PSCs.34 The unusual defect tolerance originates from the ionic nature and strong antibonding coupling between Pb 6s and I 5p orbitals. Considering vacancies as an example, the dangling bond states produce defect levels that are either close to the band edges or resonances inside the bands. However, recent experiments suggest that shallow defects play important roles in inducing unexpected doping and deteriorating carrier lifetimes.35−37 The majority of the theoretical studies focus on geometric and electronic structure of defects, and detailed atomistic insights into the influence of defect states on charge relaxation and recombination are generally lacking. Theoretical description of the nonequilibrium nature of such dynamics in condensed matter systems is still challenging.38−41 Nonadiabatic molecular dynamics (NA-MD) is the method of choice to model excited-state processes in the time domain and at the atomistic level of detail, mimicking time-resolved experiments directly. 3789



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Moreover, excited-state forces have to be calculated to determine these trajectories. Such calculations are extremely computationally demanding and can be simplified greatly using the classical path approximation (CPA).45 CPA uses a precomputed nuclear trajectory to determine the TD-DFT and SH dynamics. Usually, a short time ground-state trajectory can be used, justified by the facts that excitation of one or few electrons in condensed matter and nanoscale systems induces only a small perturbation to the electron density and nuclear forces, in particular, compared to thermal nuclear fluctuations. In order to obtain quantum dynamics on a nanosecond time scale, one can extrapolate the results of picosecond simulations using a short-time approximation to the exponential decay. Alternatively, one can assume that the picosecond simulation provides sufficient sampling of the excited-state energies and NA couplings, iterate the NA Hamiltonian multiple times, and perform quantum dynamics simulations on a long time scale. The extrapolation approach is suitable when considering transitions from initially populated states, while the Hamiltonian iteration technique is useful to study transient population and depopulation of traps. NA-MD has been applied to various perovskites systems, including low-dimensional perovskites,55,56 perovskites containing defects57−59 and GBs,60,61 perovskite surfaces passivated with organic molecules,62 interfaces with TiO2,63 etc.64−75 Please refer to other reviews for a more comprehensive description of the NA-MD methodology and surface hopping algorithms.48,76−82

effect produced by the GBs; that chemical dopants can be used to control charge recombination at MAPbI3/TiO2 interfaces; that halide content in MAPbX3 and CsPbX3 has significant influences on charge recombination; and that doping MAPbI3 with larger A-site organic cations can extend charge carrier lifetimes. The detailed time-domain atomistic investigations identify the mechanisms of energy losses, characterize the phonon modes that promote the electronic transitions and accept the lost electronic energy, and suggest design principles for controlling and eliminating unfavorable nonradiative electron−hole recombination pathways, which are of particular importance for design of high-performing perovskite materials for photovoltaic and photocatalytic applications.

Quantum dynamics involves complex interplay between electronic and nuclear degrees of freedom. Time-domain density functional theory and nonadiabatic molecular dynamics provide a unique perspective on nonequilibrium processes in condensed matter and nanoscale materials.

Iodine-rich/Pb-poor conditions introduce point defects that are benign for charge carrier lifetime. The oxidation state of the iodine vacancy strongly influences nonradiative charge recombination.

NA-MD is an efficient and reliable method for simulation of quantum dynamics of excited-state energy and charge-transfer processes in condensed matter systems.44−48 NA-MD treats electrons quantum mechanically and nuclei (semi)classically and includes transitions between electronic states. The electronic evolution is well-described by TD-DFT, which provides both good accuracy and computational efficiency. Coupling to the nuclear evolution is achieved via surface hopping (SH), which can be viewed as a master equation with time-dependent transition rates. Importantly, SH achieves detailed balance between transitions upward and down in energy, leading to thermodynamic equilibrium in the long-time limit.49 This key feature of electron−phonon relaxation is missing if electron−nuclear coupling is described by the meanfield (Ehrenfest) approach. Tully’s fewest switches SH (FSSH) is the most popular NA-MD approach.50 It is very well-suited for modeling excited-state processes involving isolated conical intersections, e.g., photoisomerization of molecules,46 and intraband relaxation characterized by rapid hops within a dense manifold of states.51 In such cases, decoherence effects can be ignored or treated in a simple way, for instance via an instantaneous wave function collapse after a hop. Extended systems provide additional challenges to FSSH due to a large number of trivial crossings. The parameter-free corrected FSSH algorithm52 and the subspace crossing correction53 to the standard FSSH address the issue. Slow transitions across large energy gaps, encountered for example during charge trapping and recombination, require semiclassical corrections for quantum decoherence.44 In such cases, we employ the decoherence-induced surface hopping (DISH) algorithm that uses decoherence as the physical mechanism of wave function collapse leading to a surface hop.54 NA-MD requires averaging over many trajectories representing both a (canonical) ensemble of initial conditions, and different realizations of the stochastic hopping process.

Inf luence of Point Defects on Nonradiative Charge Recombination. Point defects are the most investigated defect type. They create a low density of charge traps in perovskite single crystals.37 It has been demonstrated that the open-circuit voltage (Voc) deficit arises from the trap-assisted electron−hole recombination.35 The Voc decrease of about 0.36 eV from the maximum value of 1.55 eV, corresponding to the band gap, is rather small compared to inorganic semiconductors, indicating that defects play a relatively minor role in carrier losses in PSCs.83,84 At the same time, the observation of low photoluminescence quantum yields (less than 1%) in MAPbI3 perovskite thin films85 suggests that nonradiative energy losses are significant in PSCs, and some point defects are detrimental. Understanding and controlling defects, with the goal of suppressing the nonradiative charge recombination, are essential for further improvement of PSC performance. Defect concentration is related to their formation energy. Experimentally, the chemical potential for equilibrium growth can be regulated by manipulation of precursors, pressure, and temperature to alter the free energy of defect formation. The nature of point defects in perovskites is complicated, and there are several reviews on this topic which provide comprehensive accounts of defect types and energies.31,35,84,86,87 Theoretically, p-type defects, such as the Pb vacancy (PbV) and the iodine interstitial (Ii), and n-type defects, such the iodine vacancy (IV), are proposed to be dominant. Particularly, Ii and PbV can 3790



