Nuclear Quantum Effects Accelerate Charge Recombination but Boost the Stability of Inorganic Perovskites in Mild Humidity

Experiments have demonstrated that mild humidity can enhance the stability of the CsPbBr3 perovskite, though the underlying mechanism remains unclear. Utilizing ab initio molecular dynamics, ring polymer molecular dynamics, and non-adiabatic molecular dynamics, our study reveals that nuclear quantum effects (NQEs) play a crucial role in stabilizing the lattice rigidity of the perovskite while simultaneously shortening the charge carrier lifetime. NQEs reduce the extent of geometric disorder and the number of atomic fluctuations, diminish the extent of hole localization, and thereby improve the electron–hole overlap and non-adiabatic coupling. Concurrently, these effects significantly suppress phonon modes and slow decoherence. As a result, these factors collectively accelerate charge recombination by a factor of 1.42 compared to that in scenarios excluding NQEs. The resulting sub-10 ns recombination time scales align remarkably well with experimental findings. This research offers novel insight into how moisture resistance impacts the stability and charge carrier lifetime in all-inorganic perovskites

Grain Boundaries Are Benign and Suppress Nonradiative Electron–Hole Recombination in Monolayer Black Phosphorus: A Time-Domain Ab Initio Study

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that both symmetrical (GB_s) and asymmetrical grain boundaries (GB_a) significantly extend charge-carrier lifetime compared with monolayer black phosphorus. Boundaries create no deep trap states, which decrease electron–phonon coupling. As a result, GB_s increases carrier lifetime by a factor of 22, whereas GB_a extends the lifetime by a factor of 4. More importantly, the interplay between the immobile electron localized at the boundaries in the GB_s and extended excited-state lifetime facilitates a chemical reaction, which is beneficial for photocatalysts. In contrast, GB_a separates electron and hole spatially in different locations, which forms a long-lived charge-separated state and is favorable for photovoltaics. Our simulations demonstrate that grain boundaries are benign and retard nonradiative electron–hole recombination in monolayer black phosphorus, suggesting a route to reduce energy losses via rational choice of defect to realize high-performance photovoltaic and photocatalytic devices

Ab Initio Nonadiabatic Molecular Dynamics of the Ultrafast Electron Injection from a PbSe Quantum Dot into the TiO₂ Surface

Following recent experiments [<i>Science</i> <b>2010</b>, <i>328</i>, 1543; <i>PNAS</i> <b>2011</b>, <i>108</i>, 965], we report an <i>ab initio</i> nonadiabatic molecular dynamics (NAMD) simulation of the ultrafast photoinduced electron transfer (ET) from a PbSe quantum dot (QD) into the rutile TiO₂ (110) surface. The system forms the basis for QD-sensitized semiconductor solar cells and demonstrates that ultrafast interfacial ET is instrumental for achieving high efficiencies in solar-to-electrical energy conversion. The simulation supports the observation that the ET successfully competes with energy losses due to electron–phonon relaxation. The ET proceeds by the adiabatic mechanism because of strong donor–acceptor coupling. High frequency polar vibrations of both QD and TiO₂ promote the ET, since these modes can rapidly influence the donor–acceptor state energies and coupling. Low frequency vibrations generate a distribution of initial conditions for ET, which shows a broad variety of scenarios at the single-molecule level. Compared to the molecule–TiO₂ interfaces, the QD–TiO₂ system exhibits pronounced differences that arise due to the larger size and higher rigidity of QDs relative to molecules. Both donor and acceptor states are more delocalized in the QD system, and the ET is promoted by optical phonons, which have relatively low frequencies in the QD materials composed of heavy elements. In contrast, in molecular systems, optical phonons are not thermally accessible under ambient conditions. Meanwhile, TiO₂ acceptor states resemble surface impurities due to the local influence of molecular chromophores. At the same time, the photoinduced ET at both QD–TiO₂ and molecule–TiO₂ interfaces is ultrafast and occurs by the adiabatic mechanism, as a result of strong donor–acceptor coupling. The reported state-of-the-art simulation generates a detailed time-domain atomistic description of the interfacial ET process that is fundamental to a wide variety of applications

Dopants Control Electron–Hole Recombination at Perovskite–TiO₂ Interfaces: <i>Ab Initio</i> Time-Domain Study

