Superoxide/Peroxide Chemistry Extends Charge Carriers’ Lifetime but Undermines Chemical Stability of CH3NH3PbI3 Exposed to Oxygen: Time-Domain ab Initio Analysis

DL-Serine Hydrazide Hydrochloride Multiple-site Synergy Induced Effective and Stable Formamidine-Rich Perovskite Solar Cells

The efficiency and stability of solar cells are two key indicators that determine for the commercial feasibility of photovoltaic devices. Formamidine-cesium perovskite has been extensively investigated since its excellent thermal stability and has great potential in achieving high power conversion efficiency. However, during the aging process, especially under light conditions, formamidine-rich perovskites are prone to produce iodine, and the escape of iodine is one of the important factors leading to device degradation. Here, DL-Serine Hydrazide Hydrochloride containing the reducing group is introduced into the precursor solution of formamidine-cesium perovskite, which achieves multiple-site passivation. Hydrazine reacts with iodine to reduce it to iodine ions, inhibiting the escape of iodine. In addition, carbonyl groups and uncoordinated lead ions form coordination bonds to reduce defects. In the end, the perovskite solar cell with DL-Serine Hydrazide Hydrochloride added achieves a champion efficiency of 22.22%, and maintains 85.88% of the initial efficiency after continuous exposure under 1 sun for 7000 s at a relative humidity of ≈40%. Additionally, DL-Serine Hydrazide Hydrochloride added device shows good stability in air environments with relative humidity of 50%-60%. DL-Serine Hydrazide Hydrochloride improves the stability of formamidine-rich perovskite solar cells and provides a low-cost strategy for commercial development.

Defect Passivation: Physisorption or Chemisorption? A Nonadiabatic Molecular Dynamics Study

Nonradiative charge recombination, originating from defects, limits the use of semiconductors in solar energy conversion technologies. Defect passivation is an effective approach to eliminating charge recombination centers. Focusing on InSe semiconductor, we have shown that the adsorption configurations of passivators have a strong impact on the defect passivation, using nonadiabatic molecular dynamics combined with time-dependent density functional theory. The simulations demonstrate that the physisorption passivator cannot eliminate the recombination centers, resulting in fast nonradiative charge recombination. By contrast, the chemisorption passivators are able to form covalent bonds with indium, remove the charge recombination centers, thereby prolonging the charge recombination time by more than a factor of 10 because of the decreased nonadiabatic coupling and channels for charge and energy losses. This study uncovers the microscopic effects of the adsorption configurations of passivators on the photogenerated charge carrier dynamics, suggesting that chemisorption passivators are essential for defect passivation.

Synergistic Effect of Laser, Water Vapor, and Electron-Beam on the Degradation of Quasi-Two-Dimensional Ruddlesden-Popper Perovskite Flakes

Understanding the effects of laser light, water vapor, and energetic electron irradiation on the intrinsic properties of perovskites is important in the development of perovskite-based solar cells. Various phase transition and degradation processes have been reported when these agents interact with perovskites separately. However, detailed studies of their synergistic effects are still missing. In this work, the synergistic effect of three factors (exposure to laser light, water vapor, and e-beam) on the optical and physical properties of two-dimensional (2D) Ruddlesden-Popper (RP) perovskite flakes [(BA)2(MA)2Pb3Br10] has been investigated in an environmental cell. When the perovskite flakes were subjected to moderate laser irradiation in a humid environment after prior e-beam irradiation, the photoluminescence (PL) peak centered at 480 nm vanished, while a new PL peak centered at 525 nm emerged, grew, and then quenched. This indicates the degradation process of the 2D RP perovskite was a phase transition to a three-dimensional (3D) perovskite [MAPbBr3] followed by the degradation of 3D perovskite. The spatial distribution of the 525 nm PL signal shows that this phase-transition process spreads across the flake to the area as far as ∼40 μm from the laser spot. Without humidity, the phase transition happened in the laser-irritated area but did not spread, which suggests that moisture enhanced the ion migration from the laser-scanned area to the rest of the flake and accelerated the phase transition in the nearby area. Experiments with no prior e-beam irradiation show that e-beam irradiation is the key to activating the 2D-3D phase transition. Therefore, when the three factors work synergistically, a conversion from the 2D RP perovskite into the 3D perovskite is not localized and propagates through the perovskite. These findings contribute to our understanding of the complex interactions between external stimuli and perovskite materials, thereby advancing the development of efficient and stable perovskite-based solar cells.