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be easily formed under iodine-rich/lead-poor conditions because of their low formation energies.12,18 First-principles calculations suggested that the iodine interstitial defect introduces a deep trap, whereas the Pb vacancy creates only a shallow trap in MAPbI3.13,16 Using impedance spectroscopy measurements, Yang and co-workers attributed the defect level about 0.16 eV above the valence band maximum (VBM) in bulk MAPbI3 to the iodine interstitial.88 The iodine interstitial defect has a low activation energy to diffuse across the perovskite crystal under light illumination, according to De Angelis and co-workers.18 The iodine vacancy was reported to be an energetically favorable site for adsorption of molecular oxygen, leading to superoxide formation.57,89 A number of works reported measurements of carrier recombination lifetimes in PSCs in the presence of traps using time-resolved photoluminescence.23,26,90−95 Experiments showed that charge carrier lifetimes can be extended by an order of magnitude with addition of excess iodine in precursors during synthesis of both MAPbI3 and FAPbI3 perovskites.1,96 Imparting point defects with a particular charge (oxidation state) is a viable strategy to control electrical conductivity and enhance photocatalytic activity by defect engineering.97 Many of the unusual characteristics of PSCs can be ascribed to the charged defects formed under external and built-in electric fields or light.13,15,20 For example, Huang and co-workers reported that the giant switchable photovoltaic effect in MAPbI3-based devices is achieved under external electric field because of drift of charged defects.27 In the absence of a field, charged defects randomly migrate along the perovskite layer. In particular, migration of charged halide vacancies affects the current−voltage characteristics and can account for the current−voltage hysteresis in PSCs at room temperature.18 Charged defects, including charged cations and anions, may attract each other because of electrostatic Coulomb interactions and form clusters of defects that heal each other, explaining the low trap density in PSCs.15,86,98 Defects in different charge states have a strong influence on the nonradiative charge recombination, as discussed by Koster and co-workers.91 Interestingly, an uncommon improvement of PSC performance in the presence of iodide vacancies under bias voltage was reported.99 The plethora of experimental data on the influence of various defects and synthetic protocols on charge carrier trapping and relaxation in PSCs motivates theoretical studies. Time-domain ab initio simulations of excited-state dynamics provide the most direct way to model such experiments and to obtain a thorough understanding of the defect properties at the atomistic level of detail. The influence of the iodine interstitial defect on the charge carrier trapping and relaxation dynamics in MAPbI3 was examined using ab initio NA-MD.58 It was shown that an interstitial iodine atom introduces a deep trap state (relative to kBT) near the VBM (Figure 2), in line with the previous electronic structure investigations and available experimental characterization.33,88 The charge densities of both valence band (VB) and conduction band (CB) states are delocalized, while the defect level is localized. The CB minimum (CBM) is formed by Pb 6p orbitals, while the VBM is composed of Pb 6s and I 5p orbitals coupled in an antibonding manner. The defect state originates from I 5p orbitals and overlaps better with the charge density of the VBM than CBM. The hole and electron trapping occur on tens of picosecond and 100 ns time scales, respectively. The hole trapping is much faster than electron trapping for the following reasons: First, the trap state

Figure 2. Total and projected densities of states (DOS) of pristine MAPbI3 (top panel) and MAPbI3 with the iodine interstitial defect (bottom panel). The insets show charge densities of (a) VBM and (b) CBM in pristine MAPbI3 and (c) VBM, (d) CBM, and (e) trap state in MAPbI3 with the iodine interstitial defect. Adapted from ref 58. Copyright 2017 American Chemical Society.

is closer in energy to the VBM than the CBM. Second, the NA electron−phonon coupling for hole trapping is large, because the wave functions of both the defect level and the VBM originate from I 5p orbitals and overlap much better than the wave functions of the defect level and the CBM. Third, quantum decoherence between the defect state and the VBM is long. Most interestingly, the trapped holes are surprisingly long-lived. Recombination of the trapped holes with CB electrons is several times slower than recombination of VB holes with CB electrons. The trapped holes can escape into the VB prior to recombining with CB electrons. These factors can lead to reduction of charge and energy losses in MAPbI3 with the iodine interstitial defect and contribute to the high efficiencies of PSCs. The effect of the Pb vacancy on the nonradiative electron− hole recombination in FAPbI3 was investigated.100 The nonradiative recombination in pristine FAPbI3 occurred on a nanosecond time scale and was extended by an order of magnitude in the presence of the Pb vacancy (Figure 3). This result agrees with the experimental observation of long charge carrier lifetimes in FAPbI3 under iodine-rich conditions.1 The

Figure 3. Decay of excited-state population in FAPbI3 with and without the Pb vacancy. Adapted from ref 100. Copyright 2018 American Chemical Society. 3791



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Figure 4. Charge carrier trapping and relaxation dynamics in pristine MAPbI3 and MAPbI3 with iodine vacancy in different oxidation states. The +1 I−1 V defect accelerates the recombination only slightly because no trap states are created, while the IV accelerates charge carrier losses by 2 orders of magnitude. Adapted from ref 59. Copyright 2018 American Chemical Society.