TiO₂ sensitized with organohalide perovskites gives rise to solar-to-electricity conversion efficiencies reaching close to 20%. Nonradiative electron–hole recombination across the perovskite/TiO₂ interface constitutes a major pathway of energy losses, limiting quantum yield of the photoinduced charge. In order to establish the fundamental mechanisms of the energy losses and to propose practical means for controlling the interfacial electron–hole recombination, we applied <i>ab initio</i> nonadiabatic (NA) molecular dynamics to pristine and doped CH₃NH₃PbI₃(100)/TiO₂ anatase(001) interfaces. We show that doping by substitution of iodide with chlorine or bromine reduces charge recombination, while replacing lead with tin enhances the recombination. Generally, lighter and faster atoms increase the NA coupling. Since the dopants are lighter than the atoms they replace, one expects <i>a</i> <i>priori</i> that all three dopants should accelerate the recombination. We rationalize the unexpected behavior of chlorine and bromine by three effects. First, the Pb–Cl and Pb–Br bonds are shorter than the Pb–I bond. As a result, Cl and Br atoms are farther away from the TiO₂ surface, decreasing the donor–acceptor coupling. In contrast, some iodines form chemical bonds with Ti atoms, increasing the coupling. Second, chlorine and bromine reduce the NA electron–vibrational coupling, because they contribute little to the electron and hole wave functions. Tin increases the coupling, since it is lighter than lead and contributes to the hole wave function. Third, higher frequency modes introduced by chlorine and bromine shorten quantum coherence, thereby decreasing the transition rate. The recombination occurs due to coupling of the electronic subsystem to low-frequency perovskite and TiO₂ modes. The simulation shows excellent agreement with the available experimental data and advances our understanding of electronic and vibrational dynamics in perovskite solar cells. The study provides design principles for optimizing solar cell performance and increasing photon-to-electron conversion efficiency through creative choice of dopants

Time-Domain Ab Initio Analysis of Excitation Dynamics in a Quantum Dot/Polymer Hybrid: Atomistic Description Rationalizes Experiment

Hybrid organic/inorganic polymer/quantum dot (QD) solar cells are an attractive alternative to the traditional cells. The original, simple models postulate that one-dimensional polymers have continuous energy levels, while zero-dimensional QDs exhibit atom-like electronic structure. A realistic, atomistic viewpoint provides an alternative description. Electronic states in polymers are molecule-like: finite in size and discrete in energy. QDs are composed of many atoms and have high, bulk-like densities of states. We employ ab initio time-domain simulation to model the experimentally observed ultrafast photoinduced dynamics in a QD/polymer hybrid and show that an atomistic description is essential for understanding the time-resolved experimental data. Both electron and hole transfers across the interface exhibit subpicosecond time scales. The interfacial processes are fast due to strong electronic donor–acceptor, as evidenced by the densities of the photoexcited states which are delocalized between the donor and the acceptor. The nonadiabatic charge–phonon coupling is also strong, especially in the polymer, resulting in rapid energy losses. The electron transfer from the polymer is notably faster than the hole transfer from the QD, due to a significantly higher density of acceptor states. The stronger molecule-like electronic and charge-phonon coupling in the polymer rationalizes why the electron–hole recombination inside the polymer is several orders of magnitude faster than in the QD. As a result, experiments exhibit multiple transfer times for the long-lived hole inside the QD, ranging from subpicoseconds to nanoseconds. In contrast, transfer of the short-lived electron inside the polymer does not occur beyond the first picosecond. The energy lost by the hole on its transit into the polymer is accommodated by polymer’s high-frequency vibrations. The energy lost by the electron injected into the QD is accommodated primarily by much lower-frequency collective and QD modes. The electron dynamics is exponential, whereas evolution of the injected hole through the low density manifold of states of the polymer is highly nonexponential. The time scale of the electron–hole recombination at the interface is intermediate between those in pristine polymer and QD and is closer to that in the polymer. The detailed atomistic insights into the photoinduced charge and energy dynamics at the polymer/QD interface provide valuable guidelines for optimization of solar light harvesting and photovoltaic efficiency in modern nanoscale materials

Quantum Coherence Facilitates Efficient Charge Separation at a MoS₂/MoSe₂ van der Waals Junction

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

Defects Slow Down Nonradiative Electron–Hole Recombination in TiS₃ Nanoribbons: A Time-Domain Ab Initio Study

