We demonstrate that halide content strongly affects nonradiative electron–hole recombination in all-inorganic perovskite quantum dots (QDs). Using time domain density functional theory and ...nonadiabatic molecular dynamics, we show that replacing half of the bromines with iodines in a CsPbBr3 QD extends the charge carrier lifetime by a factor of 5, while complete replacement extends the lifetime by a factor of 8. Doping with iodines decreases the nonadiabatic charge–phonon coupling because iodines are heavier and slower than bromines and because the overlap between the electron and hole wave functions is reduced. In general, the nonradiative electron–hole recombination proceeds slowly, on a nanosecond time scale, due to small sub-1 meV nonadiabatic coupling and short sub-10 fs coherence times. The obtained recombination times and their dependence on the halogen content show excellent agreement with experiments. Our study suggests that the power conversion efficiencies of solar cells can be controlled by changing the halide composition in all-inorganic perovskite QDs.
Promising alternatives to three-dimensional perovskites, two-dimensional (2D) layered metal halide perovskites have proven their potential in optoelectronic applications due to improved photo- and ...chemical stability. Nevertheless, photovoltaic devices based on 2D perovskites suffer from poor efficiency owing to unfavorable charge carrier dynamics and energy losses. Focusing on the 2D Dion–Jacobson perovskite phase that is rapidly rising in popularity, we demonstrate that doping of complementary cations into the 3-(aminomethyl)piperidinium perovskite accelerates spontaneous charge separation and slows down charge recombination, both factors improving the photovoltaic performance. Employing ab initio nonadiabatic (NA) molecular dynamics combined with time-dependent density functional theory, we demonstrate that cesium doping broadens the bandgap by 0.4 eV and breaks structural symmetry. Assisted by thermal fluctuations, the symmetry breaking helps to localize electrons and holes in different layers and activates additional vibrational modes. As a result, the charge separation is accelerated. Simultaneously, the charge carrier lifetime grows due to shortened coherence time between the ground and excited states. The established relationships between perovskite composition and charge carrier dynamics provide guidelines toward future material discovery and design of perovskite solar cells.
Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab ...initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI3 without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn–I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI3 synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI3 exhibits lower photovoltaic efficiency compared to some lead-based perovskites.
Decoherence plays an important role in nonadiabatic (NA) molecular dynamics (MD) simulations because it provides a physical mechanism for trajectory hopping and can alter transition rates by orders ...of magnitude. Generally, decoherence effects slow quantum transitions, as exemplified by the quantum Zeno effect: in the limit of infinitely fast decoherence, the transitions stop. If the measurements are not sufficiently frequent, an opposite quantum anti-Zeno effect occurs, in which the transitions are accelerated with faster decoherence. Using two common NA-MD approaches, fewest switches surface hopping and decoherence-induced surface hopping, combined with analytic examination, we demonstrate that including decoherence into NA-MD slows down NA transitions; however, many realistic systems operate in the anti-Zeno regime. Therefore, it is important that NA-MD methods describe both Zeno and anti-Zeno effects. Numerical simulations of charge trapping and relaxation in graphitic carbon nitride suggest that time-dependent NA Hamiltonians encountered in realistic systems produce robust results with respect to errors in the decoherence time, a favorable feature for NA-MD simulations.
In the electrocatalytic nitrogen reduction reaction (NRR), nitrogen (N2) is chemically inert, it is difficult to break the triple bond, and the subsequent protonation step is very challenging. ...Suitable catalysts with high selectivity and high activity are needed to promote the electrocatalytic NRR. We screen a large number of clusters composed of three metal atoms embedded into a two-dimensional metal nitride, W2N3, with a N vacancy, and calculate the reaction energetics. The VNiCu cluster has the best catalytic activity among all the catalysts proposed so far. The Fe3 and Fe2Co clusters are excellent catalysts as well. In all cases, spin polarization is needed to observe the catalytic effect. We establish the optimal NRR path and confirm that it remains unchanged in the presence of a solvent. We find three groups of descriptors that can well predict the materials’ properties and exhibit linear relationships with the NRR limiting potential.
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.