on charge carrier lifetimes, rationalizing the high efficiencies of PSCs. The iodine interstitial defect in MAPbI3 even extends the charge carrier lifetime, because it introduces a gap state near the VBM that couples weakly to the CBM. Holes can be trapped and detrapped many times prior to recombining with electrons. The Pb vacancy decreases the nonradiative charge recombination rate in FAPbI3, because it localizes the VBM charge density and decreases the NA coupling. The iodide vacancy does not introduce midgap trap states and decreases charge carrier lifetimes only modestly. However, the same point defects in other, less common oxidation states can decrease carrier lifetimes by orders of magnitude, indicating that PSC efficiencies can depend strongly on nonequilibrium operation conditions involving strong currents, high carrier densities, exposure to environment and chemicals, etc.

decrease in the nonradiative electron−hole recombination rate in FAPbI3 with the Pb vacancy can be explained by considering the NA electron−phonon coupling and the energy gap. The NA electron−phonon coupling is smaller in the presence of the Pb vacancy, because the missing Pb atom localizes the VBM charge density and thereby decreases the VBM−CBM overlap. The band gap is almost unaffected by the defect, because the shallow trap state generated by the atomic vacancy merges into the VB. Kilin and co-workers studied the effect of the Pb vacancy on the charge carrier dynamics in MAPbI3 using a Redfield theory formalism.101 They also found that the Pb vacancy increases the nonradiative relaxation time of the excited electron, in agreement with the NA-MD simulations.100 Point defects in halide perovskites can exist in different oxidation states, which can be controlled by chemical or electrical doping, and altered by flowing charges under PSC operating conditions. The effect of the oxidation state on the electron−hole recombination dynamics in MAPbI3 in the presence of an iodine vacancy was investigated.59 Neutral (IV), −1 cation (I+1 V ), and anion (IV ) iodine vacancies were considered. Here, the charge refers to the charge of the missing iodine atom. Thus, I−1 V corresponds to the missing iodide anion. The calculations show that the oxidation state of the halide vacancy has a strong influence on the charge carrier dynamics (Figure 4). The iodide vacancy I−1 V has negligible influence on the electron−hole recombination, because it creates no subgap states. The neutral IV accelerates the recombination to a moderate extent, because it creates a shallow trap near the CBM, and increases the NA electron−phonon coupling. The I+1 V defect produces an additional hole trap near the VBM, compared to the neutral IV. This is because the negative charge occupying the vacancy site attracts the two nearby Pb ions that form a stable dimer and detach from iodines, leaving them chemically unsaturated. I+1 V accelerates charge recombination by 2 orders of magnitude, compared to pristine MAPbI3, by providing additional relaxation pathways via both shallow and deep trap states. The above analysis of the influence of point defects on the charge carrier trapping and relaxation dynamics, performed using NA-MD combined with real-time TD-DFT, demonstrates that the most easily formed defects have little influence

Removing nonradiative recombination sites at surfaces, interfaces, and grain boundaries is important to advance perovskite optoelectronic properties. PbI2terminated surfaces maintain long carrier lifetimes. Surface passivation with Lewis bases, salts, and other additives leads to further improvements in chemical stability and photophysics. Charge Recombination at Surfaces and Grain Boundaries. In addition to the point defects in perovskite crystals, extended defects, such as grain boundaries, surfaces, and interfaces, can influence charge carrier lifetimes and transport. The observed carrier diffusion is much longer in single crystals than polycrystalline MAPbI3 films,102 indicating that trap density at perovskite surfaces and GBs is much higher than in bulk. The low thermal stability of perovskites arises in particular because organic cations, in the form of MAI, can be lost from the crystal during thermal annealing to produce nonstoichio3792



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metric compositions.103 Experiments demonstrate that significant charge recombination takes place at perovskite surfaces.104,105 Suppression of surface recombination is of crucial importance to further advance PSCs. Experiments show that an excess of the PbI2 precursor during perovskite synthesis enhances PSC efficiency.105,106 The exact role of excess PbI2 and the mechanism of the efficiency enhancement are still matters of debate.104,107 A small excess of PbI2 in the precursor solution improves device stability and suppresses the current−voltage hysteresis and ion migration in PSCs.108 Surface passivation can notably improve PSC efficiency. Supasai and co-workers demonstrated a self-induced passivation mechanism at perovskite surfaces.109 Chen and coworkers found that upon thermal annealing of MAPbI3 films, the nonradiative recombination was greatly suppressed in the presence of a rigid PbI2 termination layer.110 Surface photovoltage spectroscopy experiments revealed that a PbI2 layer located at the perovskite/TiO2 interface could inhibit the charge recombination.111 A similar effect of excess PbI2 was observed in other works, confirming the beneficial role of the PbI2 treatment.112 A large number of computational studies were dedicated to investigation of surface properties of perovskites. For example, Haruyama and co-workers examined systematically the thermodynamics properties of MAPbI3 surfaces and concluded that the PbI2-terminated surface does not introduce subgap states.113,114 Liu and co-workers suggested that the PbI2terminated surface has a smaller band gap than the MAIterminated surface115 and that structural stability and electronic properties at the MAPbI3/TiO2 interface depend on a particular surface termination.116 Quarti and co-workers emphasized that the different surface terminations, MAI versus PbI2, affect the relative energy alignment at the perovskite/C60 interface.117 Using ab initio MD simulations, Mosconi and coworkers showed that the PbI2-terminated surface is more robust to water degradation compared with the MAI termination.118 Uratani and co-workers investigated charge carrier trapping by surface defects and concluded that surface defects are more likely to form in the PbI2-terminated than MAI-terminated surfaces.11 Whether excess PbI2 is truly essential for high-efficiency PSCs remains an open question,104,105 especially because both MAI- and PbI2-terminated surfaces form and coexist under thermodynamic growth conditions. It is important to note that PbI2-terminated surfaces contain undercoordinated Pb2+ ions, which may accumulate photogenerated charges and possibly increase recombination at the interface with a charge transport layer. Passivation of the undercoordinated Pb2+ ions at the perovskite surface can be achieved by adding organic molecules to the precursor solutions. Noel et al. reported that electron-rich Lewis bases, such as pyridine or thiophene, passivate perovskite surfaces, reducing the nonradiative recombination rate by an order of magnitude.119 Similar ideas for passivation of surface defects were adopted by other research groups, which utilized various passivation agents.120−122 A comprehensive analysis of surface passivation in PSCs is provided in a recent review.123 The experimental characterization of PSC devices and electronic structure calculations provide important information on the influence of surfaces and their passivation on the PSC performance. Further key insights are generated by timeresolved spectroscopies and quantum dynamics simulations.

