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.

Nonadiabatic Molecular Dynamics with Non-Condon Effect of Charge Carrier Dynamics.

Nonradiative multiphonon transitions play a crucial role in understanding charge carrier dynamics. To capture the non-Condon effect in nonadiabatic molecular dynamics (NA-MD), we develop a simple and accurate method to calculate noncrossing and crossing k-point NA coupling in momentum space on an equal footing and implement it with a trajectory surface hopping algorithm. Multiple k-point MD trajectories can provide sufficient nonzero momentum multiphonons coupled to electrons, and the momentum conservation is maintained during nonvertical electron transition. The simulations of indirect bandgap transition in silicon and intra- and intervalley transitions in graphene show that incorporation of the non-Condon effect is needed to correctly depict these types of charge dynamics. In particular, a hidden process is responsible for the delayed nonradiative electron-hole recombination in silicon: the thermal-assisted rapid trapping of an excited electron at the conduction band minimum by a long-lived higher energy state through a nonvertical transition extends charge carrier lifetime, approaching 1 ns, which is about 1.5 times slower than the direct bandgap recombination. For graphene, intervalley scattering takes place within about 225 fs, which can occur only when the intravalley relaxation proceeds to about 50 fs to gain enough phonon momentum. The intra- and intervalley scattering constitute energy relaxation, which completes within sub-500 fs. All the simulated time scales are in excellent agreement with experiments. The study establishes the underlying mechanisms for a long-lived charge carrier in silicon and valley scattering in graphene and underscores the robustness of the non-Condon approximation NA-MD method, which is suitable for rigid, soft, and large defective systems.

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4.

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Deciphering the In Situ Reconstruction of Metal Phosphide/Nitride Dual Heterostructures for Robust Alkaline Hydrogen Evolution Above 3 A cm-2.

Hydrogen evolution reaction (HER) in neutral or alkaline electrolytes is appealing for sustainable hydrogen production driven by water splitting, but generally suffers from unsatisfied catalytic activities at high current densities owing to extra kinetic energy barriers required to generate protons through water dissociation. In response, here, a competitive Ni3 N/Co3 N/CoP electrocatalyst with multifunctional interfacial sites and multilevel interfaces, in which Ni3 N/CoP performs as active sites to boost initial water dissociation and Co3 N/CoP accelerates subsequent hydrogen adsorption process as confirmed by density functional theory calculations and in situ X-ray photoelectron spectroscopy analysis, is reported. This hybrid catalyst possesses extraordinary HER activity in base, featured by extremely low overpotentials of 115 and 142 mV to afford 500 and 1000 mA cm-2 , respectively, outperforming most ever-reported metal phosphides-based catalysts. This catalyst presents an ultrahigh current density of 3545 mA cm-2 by a factor of 4.96 relative to noble Pt/C catalysts (715 mA cm-2 ) at 0.2 V. Assembled with Fe(PO3 )2 /Ni2 P anode, industrial-level current densities of 500/1000 mA cm-2 at ultralow cell voltages of 1.62/1.66 V for overall water electrolysis with outstanding long-term stability are actualized. More interestingly, this hybrid catalyst also performs well in acidic, neutral freshwater, and seawater requiring relatively low overpotentials of 140, 290, and 331 mV to reach 500 mA cm-2 . Particularly, this catalyst can withstand electrochemical corrosion without obvious activity decay at the industrial-level current densities for over 100 h in base. This work provides a cornerstone for the construction of advanced catalysts operated in different pH environments.

Impact of large A-site cations on electron-vibrational interactions in 2D halide perovskites: Ab initio quantum dynamics.

Using ab initio nonadiabatic molecular dynamics, we study the effect of large A-site cations on nonradiative electron-hole recombination in two-dimensional Ruddlesden-Popper perovskites HA2APb2I7, HA = n-hexylammonium, A = methylammonium (MA), or guanidinium (GA). The steric hindrance created by large GA cations distorts and stiffens the inorganic Pb-I lattice, reduces thermal structural fluctuations, and maintains the delocalization of electrons and holes at ambient and elevated temperatures. The delocalized charges interact more strongly in the GA system than in the MA system, and the charge recombination is accelerated. In contrast, replacement of only some MA cations with GA enhances disorder and increases charge lifetime, as seen in three-dimensional perovskites. This study highlights the key influence of structural fluctuations and disorder on the properties of charge carriers in metal halide perovskites, providing guidance for tuning materials' optoelectronic performance.

Rationalization of passivation strategies toward high-performance perovskite solar cells.

