Experiments show both positive and negative changes in performance of hybrid organic–inorganic perovskite solar cells upon exposure to moisture. Ab initio nonadiabatic molecular dynamics reveals the ...influence of humidity on nonradiative electron–hole recombination. In small amounts, water molecules perturb perovskite surface and localize photoexcited electron close to the surface. Importantly, deep electron traps are avoided. The electron–hole overlap decreases, and the excited state lifetime increases. In large amounts, water forms stable hydrogen-bonded networks, has a higher barrier to enter perovskite, and produces little impact on charge localization. At the same time, by contributing high frequency polar vibrations, water molecules increase nonadiabatic coupling and accelerate recombination. In general, short coherence between electron and hole benefits photovoltaic response of the perovskites. The calculated recombination time scales show excellent agreement with experiment. The time-domain atomistic simulations reveal the microscopic effects of humidity on perovskite excited-state lifetimes and rationalize the conflicting experimental observations.
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 (Ii) into the VB. The strain distorts the system and breaks the I-dimer responsible for the Ii 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 (MAI) 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 Ii and MAI defects in MAPbI3, providing a new nonchemical strategy for defect control and engineering.
Two-dimensional transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se) hold great potential in optoelectronics and photovoltaics. To achieve efficient light-to-electricity conversion, ...electron–hole pairs must dissociate into free charges. Coulomb interaction in MX2 often exceeds the charge transfer driving force, leading one to expect inefficient charge separation at a MX2 heterojunction. Experiments defy the expectation. Using time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we show that quantum coherence and donor–acceptor delocalization facilitate rapid charge transfer at a MoS2/MoSe2 interface. The delocalization is larger for electron than hole, resulting in longer coherence and faster transfer. Stronger NA coupling and higher acceptor state density accelerate electron transfer further. Both electron and hole transfers are subpicosecond, which is in agreement with experiments. The transfers are promoted primarily by the out-of-plane Mo–X modes of the acceptors. Lighter S atoms, compared to Se, create larger NA coupling for electrons than holes. The relatively slow relaxation of the “hot” hole suggests long-distance bandlike transport, observed in organic photovoltaics. The electron–hole recombination is notably longer across the MoS2/MoSe2 interface than in isolated MoS2 and MoSe2, favoring long-lived charge separation. The atomistic, time-domain studies provide valuable insights into excitation dynamics in two-dimensional transition metal dichalcogenides.
Advancing organohalide perovskite solar cells requires understanding of carrier dynamics. Electron–hole recombination is a particularly important process because it constitutes a major pathway of ...energy and current losses. Grain boundaries (GBs) are common in methylammonium lead iodine CH3NH3PbI3 (MAPbI3) perovskite polycrystalline films. First-principles calculations have suggested that GBs have little effect on the recombination; however, experiments defy this prediction. Using nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory, we show that GBs notably accelerate the electron–hole recombination in MAPbI3. First, GBs enhance the electron–phonon NA coupling by localizing and contributing to the electron and hole wave functions and by creating additional phonon modes that couple to the electronic degrees of freedom. Second, GBs decrease the MAPbI3 bandgap, reducing the number of vibrational quanta needed to accommodate the electronic energy loss. Third, the phonon-induced loss of electronic coherence remains largely unchanged and not accelerated, as one may expect from increased electron–phonon coupling. Further, replacing iodines by chlorines at GBs reduces the electron–hole recombination. By pushing the highest occupied molecular orbital (HOMO) density away from the boundary, chlorines restore the NA coupling close to the value observed in pristine MAPbI3. By introducing higher-frequency phonons and increasing fluctuation of the electronic gap, chlorines shorten electronic coherence. Both factors compete successfully with the reduced bandgap relative to pristine MAPbI3 and favor long excited-state lifetimes. The simulations show excellent agreement with experiment and characterize how GBs and chlorine dopants affect electron–hole recombination in perovskite solar cells. The simulations suggest a route to increased photon-to-electron conversion efficiencies through rational GB passivation.
We develop an efficient and accurate method for numerical evaluation of nonadiabatic (NA) coupling in the Kohn–Sham representation with projector augmented-wave (PAW) pseudopotentials that are ...commonly used in electronic structure calculations on nanoscale, condensed matter, and molecular systems. Without additional cost, the method provides an order of magnitude improvement in accuracy compared to the current technique, while it is 3–4 orders of magnitude faster than the exact evaluation. Atomic displacements over typical time steps in molecular dynamics (MD) simulations are much smaller than the size of the PAW core region, and therefore, evaluation of the NA in the core is simplified. The accuracy is demonstrated with three condensed matter systems. The method is robust to variation in the MD time step. The accurate NA coupling evaluation also helps in maintaining phase-consistency of the NA coupling and identifying trivial crossings of adiabatic states. The approach stimulates NAMD applications to modeling of modern materials and processes.