In addition to perovskite surfaces, interfaces between perovskite grains are also attracting significant attention. Many experiments show that GBs and structural disorder strongly impact the optoelectronic quality.36,102,124−129 There exist conflicting reports regarding the effect of GBs on the photovoltaic performance of perovskites. A number of experimental publications reported that solution-processed polycrystalline MAPbI3 films containing GBs have excellent charge-transporting properties, resulting in high photon conversion efficiencies, exceeding 20%.36,124 Such data imply that GBs do not open up additional channels to charge carrier losses and may even be beneficial. This phenomenon can be explained by the recent first-principles calculations of Yin et al., who showed that structural disorder at GBs, resulting, for instance, in formation of Pb−Pb bonds that are not possible in pristine perovskites, creates no subgap states because of the relatively large distance between the two Pb2+ ions.127 At the same time, several studies found that charge carrier lifetimes grow with increasing grain size in perovskite films,125,126 indicating that GBs are related to nonradiative charge recombination and deteriorate device performance. Moreover, the halide ions were demonstrated to segregate spontaneously into GBs and eliminate trap states, playing beneficial roles in passivating GBs in perovskite polycrystalline films.128 A thorough understanding of the carrier dynamics at perovskite GBs and interfaces, including mechanisms of suppression of the nonradiative recombination through proper passivation, is of crucial importance for further development of PSCs. Motived by the above experimental and theoretical observations, Tong et al. investigated the nonradiative electron−hole recombination in MAPbI3 with PbI2- and MAI-terminated surfaces representing PbI2-rich and PbI2poor conditions.130 The simulations demonstrated that the charge carrier relaxation is slow in the PbI2-terminated MAPbI3 compared to the MAI-terminated MAPbI3 (Figure 5),

Figure 5. Simulation structure and energy level diagram for nonradiative recombination at the PbI2- and MAI-terminated perovskite surfaces. Adapted from ref 130. Copyright 2018 American Chemical Society. The MAI-terminated MAPbI3 surface has more mobile MA cations, which couple electrostatically to charge carriers through high-frequency modes.

primarily because of a smaller NA electron−phonon coupling in the PbI2-terminated surface. The MA cations exposed at the MAI-terminated surface are more mobile; couple electrostatically to the charge carriers; and through high-frequency modes, accelerate charge−phonon energy exchange. The MA cations in the PbI2-terminated MAPbI3 are much less mobile and contribute little to the charge carrier recombination. It is interesting that even though the electrons and holes at the 3793



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breaking the perovskite crystalline symmetry, GBs create localized states which overlap less than the CBM and VBM in the pristine perovskite (Figure 7). In the absence of midgap states,127 such localization can reduce NA coupling and slow down the recombination, leading to efficient PSCs.36,124 The calculated NA couplings were larger at the GB than in the pristine perovskite because of enhanced atomic motions within the less structured boundary region.61 The simulations suggest that charge recombination depends strongly on the type of created GBs and that annealing of perovskite films can heal GB defects, eliminating midgap states, and stabilize boundary structures, reducing atomic fluctuations, thereby extending charge carrier lifetimes.60 GB defects can be also healed by doping, e.g. with Cl atoms. Cl is more electronegative than I, and its orbitals are located deeper inside the VB. Thus, Cl dopants do not contribute to the VBM charge density. Because dopants are most easily deposited at boundaries and surfaces, doping of a MAPbI3 GB with chlorines pushes the VBM charge density away from the boundary (Figure 7), removing the trap states. Further, by moving faster, the lighter chlorines reduce quantum coherence time and slow down quantum dynamics, as exemplified by the quantum Zeno effect.131 The examples of the NA-MD studies of the nonradiative charge carrier recombination in the perovskites containing original and passivated surfaces and GBs demonstrate a complex interplay of several factors, including midgap states, charge localization, and involvement of additional motions due to both passivating species and reduced structural stability at surfaces. Modifications of the electronic structure in the extended defect regions alter energy gaps and rates of inelastic and elastic charge−phonon scattering. A thorough understanding of these factors allows one to engineer benign surfaces that reduce carrier losses and maintain high PSC efficiencies.

VBM and CBM are localized on the inorganic lattice, the mobile organic MA species are able to participate in the charge recombination because of their large dipole moments, which generate long-range oscillating electrostatic fields, and by influencing motions of the inorganic lattice. The mechanism of the decreased nonradiative charge recombination in perovskites passivated by Lewis base molecules, such as thiophene and pyridine, was investigated by ab initio NA-MD.62 The simulations showed that both pyridine and thiophene passivations extend charge carrier lifetimes by reducing the NA electron−phonon coupling and quantum coherence time, while retaining the energy gap. The NA coupling is reduced because pyridine and thiophene localize the charge density (Figure 6) and decrease the overlap

Figure 6. CBM charge densities in (a) the pristine MAPbI3−xClx (001) surface and the surface passivated by (b) pyridine and (c) thiophene molecules. Adapted from ref 62. Copyright 2018 American Chemical Society.

between the VBM and CBM wave functions. The localization occurs by different mechanisms, though. Thiophene pushes the electron density away from the surface because of elimination of the unsaturated chemical bonds. This is the traditional passivation mechanism. In comparison, pyridine introduces a localized surface state by forming a chemical bond between the N atom of the molecule and the under-coordinated Pb ion of the perovskite surface. Importantly, this surface state is very close to the CB edge and does not trap electrons. Both Lewis base molecules introduce high-frequency vibrational modes that shorten coherence time and slow down the quantum dynamics131 of phonon-driven charge recombination.