Lead halide perovskite solar cells (PSCs) have shown unprecedented development in efficiency and progressed relentlessly in improving stability. All the achievements have been accompanied by diverse passivation strategies to circumvent the pervasive defects in perovskite materials, which play crucial roles in the process of charge recombination, ion migration, and component degradation. Among the tremendous efforts made to solve these issues and achieve high-performance PSCs, we classify and review both well-established and burgeoning passivation strategies to provide further guidance for the passivation protocols in PSCs, including chemical passivation to eliminate defects by the formation of chemical bonds, physical passivation to eliminate defects by strain relaxation or physical treatments, energetic passivation to improve the stability toward light and oxygen, and field-effect passivation to regulate the interfacial carrier behavior. The subtle but non-trivial consequences from various passivation strategies need advanced characterization techniques combining synchrotron-based X-ray analysis, capacitance-based measurements, spatially resolved imaging, fluorescent molecular probe, Kelvin probe force microscope, etc., to scrutinize the mechanisms. In the end, challenges and prospective research directions on advancing these passivation strategies are proposed. Judicious combinations among chemical, physical, energetic, and field-effect passivation deserve more attention for future high-efficiency and stable perovskite photovoltaics.

Substrate Ferroelectric Proximity Effects Have a Strong Influence on Charge Carrier Lifetime in Black Phosphorus.

By stacking monolayer black phosphorus (MBP) with nonpolarized and ferroelectric polarized bilayer hexagonal boron nitride (h-BN), we demonstrate that ferroelectric proximity effects have a strong influence on the charge carrier lifetime of MBP using nonadiabatic (NA) molecular dynamics simulations. Through enhancing the motion of phosphorus atoms, ferroelectric polarization enhances the overlap of electron-hole wave functions that improves NA coupling and decreases the bandgap, resulting in a rapid electron-hole recombination completing within a quarter of nanoseconds, which is two times shorter than that in nonpolarized stackings. In addition to the dominant in-plane Ag2 mode in free-standing MBP, the out-of-plane high-frequency Ag1 and low-frequency interlayer breathing modes presented in the heterojunctions drive the recombination. Notably, the resonance between the breathing mode within bilayer h-BN and the B1u mode of MBP provides an additional nonradiative channel in ferroelectric stackings, further accelerating charge recombination. These findings are crucial for charge dynamics manipulation in two-dimensional materials via substrate ferroelectric proximity effects.

Electron- versus Spin-Phonon Coupling Governs the Temperature-Dependent Carrier Dynamics in the Topological Insulator Bi2Te3.

Ultrafast charge and spin dynamics have immense effects on the applications of topological insulators (TIs). By performing spin-adiabatic nonadiabatic molecular dynamics simulations in the presence of electron-phonon (e-ph) and spin-phonon couplings, we investigate temperature-dependent intra- and interband charge and spin relaxation dynamics via the bulk and surface paths in the three-dimensional TI Bi2Te3. The e-ph coupling dominates charge relaxation in the bulk path, and the relaxation rate is positively correlated with temperature due to the large energy gaps and weak spin polarization. Conversely, the relaxation dynamics exhibits an opposite temperature dependence in the surface path because of electron re-excitation and spin mismatching induced by spin-phonon coupling, which arises from small energy gaps and strong spin polarization. The two mechanisms rationalize the charge carriers being long-lived in the bulk and surface phases at low and room temperature, respectively. Additionally, strong thermal fluctuations of the topological states' magnetic moments destroy the spin-momentum locking and trigger backscattering at room temperature.

Photolysis versus Photothermolysis of N2O on a Semiconductor Surface Revealed by Nonadiabatic Molecular Dynamics.

Identifying photolysis and photothermolysis during a photochemical reaction has remained challenging because of the highly non-equilibrium and ultrafast nature of the processes. Using state-of-the-art ab initio adiabatic and nonadiabatic molecular dynamics, we investigate N2O photodissociation on the reduced rutile TiO2(110) surface and establish its detailed mechanism. The photodecomposition is initiated by electron injection, leading to the formation of a N2O- ion-radical, and activation of the N2O bending and symmetric stretching vibrations. Photothermolysis governs the N2O dissociation when N2O- is short-lived. The dissociation is activated by a combination of the anionic excited state evolution and local heating. A thermal fluctuation drives the molecular acceptor level below the TiO2 band edge, stabilizes the N2O- anion radical, and causes dissociation on a 1 ps timescale. As the N2O- resonance lifetime increases, photolysis becomes dominant since evolution in the anionic excited state activates the bending and symmetric stretching of N2O, inducing the dissociation. The photodecomposition occurs more easily when N2O is bonded to TiO2 through the O rather than N atom. We demonstrate further that a thermal dissociation of N2O can be realized by a rational choice of metal dopants, which enhance p-d orbital hybridization, facilitate electron transfer, and break N2O spontaneously. By investigating the charge dynamics and lifetime, we provide a fundamental atomistic understanding of the competition and synergy between the photocatalytic and photothermocatalytic dissociation of N2O and demonstrate how N2O reduction can be controlled by light irradiation, adsorption configuration, and dopants, enabling the design of high-performance transition-metal oxide catalysts.