Electron-Phonon Coupling-Mediated Ultralong Carrier Lifetime in an All-Inorganic Two-Dimensional Cs2PbI2Cl2 Perovskite: Explanation for the High Antisite Defect Tolerance

Two-dimensional (2D) halide perovskite are appealing candidates for applications in optoelectronics and photovoltaics, but their energy conversion efficiency is severely limited by nonradiative electron-hole recombination. In most investigations, point defects with deep defect levels and deep charge-state transition levels in the band gap are treated as the carrier recombination centers. For the all-inorganic 2D Css 2PbI2Cl2, the IPb antisite defect is the most likely to form and cause nonradiative electron-hole recombination. By using density functional theory and ab initio nonradiative molecular dynamics calculations, we found that the IPb defect can introduce the deep acceptor and donor levels into the band gap. Because electron-phonon coupling gives rise to weak nonadiabatic coupling and rapid loss of electronic coherence, those levels lead to a reduction of the carrier loss and the prolongation of the excited-state carrier lifetime, thereby enhancing the photoelectric and defect tolerance properties of the Cs2PbI2Cl2 material. These results could deepen the understanding of the chemistry of defects and carrier dynamics in perovskite materials.

Synergistic Role of Water and Oxygen Leads to Degradation in Formamidinium-Based Halide Perovskites

Mixed-cation metal halide perovskites have shown remarkable progress in photovoltaic applications with high power conversion efficiencies. However, to achieve large-scale deployment of this technology, efficiencies must be complemented by long-term durability. The latter is limited by external factors, such as exposure to humidity and air, which lead to the rapid degradation of the perovskite materials and devices. In this work, we study the mechanisms causing Cs and formamidinium (FA)-based halide perovskite phase transformations and stabilization during moisture and air exposure. We use in situ X-ray scattering, X-ray photoelectron spectroscopy, and first-principles calculations to study these chemical interactions and their effects on structure. We unravel a surface reaction pathway involving the dissolution of FAI by water and iodide oxidation by oxygen, driving the Cs/FA ratio into thermodynamically unstable regions, leading to undesirable phase transformations. This work demonstrates the interplay of bulk phase transformations with surface chemical reactions, providing a detailed understanding of the degradation mechanism and strategies for designing durable and efficient perovskite materials.

Insights into Photogenerated Carrier Dynamics and Overall Water Splitting of the CrS3/GeSe Heterostructure

Based on the nonadiabatic molecular dynamics (NAMD) simulations and the first-principles calculations, we explore the overall water-splitting schemes and the photogenerated carrier dynamics for two configurations (CG and CyG) of the CrS3/GeSe van der Waals heterostructures. The photocatalytic direct Z-schemes and carrier migration pathways for hydrogen and oxygen evolution reactions (HER/OER) are constructed based on the electronic properties. The solar-to-hydrogen efficiency (η'STH values) of the schemes can reach 10.60% and 10.17% and further rise under tensile strain. The NAMD results demonstrate similar transfer times of the electron/hole for HER/OER and more rapid electron-hole recombination in CG enables it to be superior to CyG in photocatalytic performance. Moreover, the Gibbs free energy indicates that both the HERs and OERs turn to spontaneously proceed with CG and CyG at pH = 0-12.37 and pH = 2.55-11.01, respectively. These facts reveal that the CrS3/GeSe heterostructure is promising in photocatalytic overall water splitting.

Great Influence of Pressure and Isotope Effects on Nonradiative Charge Loss in Hybrid Organic-Inorganic Perovskites

The intrinsic softness of hybrid organic-inorganic perovskites (HOIPs) allows their lattice and optoelectronic performance to be tunable to external pressure. Using nonadiabatic (NA) molecular dynamics, we demonstrate that a mild pressure accelerates hot electron relaxation and suppresses nonradiative electron-hole recombination in CH3NH3PbI3. Both processes are governed by NA coupling, which is enhanced between the electronic states of the quasi-continuous bands while is decreased between the band-edge states by reducing the electron-hole wave function overlap. Hydrogen/deuterium isotope exchange alleviates the pressure-induced NA coupling by increasing lattice rigidity and decreasing wave function overlap, slowing down both the hot electron relaxation and electron-hole recombination processes. The simulated time scales of sub-3 ps for hot electron relaxation and half nanoseconds for recombination agree well with the experiments. The study suggests that the isotope exchange can mitigate the pressure-caused fast losses of hot electrons and further prolong the charge carrier lifetime in HOIPs.