Tuning chemical composition, for example, mixing of halides or Asite cations, is a viable way to modulate optoelectronic properties. Halides affect electronic properties and carrier lifetimes, because they vary in electronegativity and introduce different phonon frequencies. A-site cations do not contribute to band edges, but they influence inorganic lattice parameters, vibrational dynamics, and (thermal) disorder.

The role of grain boundaries in perovskite solar cells is still under debate. They facilitate charge separation while also providing carrier recombination sites. Some boundaries are more detrimental than others. Boundary annealing and doping can repair the detrimental effects.

Optimizing Composition To Control Perovskite Nonradiative Electron−Hole Recombination. The family of halide perovskites with the ABX3 general chemical formula contains a large number of compounds obtained by varying the three components: the A-site monovalent cation, the B-site divalent cation, or the halide anion. Additional materials can be created by mixing multiple components, e.g., A, A′ or X, X′, within the same structure. Such variability provides an important means to introduce doping impurities into the crystal lattice or to synthesize new materials by composition engineering. Here,

NA-MD calculations demonstrated that GBs accelerate the nonradiative electron−hole recombination, while chlorine doping of the boundary can repair the detrimental effects produced by the GBs.61 Creation of midgap trap states is the main mechanism of the accelerated recombination. By 3794



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Figure 7. VBM and CBM charge density in (a) pristine MAPbI3, (b) Σ5 (012) GB, and (c) Cl-doped Σ5 (012) GB. VBM and CBM are delocalized in panel a and become localized on inorganic atoms in the vicinity of the GB in panels b and c. Doping by Cl atoms pushes charge density away from the GB. Adapted from ref 61. Copyright 2016 American Chemical Society.

in the configurational entropy.148 First-principles calculations revealed that the band edge states in perovskites originate from the PbX3− inorganic sublattice, suggesting that the A-site cation should not influence the optoelectronic properties directly. In particular, the VBM is dominated by 5p orbitals of the halide and CBM is composed of 6p orbitals of Pb.19,33,149,150 However, band gap changes induced by A-site cations were reported. This is likely correlated with the steric effects: motions of the A-site cations couple to deformation of the inorganic lattice, leading to changes in the Pb−I bond lengths and the Pb−I−Pb bond angle.151,152 Recent photoluminescence experiments demonstrated that the carrier lifetimes are significantly different in perovskites containing MA, FA, GA, and Cs.153 Moreover, partial replacement of MA with the larger FA154 and GA155 cations can significantly extend carrier lifetimes. These and other observations require a fundamental understanding that is achieved by quantum dynamics simulations. Motivated by the above-mentioned experiments, Liu and Prezhdo simulated charge carrier dynamics in Cl-doped MAPbI3.156 It was shown that replacement of iodine atoms with chlorines reduces the charge recombination rate, reproducing the experimental observations. The charge recombination was slowed down by the Cl doping because of decreased NA electron−phonon coupling and shortened quantum coherence time. Cl doping breaks the symmetry of the perovskite crystal structure, inducing localization in the VBM and CBM wave functions. The localization is further enhanced by the fact that chlorine is more electronegative than iodine and contributes little to the VBM. The decreased VBM−CBM wave function overlap weakens the NA coupling (Figure 8). Further, the quantum coherence time is shorter in the presence of chlorines, both because the wave function localization and because lighter chlorines, compared to iodines, introduce higher frequency phonon modes that lead to faster loss of coherence within the electronic subsystem. The influence of halide composition on the nonradiative charge recombination in CsPbBr3 perovskite QDs was studied.56 It was shown that replacement of Br atoms with I atoms in CsPbBr3 QDs extends the charge carrier lifetime by a factor of almost 8, whereas half replacement extends the lifetime nearly 5 times (Figure 9). The mechanism of the reduction of the nonradiative charge recombination rate was

the goal is to change the optoelectronic properties of the semiconductor and to enhance its long-term stability and efficiency. In particular, mixing the halide composition is the most common approach to tune the optoelectronic properties. It is interesting to note that the best efficiency PSCs are based on the hybrid perovskites with mixed MA/FA cations and I/Br halide anions.132 Halide mixing is well-illustrated by doping MAPbI3 with chlorine. Chlorine can be incorporated into MAPbI3 films at relatively large concentration,133 leading to the release of MA and partial formation of the MACl compound. As a result, mixed halide MAPbI3−xClx perovskite are produced.134 Theoretically, the band gap is almost unaltered by incorporation of Cl into MAPbI3 films.135 Recent experiments found that Cl doping optimizes film morphology,136 improves carrier mobility,10 and delays luminescence decay.137 It is thus expected that incorporation of Cl into perovskite materials affects the fundamental electronic properties. Replacement of iodine at the MAPbI3/TiO2 interface with Cl redistributes charges and changes electronic coupling between CB states of MAPbI3 and TiO2, which may influence the interface charge injection rate, according to De Angelis and co-workers.138 As a result, Cl doping of the MAPbI3/TiO2 interface delays charge recombination, as shown by Long and Prezhdo.63 Mixed-halide MAPbI3−xClx perovskites show a comparably better performance than the pristine MAPbI3 perovskite.139 A similar beneficial role of halide substitution was also observed for Br doping.140 Tuning halide composition in nanocrystals of allinorganic perovskites received much attention as well.141−143 Bulk CsPbI3 has low phase stability because it does not satisfy the Goldschmidt tolerance factor rule. It can be stabilized in the quantum dot (QD) form because of the high surface-tovolume ratio. It was shown that replacement of iodine atoms with bromines in CsPbI3 QDs decreases the carrier diffusion length.144 Tong et al. measured the averaged carrier lifetime in CsPbX3 QDs and demonstrated that it decreases as the chemical compositions of the halide anions changes from I to Br to Cl.145 Similar studies were reported with hybrid organic− inorganic perovskites.146,147 In addition to manipulation of the halide compositions, tuning the A-site cations for targeted properties draws attention. It was demonstrated that mixed A-site cations could stabilize the perovskite crystal phase because of changes 3795