Two-dimensional (2D) Ruddlesden–Popper perovskites form a new class of solar energy materials with high performance, low cost and good stability. Nonradiative electron–hole recombination is the main ...source of charge and energy losses, limiting material efficiency. Experiments show that edge states in 2D halide perovskites accelerate exciton dissociation into long-lived charge carriers, improving performance. Using a combination of nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that unsaturated chemical bonds of iodine atoms at perovskite edges is the main driving force for hole localization. Chemically unsaturated Pb atoms confine electrons to a much lesser extent, because they more easily support different oxidation states and heal chemical defects. This difference between defects associated with metals and nonmetals is general to many nanoscale systems. Thermal atomic fluctuations play important roles in charge localization, even in the bulk region of 2D perovskite films, a phenomenon that is different from polaron formation. Charge localization at edges is robust to thermal excitation at ambient conditions. The separated charges live a long time, because the nonadiabatic coupling between the excited and ground states is small, under 1 meV, and quantum coherence is short, less than 10 fs. The calculations agree very well with the time-resolved optical measurements on both luminescence lifetime and line width. The detailed understanding of the excited state dynamics in the 2D halide perovskites generated by the simulations highlights the unique chemical properties of these materials, and provides guidelines for design of efficient and inexpensive solar energy materials.
Recent Progress in Surface Hopping: 2011–2015 Wang, Linjun; Akimov, Alexey; Prezhdo, Oleg V
The journal of physical chemistry letters,
06/2016, Letnik:
7, Številka:
11
Journal Article
Recenzirano
Developed 25 years ago, Tully’s fewest switches surface hopping (FSSH) has proven to be the most popular approach for simulating quantum-classical dynamics in a broad variety of systems, ranging from ...the gas phase, to the liquid and solid phases, to biological and nanoscale materials. FSSH is widely adopted as the fundamental platform to introduce modifications as needed. Significant progress has been made recently to enhance the accuracy and efficiency of the surface hopping technique. Various limitations of the standard FSSHassociated with quantum nuclear effects, interference and decoherence, trivial or “unavoided” crossings, superexchange, and representation dependencehave been lifted. These advances are needed to allow one to treat many important phenomena in chemistry, physics, materials, and related disciplines. Examples include charge transport in extended systems such as organic solids, singlet fission in molecular aggregates, Auger-type exciton multiplication, recombination and relaxation in quantum dots and other nanoscale materials, Auger-assisted charge transfer, nonradiative luminescence quenching, and electron–hole recombination. This Perspective summarizes recent advances in the surface hopping formulation of nonadiabatic dynamics and provides an outlook on the future of surface hopping.
In our previous work J. Chem. Theory Comput. 2013, 9, 4959, we introduced the PYXAID program, developed for the purpose of performing nonadiabatic molecular dynamics simulations in large-scale ...condensed matter systems. The methodological aspects and the basic capabilities of the program have been extensively discussed. In the present work, we perform a thorough investigation of advanced capabilities of the program, namely, the advanced integration techniques for the time-dependent Schrodinger equation (TD-SE), the decoherence corrections via decoherence-induced surface hopping, the use of multiexciton basis configurations, and the direct simulation of photoexcitation via explicit light–matter interaction. We demonstrate the importance of the mentioned features by studying the electronic dynamics in a variety of systems. In particular, we demonstrate that the advanced integration techniques for solving TD-SE may lead to a significant speedup of the calculations and provide more stable solutions. We show that decoherence is necessary for accurate description of slow relaxation processes such as electron–hole recombination in solid C60. By using multiexciton configurations and direct, nonperturbative treatment of field–matter interactions, we found nontrivial optimality conditions for the multiple exciton generation in a small silicon cluster.
Although all‐inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic–inorganic MHPs in efficiency. Recently, a conceptually new ...β‐CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non‐adiabatic molecular dynamics to show that native point defects in β‐CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect‐mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low‐frequency vibrations. Both factors decrease notably the non‐adiabatic coupling and slow down the dissipation of energy to heat.
The nonradiative electron–hole recombination based on native point defects in β‐CsPbI3 was investigated. Soft lattice and defect covalency localize defect states and decouple them from free charges, rationalizing defect tolerance. The conclusion applies to metal halide perovskites in general, and perhaps to other classes of semiconductors that exhibit similar structural properties.
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