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.

Insights into Photoinduced Carrier Dynamics and Overall Water Splitting of Z-Scheme van der Waals Heterostructures with Intrinsic Electric Polarization

Using first-principles calculations in combination with nonadiabatic molecular dynamics (NAMD), we propose novel heterostructures of carbon nitride (C₇N₆) and the Janus GaSnPS monolayer as promising direct Z-scheme photocatalysts for solar-driven overall water splitting. The out-of-plane electric field due to the electric polarization which is dependent on the stacking pattern alters the band alignment and catalytic activity of the heterostructures. The relatively strong interfacial nonadiabatic coupling and long quantum coherence time accelerate the interlayer carrier recombination, enabling a direct Z-scheme photocatalytic mechanism. More importantly, the redox ability of the remanent photogenerated carriers in the Z scheme is strong enough to trigger both the hydrogen evolution reaction (HER) and oxygen reduction reaction (OER) simultaneously without the help of sacrificial agents. Our work reveals a fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as new design prospects for highly efficient direct Z-scheme photocatalysts.

Improved Defect Tolerance and Charge Carrier Lifetime in Tin-Lead Mixed Perovskites: Ab Initio Quantum Dynamics

Simulations by nonadiabatic (NA) molecular dynamics demonstrate that mixing tin with lead in CH₃NH₃PbI₃ can passivate the midgap state created by an interstitial iodine (I_i) via the imposed compressive strain and upshifted valence band maximum, reduce NA coupling by decreasing electron-hole wave functions overlap, and shortens pure-dephasing time by introducing high-frequency phonon modes. Thus, the charge carrier lifetime extends to 3.6 ns due to the significantly reduced nonradiative electron-hole recombination, which is an order of magnitude longer than the I_i-containing CH₃NH₃PbI₃, over 2.5 times longer than the pristine CH₃NH₃PbI₃ (1.4 ns), and even 1.7 times longer than the tin-lead mixed perovskite without the I_i defects (2.1 ns). Tin-lead alloying simultaneously increases the I_i defect formation energy to 0.402 eV from 0.179 eV in CH₃NH₃PbI₃, which effectively enhances defect tolerance by reducing the defect concentration. The study reveals the factors controlling the enhanced performance of tin-lead mixed perovskite solar cells.

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.

20.67%-Efficiency Inorganic CsPbI3 Solar Cells Enabled by Zwitterion Ion Interface Treatment

All-inorganic CsPbI₃ perovskite solar cells (PSCs) have been extensively studied due to their high thermal stability and unprecedented rise in power conversion efficiency (PCE). Recently, the champion PCE of CsPbI₃ PSCs has reached up to 21%; however, it is still much lower than that of organic-inorganic hybrid PSCs. Interface modification to passivate surface defects and minimize charge recombination and trapping is important to further improve the efficiency of CsPbI₃ PSCs. Herein, a new zwitterion ion is deposited at the interface between electron transporting layer (ETL) and perovskite layer to passivate the defects therein. The zwitterion ions can not only passivate oxygen vacancy (V_O ) and iodine vacancy (V_I ) defects, but also improve the band alignment at the ETL-perovskite interface. After the interface treatment, the PCE of CsPbI₃ device reaches up to 20.67%, which is among the highest values of CsPbI₃ PSCs so far. Due to the defect passivation and hydrophobicity improvement, the PCE of optimized device remains 94% of its original value after 800 h storing under ambient condition. These results provide an efficient way to improve the quality of ETL-perovskite interface by zwitterion ions for achieving high performance inorganic CsPbI₃ PSCs.

Anion Doping Delays Nonradiative Electron-Hole Recombination in Cs-Based All-Inorganic Perovskites: Time Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that composition engineering of the X-site anions has a strong influence on the nonradiative electron-hole recombination and thermodynamic stability of cesium-based all-inorganic perovskites. Partial substitution of iodine(I) with bromine (Br) and acetate (Ac) anions reduces the NA electron-vibrational coupling by minimizing the overlap between the electron and hole wave functions and suppressing atomic fluctuations. The doping also widens the energy gap to further reduce the NA coupling and to enhance the open-circuit voltage of perovskite solar cells. These factors increase the charge carrier lifetime by an order of magnitude and improve structural stability in the series CsPbI_1.88BrAc_0.12 > CsPbI₂Br > CsPbI₃. The fundamental atomistic insights into the influence of anion doping on the photophysical properties of the all-inorganic lead halide perovskites guide the design of efficient optoelectronic materials.