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Figure 10. Charge recombination dynamics in MAPbI 3 , MA0.75FA0.25PbI3, and MA0.75GA0.25PbI3 perovskites. Adapted from ref 157. Copyright 2018 American Chemical Society. Figure 8. VBM and CBM charge densities in MAPbI3 and MAPbI2Cl. Adapted from ref 156. Copyright 2015 American Chemical Society.

lifetimes. The opposite effect was seen in the GB system61 discussed above. The looser structure around the GB region exhibited larger thermal atomic fluctuations and increased NA electron−phonon coupling. Similarly, another work of Long and co-workers investigated the influence of the A-site cations on the charge carrier dynamics in lead bromide perovskites.65 The excited-state lifetime in the CsPbBr3 perovskite was compared to those in MAPbBr3 and FAPbBr3 perovskites. It was shown that the nonradiative charge recombination is slowest in FAPbBr3, followed by MAPbBr3 and CsPbBr3 (Figure 11). The

Figure 9. Charge recombination dynamics in CsPbBr3, CsPbBr1.5I1.5, and CsPbI3 QDs. Adapted from ref 56. Copyright 2018 American Chemical Society.

rationalized by the smaller NA electron−phonon coupling. Iodines are heavier than bromines and have a slower velocity. The NA electron−phonon coupling is proportional to the nuclear velocity, and therefore, it is smaller in CsPbI3 than CsPbBr 3 . The velocity argument also applies to the CsPbI1.5Br1.5 mixed perovskite, compared to CsPbI3. The NA coupling is reduced further in the mixed system because of the lowered symmetry and the disorder and charge localization introduced by the mixing. NA-MD calculations demonstrated that nonradiative charge recombination in MAPbI3 is reduced by partial replacement of MA with the larger FA and GA cations (Figure 10).157 The larger cations stiffen the perovskite structure and decrease the extent of thermal atomic fluctuations. In particular, replacement of 25% MA with FA or GA decreases fluctuations of the inorganic PbI lattice by 10% and nearly 50%, respectively. At the same time, the band gap remains unchanged. Because the inorganic lattice supports the VBM and CBM wave functions, and the NA coupling depends on overlap of these wave functions at sequential MD time steps, the decreased fluctuations give smaller NA coupling and slower charge recombination. The stiffening of the perovskite structure, leading to reduction of atomic fluctuations, provides an important design principle for extending charge carrier

Figure 11. Charge recombination dynamics in CsPbBr3, MAPbBr3, and FAPbBr3. Adapted from ref 65. Copyright 2018 American Chemical Society.

difference in the recombination rates can be mainly attributed to the smaller NA electron−phonon coupling, because the energy gaps and coherence times are almost the same in the investigated systems. The larger FA and MA cations suppress motions of the inorganic lattice, and correspondingly, the NA coupling is reduced. The influence of doping on the charge recombination across the MAPbI3/TiO2 interface was investigated (Figure 12).63 The recombination occurs after photoexcitation of MAPbI3 and subsequent charge collection by TiO2, which is used as an electron acceptor in many types of solar cells. The recombination is much slower than the injection51 and constitutes the main mechanism of carrier losses in such heterostructures. It was found that Cl and Br doping slow down the nonradiative charge recombination across the interface, whereas Sn doping accelerates the recombination, 3796



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Figure 12. Electron (top) and hole (bottom) states for electron−hole recombination across the MAPbI3/TiO2 interface. Adapted from ref 63. Copyright 2015 American Chemical Society.

in agreement with the experiments.2,158−160 The influence of the Cl, Br, and Sn doping on the interfacial charge recombination was rationalized by the following arguments: First, the Cl and Br atoms have smaller radii than I. Replacing I atoms with Cl and Br at the interface by the doping decreases the bonding interactions between MAPbI3 and TiO2. A similar effect occurs when Pb is replaced with Sn. Second, Cl and Br are more electronegative than I, and therefore, their states are deep inside the VB. The VBM is formed primarily by atomic orbitals of iodine atoms in both pristine and doped MAPbI3/ TiO2. Replacing interfacial iodines with Cl and Br pushes the VBM, and hence the photogenerated hole, away from the interfacial region, reducing the electron−hole overlap and the NA coupling. In contrast, Sn contributes to the frontier orbitals, and being lighter than Pb, it has a larger nuclear velocity and produces a stronger NA coupling. The recombination is driven primarily by low-frequency phonon modes which can be attributed to bending and stretching of the inorganic lattice.

coupling decreased. It is important not to modify the structure significantly. For example, changing the lattice parameters or electronegativity of the inorganic atoms can alter the fundamental band gap and change the light absorption characteristics. Thus, doping provides an important means to fine-tune the perovskite properties.

Time-domain ab initio nonadiabatic molecular dynamics provides an atomistic description of the nonequilibrium processes as they occur in real time and at the atomistic level. It is computationally demanding, and its success in applications to perovskites and other modern materials relies on efficient and reliable approximations and algorithms.

Controlled symmetry breaking without destroying the inorganic lattice can separate electrons and holes, reducing their interaction while maintaining efficient charge transport.