Layered TiS₃ materials hold appealing potential in photovoltaics and optoelectronics due to their excellent electronic and optical properties. Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that the electron–hole recombination in pristine TiS₃ nanoribbons (NRs) occurs in tens of picoseconds and is over 10-fold faster than the experimental value. By performing an atomistic ab initio simulation with a sulfur vacancy, we demonstrate that a sulfur vacancy greatly reduces electron–hole recombination, achieving good agreement with experiment. Introduction of a sulfur vacancy increases the band gap slightly because the NR’s highest occupied molecular orbital is lowered in energy. More importantly, the sulfur vacancy partially diminishes the electron and hole wave functions’ overlap and reduces NA electron–phonon coupling, which competes successfully with the longer decoherence time, slowing down recombination. Our study suggests that a rational choice of defects can control nonradiative electron–hole recombination in TiS₃ NRs and provides mechanistic principles for photovoltaic and optoelectronic device design

Photoinduced Localized Hole Delays Nonradiative Electron–Hole Recombination in Cesium–Lead Halide Perovskites: A Time-Domain Ab Initio Analysis

All-inorganic perovskites have attracted intense interest as promising photovoltaic materials due to their excellent performance. Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that a photoinduced localized polaron-like hole greatly delays the nonradiative electron–hole recombination relative to the structure with delocalized free charge of the CsPbBr₃. This is because localized charge carriers diminish overlap between electron and hole wave functions and decrease the NA coupling by a factor of 6. In addition, polaron formation increases the band gap of CsPbBr₃, slowing down recombination further. The smaller NA coupling and larger band gap compete successfully with the longer decoherence time, extending the recombination to tens of nanoseconds. The calculated recombination times show excellent agreement with experiment. Our study reveals the atomistic mechanisms underlying the suppression of recombination upon formation of localized polaron-like holes and advances our understanding of the excited-state dynamics of all-inorganic perovskite solar cells

Disparity in Photoexcitation Dynamics between Vertical and Lateral MoS₂/WSe₂ Heterojunctions: Time-Domain Simulation Emphasizes the Importance of Donor–Acceptor Interaction and Band Alignment

Two-dimensional transition metal dichalcogenides (TMDs) heterojunctions are appealing candidates for optoelectronics and photovoltaics. Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that photoexcitation dynamics exhibit a significant difference in the vertical and lateral MoS₂/WSe₂ heterojunctions arising from the disparity in the donor–acceptor interaction and fundamental band alignment. The obtained electron transfer time scale in the vertical heterojunction shows excellent agreement with experiment. Hole transfer proceeds 1.5 times slower. The electron–hole recombination is 3 orders of magnitude longer than the charge separation, which favors solar cell applications. On the contrary, the lateral heterojunction shows no band offsets steering charge separation. The excited electron is localized at the interface that attracts holes to form an exciton-like state due to Coulomb interaction, suggesting potential applications in light-emitting devices. The coupled electron and hole wave functions increase NA coupling and the coherence time, accelerating electron–hole recombination by a factor of 3 compared with the vertical case. The atomistic studies advance our understanding of the photoinduced charge–phonon dynamics in TMDs heterojunctions

Defects Are Needed for Fast Photo-Induced Electron Transfer from a Nanocrystal to a Molecule: Time-Domain <i>Ab Initio</i> Analysis

Quantum dot (QD) solar cells constitute an attractive alternative to traditional solar cells due to unique electronic and optical properties of QDs. In order to achieve high photon-to-electron conversion efficiency, rapid charge separation and slow charge recombination are required. We use nonadiabatic molecular dynamics combined with time-domain density functional theory to study electron transfer from a PbS QD to the rhodamine B (RhB) molecule and subsequent electron return from RhB to the QD. The time scale for the electron–hole recombination obtained for the system without defects agrees well with the experiment, while the simulated time scale for the charge separation is 10-fold longer than the experimental value. By performing an atomistic simulation with a sulfur vacancy, which is a common defect in PbS systems, we demonstrate that the defect accelerates the charge separation. This result is supported further by scaling arguments. Missing sulfur creates unsaturated chemical bonds on Pb atoms, which form the PbS conduction band. As a result, the QD lowest unoccupied molecular orbital (LUMO) is lowered in energy, and the LUMO density extends onto the adsorbed molecule, increasing the donor–acceptor interaction. The counterintuitive conclusion that defects are essential rather than detrimental to functioning of QD solar cells generates an unexpected view on the QD surface chemistry