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.

Photoinduced Small Hole Polarons Formation and Recombination in All-Inorganic Perovskite from Quantum Dynamics Simulation

We conducted ab initio molecular dynamics (AIMD) and nonadiabatic MD to simulate polaron formation and recombination in all-inorganic Cs₃Bi₂Br₉ perovskite. The meticulously designed AIMD simulations show that two types of small hole polaron, including localized and semidelocalized small hole polaron on either an intralayer or an interlayer Br dimer, are adiabatically formed within 1.71 ps. The localized small hole polaron reduces nonadiabatic coupling and decoherence time and, thus, delays charge recombination to 213 ns. In contrast, the dominant semidelocalized polaron increases nonadiabatic coupling by enhancing electron-hole overlap and restores the energy gap and decoherence time to the pristine system, accelerating recombination to 4.7 ns compared to a 10 ns charge carrier lifetime in the pristine system. All the obtained time scales agree well with experiments. The study offers a fundamental understanding of the excited-state dynamics of small hole polaron in Cs₃Bi₂Br₉ and helps to design high-performance perovskite optoelectronics and photovoltaics.

Atomistic Mechanism of Surface-Defect Passivation: Toward Stable and Efficient Perovskite Solar Cells

Molecular engineering has been demonstrated to be a predominant strategy for augmenting the long-term stability and passivating adverse defects for perovskite solar cells (PSCs). Here, using density functional theory calculations combined with ab initio molecular dynamics (AIMD) simulations, the passivation effects of bidentate passivation molecules, 2-MP and 2-MDEP, on the iodine vacancy MAPbI₃ were comprehensively investigated. We demonstrate that 2-MDEP engenders stronger adsorption and localized charges on Pb atoms because the separated binding sites match with the MAPbI₃ lattice. Moreover, the activation barriers for ion migrations are improved by the passivation of 2-MP and 2-MDEP. Furthermore, AIMD simulations verify the improved structural stability and restrained nonradiative recombination after passivation. More importantly, the durable Pb-heteroatom interactions at the interface and stronger hydrophobicity endow 2-MDEP with more remarkable shielding effects against moisture compared to those of 2-MP. This work deepens our understanding of the passivation effects and paves the way for the design of passivation molecules toward the attainment of efficient and stable PSCs.

Electron-Volt Fluctuation of Defect Levels in Metal Halide Perovskites on a 100 ps Time Scale

Metal halide perovskites (MHPs) have gained considerable attention due to their excellent optoelectronic performance, which is often attributed to unusual defect properties. We demonstrate that midgap defect levels can exhibit very large and slow energy fluctuations associated with anharmonic acoustic motions. Therefore, care should be taken classifying MHP defects as deep or shallow, since shallow defects may become deep and vice versa. As a consequence, charges from deep levels can escape into bands, and light absorption can be extended to longer wavelengths, improving material performance. The phenomenon, demonstrated with iodine vacancy in CH₃NH₃PbI₃ using a machine learning force field, can be expected for a variety of defects and dopants in many MHPs and other soft inorganic semiconductors. Since large-scale anharmonic motions can be precursors to chemical decomposition, a known problem with MHPs, we propose that materials that are stiffer than MHPs but softer than traditional inorganic semiconductors, such as Si and TiO₂, may simultaneously exhibit excellent performance and stability.

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.

The Important Role of Optical Absorption in Determining the Efficiency of Intermediate Band Solar Cells and a Design Principle for Perovskite Doping

The efficiency of solar cells can be increased by introducing an intermediate band (IB) in the band gap. Considering that absorption within the band gap is typically weak, the efficiency of IB solar cells was overestimated previously, with a strong enough optical absorption assumed. Here, we propose a new formulism to calculate the limit of the efficiency of IB solar cells with the ideal absorption assumption removed, which can be used to evaluate the effect of absorption. New IB materials are designed via doping double perovskite, which has a relatively strong absorption within the band gap with both d-p and s-p transitions. The limit of the efficiency of a 2 μm thick Sn-doped Cs₂AgBiBr₆ is 38.6% under the AM1.5G spectrum, which is only ∼6% smaller than the ideal-absorption estimation. Results presented here provide a new dimension in the rational design of IB solar cell materials.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Rational Unraveling of Alkali Metal Concentration-Dependent Photovoltaic Performance of Halide Perovskites: Octahedron Distortion vs Surface Reconstruction