In summary, hybrid organic−inorganic perovskites constitute a promising class of materials that attract a great number of investigations for a variety of applications. The compositional complexity of perovskites increases the possibility of introduction of point defects, boundaries, dopants, partial chemical segregation, and other imperfections. Deep understanding and identification of the defects that are responsible for nonradiative electron−hole recombination are very important for further advancement of PSCs and other devices. Many publications demonstrate that hybrid perovskites exhibit an unusual defect science that makes many defects benign for charge transport. Other publications show that local and extended defects can create charge traps and accelerate nonradiative electron−hole recombination. Charge recombination limits the ability of carrier extraction to charge transport layers and determines the overall PSC performance. In this Perspective, we discussed the role of defects in charge and energy losses in hybrid perovskites, considered point defects, surfaces, grain boundaries, interfaces, dopants, and nonstoichiometric compositions and analyzed the charge trapping and recombination dynamics on the basis of NAMD and TD-DFT calculations. The results highlighted that

Analysis of the NA-MD simulation of the nonradiative charge carrier recombination in the doped and mixed perovskites reveals several design principles that can be used to improve PSC performance. Mixed systems break the perfect symmetries of single-component perovskites, leading to a controlled localization and separation of electrons and holes, reducing their interaction. At the same time, the periodic structure of the inorganic lattice is not destroyed, and efficient charge transport along the lattice is sustained. The NA electron−phonon coupling responsible for the nonradiative electron−hole recombination depends on the extent of thermal atomic motions, mostly of the atoms of the inorganic lattice. Making the perovskite structure stiffer by judicious choice of the perovskite components, e.g. by mixing in larger A-site cations, the lattice fluctuations can be reduced and the NA 3797



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perfect systems, and have to be tested carefully for defects involving unsaturated chemical bonds. In many situations, nuclear dynamics is dominated by thermal fluctuations that can be sampled within a few picoseconds. In such cases the NA Hamiltonian can be iterated multiple times to extend the quantum dynamics simulation to nanoseconds.170,171 In order to reduce the computational cost of ab initio NA-MD, Akimov developed an efficient quasi-stochastic Hamiltonian NA-MD for simulation of quantum dynamics on long time scales.172 Proper description of the electronic structure of perovskites is among the key components of an accurate NA-MD calculation. The Kohn−Sham description of electronic excitations provides a very efficient and reasonably accurate way to model excited-state properties.46,173 The lowest-energy excited states can be described by the delta self-consistent field (ΔSCF) methods.174,175 The next level of accuracy is provided by the GW and Bethe−Salpeter theories and, alternatively, by linear-response TD-DFT.176,177 The latter is more suitable for NA-MD, because it offers explicit expressions for excited-state wave functions and NA couplings. SOC effects are significant in perovskites because of the presence of heavy elements such as Pb and I. Fortunately, bare DFT functionals exhibit a cancelation of errors associated with electron self-interaction and SOC and give sufficiently accurate results.17,19,98,178−180 At the same time, NA coupling may be influenced by SOC,181 and this effect should be investigated systematically further. If SOC is explicitly included into the calculations, then either hybrid DFT functionals or the many-body GW theory should be applied,182 increasing significantly the computational effort. Despite the significant progress, further development of accurate, reliable, and efficient methods is required for largescale simulation of nonequilibrium processes in perovskites and other condensed matter and nanoscale systems. Perovskites exhibit a broad range of defects, and it is important to sample properly various defect types. The point defects discussed in this Perspective are well-defined, and their structure fluctuates around the equilibrium geometry. More complex defects, such as combinations of point defects and/or dopants, nonstoichiometric systems, surfaces, GBs, etc., can take a large number of energetically close conformations. They can be sampled by combining ab initio electronic structure with Monte Carlo simulations and modern structureprediction tools.

precise control of defect formation by regulating crystal growth conditions is very important for eliminating charge traps. Characterization of defects is challenging experimentally. Usually, defect species appear in low concentrations and present small densities of states, making them hard to study by optical or electrochemical means. Optical transitions between ground and defect states have small transition dipole moments, because the ground-state wave function is delocalized, while defect states are localized. Thus, most defects participating in the nonradiative relaxation are dark, and straightforward optical measurements cannot capture their properties. Moreover, it is hard to know experimentally whether predominant defect states trap electrons or holes, or if electrons and holes recombine bypassing defect states. Complementary characterization methods and more advanced techniques are required for such tasks. For example, temperature-dependent admittance spectroscopy and combination of transient and steadystate photocurrent and optical absorption spectroscopies have been used to identify defect types and energy distributions in PSCs.88,161 Time-resolved two-photon photoemission spectroscopy is useful to detect subgap states.162 Time-resolved terahertz photoconductivity and photoluminescence characterizations have been used to measure the decay of excited carriers after photoexcitation in hybrid perovskites containing defects.10,137 The diverse experimental approaches correlate defect structure with photoluminescence properties and charge carrier transport in the resultant film and provide data used for phenomenological models. Time-domain ab initio NA-MD simulates quantum dynamics of charge carriers coupled to vibrations motions. NA-MD mimics closely time-resolved experiments and provides an atomistic description of the nonequilibrium processes as they occur in real time. Ab initio NA-MD and related techniques enable accurate determination of state energies and simulation of electron-vibrational dynamics in realistic systems including defects, interfaces, mixed and hybrid structures, etc. Such simulations are particularly useful to achieve fundamental understanding of nonradiative charge recombination that constitutes the major pathway for energy losses in photovoltaic and photocatalytic devices. However, ab initio NA-MD can be applicable only to relatively small systems composed of hundreds of atoms and time scales limited to a few to tens of picoseconds. The difficulty resides in the calculation of excited-state energies, forces, NA couplings, and decoherence times. Several computationally efficient approximations and algorithms have been introduced, drastically reducing the computational cost. They include the Kohn−Sham representation of electronic excitations,46 the classical path approximation for neglect of back-reaction,45 and correlation function evaluation of coherence times under the second-order cumulant approximation.163 Care must be taken in the proper treatment of the sign/phase of the NA coupling.164 Generally, evaluation of NA couplings requires higher accuracy of wave function convergence and smaller MD time steps than evaluation of electronic energies and forces. The trivial avoided crossing problem has to be resolved in NA-MD simulations involving long-range transport.165 Quantum dynamics methods based on semiempirical extended Hückel theory (EHT)166−168 and tight-binding DFT (DFTB)169 provide alternatives to ab initio simulations, allowing one to increase the simulation scale by over an order of magnitude. Semiempirical electronic structure theories require careful parametrization, work better for chemically