Auger-type processes are ubiquitous in nanoscale materials because quantum confinement enhances Coulomb interactions, and there exist large densities of states. Modeling Auger processes requires the ...modification of nonadiabatic (NA) molecular dynamics algorithms to include transitions caused by both NA and Coulomb couplings. The system is split into quantum and classical subsystems, e.g., electrons and vibrations, and as a result, energy conservation becomes nontrivial. In surface hopping, an electronic transition induced by NA coupling is accompanied by a classical velocity readjustment to ensure conservation of the total quantum-classical energy. A different treatment is needed for Auger transitions driven by Coulomb interactions. We develop a nonadiabatic molecular dynamics methodology that meticulously differentiates the energy redistribution accompanying hops induced by the NA coupling and the Coulomb interaction and correctly conserves the total energy at each transition. If the transition is driven by a Coulomb interaction, the hop energy is redistributed within the quantum electronic subsystem only. If the transition is NA, the energy is redistributed between the quantum and classical subsystems. Properly maintaining energy conservation for both types of transitions is crucial to generate a correct order of events, obtain accurate transition times, maintain a proper statistical distribution of state populations, and reach thermodynamic equilibrium. We test the method with biexciton annihilation and Auger-assisted hot electron relaxation in a CdSe quantum dot. The sequence of Auger and phonon-driven processes and the calculated time scales are in excellent agreement with the experimental results. The developed approach can be coupled with any surface-hopping method and provides a crucial practical advance to study charge-carrier dynamics in the nanoscale and condensed matter systems.
Ion migration, hole trapping, and electron–hole recombination are common processes in metal halide perovskites. We demonstrate using ab initio non-adiabatic molecular dynamics and time-domain density ...functional theory that they are intricately related and strongly influence each other. The hole injection accelerates ion migration by decreasing the diffusion barrier and shortening the migration length. The injected hole also promotes the nonradiative charge recombination by strengthening electron–phonon interactions in the low-frequency region and prolonging the quantum coherence time. The synergy stems from the soft perovskite lattice and response of the valence band maximum to the Pb–I lattice distortion induced by the hole. This work provides important insights into the influence of ion mobility and hole injection on the performance of perovskite solar cells and suggests that high concentration of holes should be avoided.
A novel nonadiabatic molecular dynamics scheme is applied to study the singlet fission (SF) process in pentacene dimers as a function of longitudinal and lateral displacements of the molecular ...backbones. Detailed two-dimensional mappings of both instantaneous and long-term triplet yields are obtained, characterizing the advantageous and unfavorable stacking arrangements, which can be achieved by chemical substitutions to the bare pentacene molecule. We show that the SF rate can be increased by more than an order of magnitude through tuning the intermolecular packing, most notably when going from cofacial to the slipped stacked arrangements encountered in some pentacene derivatives. The simulations indicate that the SF process is driven by thermal electron–phonon fluctuations at ambient and high temperatures, expected in solar cell applications. Although charge-transfer states are key to construct continuous channels for SF, a large charge-transfer character of the photoexcited state is found to be not essential for efficient SF. The reported time domain study mimics directly numerous laser experiments and provides novel guidelines for designing efficient photovoltaic systems exploiting the SF process with optimum intermolecular packing.
Interfacial electron transfer (ET) plays a key role in the operation of solar cells based on TiO2 sensitized with organohalide perovskites, since it leads to separation of the photogenerated ...electrons and holes into different materials. The reported experimental ET times range by over 3 orders of magnitude, from sub-200 fs to over 300 ps. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we demonstrate that ET at a CH3NH3PbI3/TiO2 interface can be complete within 100 fs, indicating that the longer time scales reflect other processes, such as charge and exciton diffusion in perovskite bulk. The electron injection is fast because the interaction between the donor and acceptor species is strong at ambient conditions. Photoexcitation directly at the interface can create a charge-separated state. Electrons generated farther away inject by a combination of adiabatic and nonadiabatic mechanisms. Thermally activated low frequency vibrational motions at the interface modulate the CH3NH3PbI3/TiO2 separation, creating opportunities for chemical bonding and generating channels for adiabatic ET. Higher-frequency modes create large nonadiabatic coupling. The interaction between perovskite and TiO2 is purely van der Waals at 0 K, whereas at ambient temperatures I–Ti covalent bonds can form transiently at the interface. The covalent bonding is particularly important for photoexcitation of charge-separated states and adiabatic ET. The ET occurs prior to nonradiative electronic energy losses that can lead to charge trapping and recombination. The ultrafast interfacial charge separation contributes to the high efficiencies of perovskite-sensitized TiO2 solar cells. The reported simulations provide a detailed time-domain atomistic description of the interfacial ET and advance our understanding of carrier dynamics in perovskite solar cells.