Rapid Interlayer Charge Separation and Extended Carrier Lifetimes due to Spontaneous Symmetry Breaking in Organic and Mixed Organic-Inorganic Dion-Jacobson Perovskites.

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.

Nuclear Quantum Effects Prolong Charge Carrier Lifetimes in Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) contain light hydrogen atoms that exhibit significant nuclear quantum effects (NQEs). We demonstrate that NQEs have a strong effect on HOIP geometry and electron-vibrational dynamics at both low and ambient temperatures, even though charges in HOIPs reside on heavy elements. By combining ring-polymer molecular dynamics (MD) and ab initio MD with nonadiabatic MD and time-dependent density functional theory and focusing on the most studied tetragonal CH3NH3PbI3, we show that NQEs increase the disorder and thermal fluctuations through coupling of the light inorganic cations to the heavy inorganic lattice. The additional disorder induces charge localization and decreases electron-hole interactions. As a result, the nonradiative carrier lifetimes are extended by a factor of 3 at 160 K and 1/3 at 330 K. The radiative lifetimes are increased by 40% at both temperatures. The fundamental band gap decreases by 0.10 and 0.03 eV at 160 and 330 K, respectively. By enhancing atomic motions and introducing new vibrational modes, NQEs strengthen electron-vibrational interactions. Decoherence, determined by elastic scattering, accelerates almost by a factor of 2 due to NQEs. However, the nonadiabatic coupling, driving nonradiative electron-hole recombination, decreases because it is more sensitive to structural distortions than atomic motions in HOIPs. This study demonstrates, for the first time, that NQEs should be considered to achieve an accurate understanding of geometry evolution and charge carrier dynamics in HOIPs and provides important fundamental insights for the design of HOIPs and related materials for optoelectronic applications.

Twist Angle-Dependent Intervalley Charge Carrier Transfer and Recombination in Bilayer WS2.

A twist angle at a van der Waals junction provides a handle to tune its optoelectronic properties for a variety of applications, and a comprehensive understanding of how the twist modulates electronic structure, interlayer coupling, and carrier dynamics is needed. We employ time-dependent density functional theory and nonadiabatic molecular dynamics to elucidate angle-dependent intervalley carrier transfer and recombination in bilayer WS2. Repulsion between S atoms in twisted configurations weakens interlayer coupling, increases the interlayer distance, and softens layer breathing modes. Twisting has a minor influence on K valleys while it lowers Γ valleys and raises Q valleys because their wave functions are delocalized between layers. Consequently, the reduced energy gaps between the K and Γ valleys accelerate the hole transfer in the twisted structures. Intervalley electron transfer proceeds nearly an order of magnitude faster than hole transfer. The more localized wave functions at K than Q values and larger bandgaps result in smaller nonadiabatic couplings for intervalley recombination, making it 3-4 times slower in twisted than high-symmetry structures. B2g breathing, E2g in-plane, and A1g out-of-plane modes are most active during intervalley carrier transfer and recombination. The faster intervalley transfer and extended carrier lifetimes in twisted junctions are favorable for optoelectronic device performance.

Suppressing Oxygen-Induced Deterioration of Metal Halide Perovskites by Alkaline Earth Metal Doping: A Quantum Dynamics Study.

Exposure to oxygen undermines stability and charge transport in metal halide perovskites, because molecular oxygen, as well as photogenerated superoxide and peroxide, erodes the perovskite lattice and creates charge traps. We demonstrate that alkaline earth metals passivate the oxygen species in CH3NH3PbI3 by breaking the O-O bond and forming new bonds with the oxygen atoms, shifting the trap states of the antibonding O-O orbitals from inside the bandgap into the bands. In addition to eliminating the oxidizing species and the charge traps, doping with the alkaline earth metals slightly increases the bandgap and partially localizes the electron and hole wavefunctions, weakening the electron-hole and charge-phonon interactions and making the charge carrier lifetimes longer than even those in pristine CH3NH3PbI3. Relative to CH3NH3PbI3 exposed to oxygen and light, the charge carrier lifetime of the passivated CH3NH3PbI3 increases by 2-3 orders of magnitude. The ab initio quantum dynamics simulations demonstrate that alkaline earth metals passivate efficiently not only intrinsic perovskite defects, but also the foreign species, providing a viable strategy to suppress perovskite degradation.