Further research efforts should focus on investigation of excitedstate dynamics in larger, more complex systems and include more complex processes. Future NA-MD studies of perovskites should investigate the charge carrier dynamics in larger, more complex systems and include more complex processes. A more thorough investigation of the types of systems discussed in the Perspective is needed as well. For example, the published work61 rationalizes the detrimental influence of GBs on perovskite properties.183 However, only a single GB was considered because of the high computational efforts associated with NA-MD. In reality the influence of GBs is more complex184 and can be both negative and benign.185 Thus, more GB systems should be analyzed. The above examples focused on 3D perovskites, and very few 3798



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point for the analysis of quantum dynamics of charge carriers by means of NA-MD that remains a state-of-the-art approach.

quantum dynamics studies of the now popular 2D perovskites have been reported thus far.186 2D systems exhibit new features, such as edge states that facilitate charge separation. The reduced dimensionality can lead to higher concentrations of charge carriers that can exchange energy by Auger-type interactions known to be important in 0D semiconductor QDs, 1D carbon nanotubes, and 2D transition-metal dichalcogenides. With a notable exception,187 Auger-type effects have been simulated by NA-MD using NA coupling between manyparticle states,188−190 and inclusion of explicit Coulomb interactions between charge carriers is desirable. While existing NA-MD approaches already provide very important insights into the charge carrier dynamics in perovskites and other novel materials, the applications motivate development of novel techniques.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (W.L.). [email protected] (R.L.). [email protected] (J.T.). [email protected] (O.V.P.).

ORCID

Wei Li: 0000-0002-9999-5081 Run Long: 0000-0003-3912-8899 Oleg V. Prezhdo: 0000-0002-5140-7500 Notes

The authors declare no competing financial interest.

Control of quantum coherence provides an intriguing and often overlooked means to reduce charge carrier recombination.

Biographies Wei Li received a Ph.D. from the Institute of Theoretical Chemistry from Jilin University in 2017 and is currently a professor of Chemistry at Hunan Agricultural University. Dr. Li’s research focuses on excitation dynamics including charge and energy transfer in nanoscale systems, especially hybrid perovskite materials. Run Long obtained a Ph.D. in atomic and molecular physics from Shandong University and is currently a professor of Chemistry at Beijing Normal University. Dr. Long’s research interests focus on excitation dynamics in nanoscale systems, in particular energy and charge-transfer dynamics in condensed phases and at interfaces.

Control of quantum coherence provides an intriguing and often overlooked means to reduce charge carrier recombination.186 Transition from the excited to the ground electronic state requires formation of quantum coherence between the two states. This coherence is destroyed by coupling of the electronic subsystem to phonons. Shorter coherence times slow down quantum dynamics,131 reducing the recombination rate. It is important to keep in mind that faster decoherence implies stronger electron−phonon coupling, and stronger electron−phonon coupling should lead to faster nonradiative recombination. This apparent contradiction disappears when one is reminded that nonradiative recombination corresponds to inelastic charge−phonon scattering, while decoherence occurs by elastic scattering (pure-dephasing). The inelastic scattering is governed by the NA coupling, which depends on the matrix element between the VBM and CBM wave functions.45 No such matrix element enters expressions for the decoherence rates.163 For example, replacing some iodine atoms with the more electronegative chlorines and bromines has little effect on the VBM wave function, which continues to reside on the iodines. At the same time, the lighter Cl and Br introduce faster phonons and accelerate coherence loss. Decoherence is a quantum dynamics phenomenon, requiring analysis extending beyond the now routine electronic structure calculations. The NA-MD calculations indicate that electronic structure calculations should be performed not only with optimized structures but also for room-temperature geometries. Thermal effects can alter significantly electronic energy levels of defect states. Atomic motions induce symmetry breaking and localization of the electronic states, decreasing electron−hole interactions. Room-temperature adiabatic MD trajectories allow one to estimate the stiffness and structural stability of perovskites, the importance of polaronic effects, the extent of structural and electronic disorder, etc. Requiring more computational effort than electronic structure calculations on optimized geometries, adiabatic MD can also be regarded as a routine simulation and should be performed with perovskites on a regular basis. The insights obtained with electronic structure calculations and adiabatic MD provide the starting

Jianfeng Tang obtained his Ph.D. from Hunan University and is currently a professor of Physics in the College of Science in Hunan Agricultural University. His research interests are multiscale computer simulations of thermodynamic, mechanical, and electronic properties of nanoscale materials. Oleg V. Prezhdo is a Professor of Chemistry, Physics and Astronomy at the University of Southern California. He is Editor for The Journal of Physical Chemistry Letters, Journal of Physical Chemistry, and Surface Science Reports. His research interests range from fundamental aspects of semiclassical physics to excitation dynamics in nanoscale and biological systems.

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ACKNOWLEDGMENTS W.L. acknowledges startup funding from Hunan Agricultural University (Grant Nos. 540499818006 and 18QN02). O.V.P. acknowledges funding from the U.S. National Science Foundation (Grant No. CHE-1900510) for methods development support and the U.S. Department of Energy (Grant No. DE-SC0014429) for funding the applications studies. R.L. acknowledges support of the National Science Foundation of China, Grant Nos. 21573022 and 51861135101. R.L. also acknowledges financial support by the Fundamental Research Funds for the Central Universities, the Recruitment Program of Global Youth Experts of China, and the Beijing Normal University Startup.

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