Structural Disorder in Higher-Temperature Phases Increases Charge Carrier Lifetimes in Metal Halide Perovskites.

Solar cells and optoelectronic devices are exposed to heat that degrades performance. Therefore, elucidating temperature-dependent charge carrier dynamics is essential for device optimization. Charge carrier lifetimes decrease with temperature in conventional semiconductors. The opposite, anomalous trend is observed in some experiments performed with MAPbI3 (MA = CH3NH3+) and other metal halide perovskites. Using ab initio quantum dynamics simulation, we establish the atomic mechanisms responsible for nonradiative electron-hole recombination in orthorhombic-, tetragonal-, and cubic MAPbI3. We demonstrate that structural disorder arising from the phase transitions is as important as the disorder due to heating in the same phase. The carrier lifetimes grow both with increasing temperature in the same phase and upon transition to the higher-temperature phases. The increased lifetime is rationalized by structural disorder that induces partial charge localization, decreases nonadiabatic coupling, and shortens quantum coherence. Inelastic and elastic electron-vibrational interactions exhibit opposite dependence on temperature and phase. The partial disorder and localization arise from thermal motions of both the inorganic lattice and the organic cations and depend significantly on the phase. The structural deformations induced by thermal fluctuations and phase transitions are on the same order as deformations induced by defects, and hence, thermal disorder plays a very important role. Since charge localization increases carrier lifetimes but inhibits transport, an optimal regime maximizing carrier diffusion can be designed, depending on phase, temperature, material morphology, and device architecture. The atomistic mechanisms responsible for the enhanced carrier lifetimes at elevated temperatures provide guidelines for the design of improved solar energy and optoelectronic materials.

Substitutional alkaline earth metals delay nonradiative charge recombination in CH3NH3PbI3 perovskite: A time-domain study.

Experiments reported that alkaline earth metal dopants greatly prolong carrier lifetime and improve the performance of perovskite solar cells. Using state-of-the-art ab initio time-domain nonadiabatic molecular dynamics (NAMD), we demonstrate that incorporation of alkaline earth metals, such as Sr and Ba, into MAPbI3 (MA = CH3NH3 +) lattice at the lead site is energetically favorable due to the largely negative formation energies about -7 eV. The replacement widens the bandgap and increases the open-circuit voltage by creating no trap states. More importantly, the substitution reduces the mixing of electron and hole wave functions by pushing the hole charge density away from the dopant together with no contribution of Sr and Ba to the conduction band edge state, thus decreasing the NA coupling. The high frequency phonons generated by enhanced atomic motions and symmetry breaking accelerate phonon-induced loss of coherence. The synergy of the three factors reduces the nonradiative recombination time by a factor of about 2 in the Sr- and Ba-doped systems with respect to pristine MAPbI3, which occurs over 1 ns and agrees well with the experiment. The study highlights the importance of various factors affecting charge carrier lifetime, establishes the mechanism of reduction of nonradiative electron-hole recombination in perovskites upon alkaline earth metal doping, and provides meaningful insights into the design of high performance of perovskite solar cells and optoelectronics.

Phonon-Mediated Interlayer Charge Separation and Recombination in a MoSe2/WSe2 Heterostructure.

Monolayer transition metal dichalcogenides bear great potential for photodetection and light harvesting due to high absorption coefficients. However, these applications require dissociation of strongly bound photogenerated excitons. The dissociation can be achieved by vertically stacking different monolayers to realize band alignment that favors interlayer charge transfer. In such heterostructures, the reported recombination times vary strongly, and the charge separation and recombination mechanisms remain elusive. We use two color pump-probe microscopy to demonstrate that the charge separation in a MoSe2/WSe2 heterostructure is ultrafast (∼200 fs) and virtually temperature independent, whereas the recombination accelerates strongly with temperature. Ab initio quantum dynamics simulations rationalize the experiments, indicating that the charge separation is temperature-independent because it is barrierless, involves dense acceptor states, and is promoted by higher-frequency out-of-plane vibrations. The strong temperature dependence of the recombination, on the other hand, arises from a transient indirect-to-direct bandgap modulation by low-frequency shear and layer breathing motions.

Elimination of Charge Recombination Centers in Metal Halide Perovskites by Strain.

Metal halide perovskites exhibit enhanced photoluminescence and long-lived carriers in experiments under strain. Using ab initio nonadiabatic molecular dynamics, we demonstrate that compressive and tensile strain can eliminate charge recombination centers created by defect states, by shifting traps from bandgap into bands. A compressive strain enhances coupling of Pb-s and I-p orbitals, pushes the valence band (VB) up in energy, and moves the trap state due to iodine interstitial (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.

Why Oxygen Increases Carrier Lifetimes but Accelerates Degradation of CH3NH3PbI3 under Light Irradiation: Time-Domain Ab Initio Analysis.

Exposure to oxygen and light undermines chemical stability of metal halide perovskites, while it surprisingly improves their optical properties. Focusing on CH3NH3PbI3, we demonstrate that material degradation and charge carrier lifetimes depend strongly on the oxidation state of the oxygen species. Nonadiabatic molecular dynamics simulations combined with time-domain density functional theory show that a neutral oxygen molecule has little influence on the perovskite stability, while the superoxide and the peroxide accelerate degradation by breaking Pb-I chemical bonds and enhancing atomic fluctuations. Creating electron and/or hole traps, the neutral oxygen and the superoxide decrease charge carrier lifetimes by over 1 and 2 orders of magnitude, respectively. Importantly, photoinduced reduction of oxygen to the peroxide eliminates trap states and extends carrier lifetimes by more than a factor of 2 because it decreases the nonadiabatic coupling and shortens quantum coherence. The simulations indicate that the superoxide should be strongly avoided, for example, by full reduction to the peroxide because it causes simultaneous degradation of perovskite stability and optical properties. The detailed simulations rationalize the complex interplay between the influence of atmosphere and light on perovskite performance, apply to other solar cell materials exposed to natural elements, and provide valuable insights into design of high-performance solar cells.

Pb dimerization greatly accelerates charge losses in MAPbI3: Time-domain ab initio analysis.

Metal halide perovskites constitute a new type of semiconducting materials with long charge carrier lifetimes and efficient light-harvesting. The performance of perovskite solar cells and related devices is limited by nonradiative charge and energy losses, facilitated by defects. Combining nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that charge losses depend strongly on the defect chemical state. By considering an extra Pb atom in CH3NH3PbI3, which is a common defect in lead halide perovskites, we investigate its influence on charge trapping and recombination. In a chemically inert form as a Pb interstitial, the extra Pb atom has only a mild influence on charge recombination. However, if the extra Pb atom binds to a native Pb atom to form a dimer, the charge trapping and recombination are greatly accelerated because the Pb-dimer creates a localized midgap trap state that couples strongly to the perovskite valence band edge. Holes disappear from the valence band two orders of magnitude faster than in the pristine perovskite and recombine with conduction band electrons one order of magnitude faster. The simulations identify the phonon modes involved in the nonradiative charge trapping and recombination and highlight the importance of rapid decoherence within the electronic subsystem for long carrier lifetimes. The detailed atomistic analysis of the charge trapping and recombination mechanisms enriches the understanding of defect properties and provides theoretical guidance for improving perovskite performance.

Quantum dynamics origin of high photocatalytic activity of mixed-phase anatase/rutile TiO2.

Mixed anatase/rutile TiO2 exhibits high photocatalytic activity; however, the mechanism underlying the high performance of the mixed phases is not fully understood. We have performed time-domain ab initio calculations to study the exited state dynamics in mixed phase TiO2 and to investigate the impact of an oxygen vacancy on the dynamics. The anatase(100)/rutile(001) heterostructures with and without an oxygen vacancy used in this work exhibit type II band alignment with the conduction band of rutile residing above that of anatase. The oxygen vacancy introduces a hole trap state inside the bandgap. Owing to a strong coupling between the donor and acceptor states, the electron and hole transfers across the anatase/rutile interface occur on an ultrafast 100 fs timescale in both systems. The decoupling of electron and hole favors a long-lived charge separated state. The electron-hole recombination across the pristine anatase/rutile interface takes 6.6 ns and is significantly slower than that in the pure anatase and rutile phases, showing good agreement with experiments. The electron transfer dynamics is independent of the oxygen vacancy, which has some influence on the hole transfer and a strong effect on carrier recombination. By creating a hole trap state, the vacancy accelerates carrier losses by over an order of magnitude. The fast charge separation and the long lifetime of the charge separated state rationalize the enhanced photocatalytic performance of mixed phase TiO2 compared to the pure phases.