Adding alkali metal in organic-inorganic halide perovskites effectively improves its photovoltaic performance, while excessive alkali metal incorporation would produce a detrimental effect. Through density functional theory and nonadiabatic molecular dynamics simulations, we demonstrate how and why the photogenerated carrier lifetime mutates with the increase of alkali metal concentration. A small amount of Rb doping in the lattice introduces a slight distortion of the octahedron, reducing the overlap of frontier orbitals and decreasing the nonadiabatic coupling, effectively enhancing the photogenerated carrier lifetime. In contrast, excessive Rb will introduce defect states, resulting in the low carrier lifetime by a factor of 2-3 orders of magnitude. Strikingly, the surface formamidinium (FA) cations exhibit unexpected responsibility for the carrier dynamics since its high-frequency thermal vibration strongly leads to the ultrafast hole trapping and carrier recombination. Our results provide new insight into the concentration-dependent photovoltaic performance of alkali metal cations in organic-inorganic halide perovskites.

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.

Extending the Time Scales of Nonadiabatic Molecular Dynamics via Machine Learning in the Time Domain

A novel methodology for direct modeling of long-time scale nonadiabatic dynamics in extended nanoscale and solid-state systems is developed. The presented approach enables forecasting the vibronic Hamiltonians as a direct function of time via machine-learning models trained directly in the time domain. The use of periodic and aperiodic functions that transform time into effective input modes of the artificial neural network is demonstrated to be essential for such an approach to work for both abstract and atomistic models. The best strategies and possible limitations pertaining to the new methodology are explored and discussed. An exemplary direct simulation of unprecedentedly long 20 picosecond trajectories is conducted for a divacancy-containing monolayer black phosphorus system, and the importance of conducting such extended simulations is demonstrated. New insights into the excited states photophysics in this system are presented, including the role of decoherence and model definition.

Modeling Non-adiabatic Dynamics in Nanoscale and Condensed Matter Systems

Rapid, far-from-equilibrium processes involving excitation of electronic, vibrational, spin, photon, topological, and other degrees of freedom form the basis of modern technologies, including electronics and optoelectronics, solar energy harvesting and conversion to electrical and chemical energy, quantum information processing, spin- and valleytronics, chemical detection, and medical therapies. Such processes are studied experimentally with various time-resolved spectroscopies that allow scientists to track system's evolution on ultrafast time scales and at close to atomistic level of detail. The availability of various forms of lasing has made such measurements easily accessible to many experimental groups worldwide, to study atoms and small molecules, nanoscale and condensed matter systems, proteins, cells, and mesoscopic materials. The experimental work necessitates parallel theoretical efforts needed to interpret the experiments and to provide insights that cannot be gained through measurements due to experimental limitations.Non-adiabatic (NA) molecular dynamics (MD) allows one to study processes at the atomistic level and in the time domain most directly mimicking the time-resolved experiments. Atomistic modeling takes full advantage of chemical intuition and principles that guide design and fabrication of molecules and materials. It provides atomistic origins of quasi-particles, such as holes, excitons, trions, plasmons, phonons, polarons, polaritons, spin-waves, momentum-resolved and topological states, electrically and magnetically polarized structures, and other abstract concepts. An atomistic description enables one to study realistic aspects of materials, which necessarily contain defects, dopants, surfaces, interfaces, passivating ligands, and solvent layers. Often, such realistic features govern material properties and are hard to account for phenomenologically. NA-MD requires few approximations and assumptions. It does not need to assume that atomic motions are harmonic, that electrons are Drude oscillators, that coupling between different degrees of freedom is weak, that dynamics is Markovian or has short memory, or that evolution occurs by exponential kinetics of transitions between few states. The classical or semiclassical treatment of atomic motions constitutes the main approximation of NA-MD and is used because atoms are 3-5 orders of magnitude heavier than electrons. NA-MD is limited by system size, typically hundreds or thousands of atoms, and time scale, picoseconds to nanoseconds. The quality of NA-MD simulations depends on the electronic structure method used to obtain excited state energies and NA couplings.NA-MD has been largely popularized and advanced in the chemistry community that focuses on molecules. Modeling far-from-equilibrium dynamics in nanoscale and condensed matter systems often has to account for other types of physics. At the same time, condensed phase NA-MD allows for approximations that may not work in molecules. Focusing on the recent NA-MD developments aimed at studying excited state processes in nanoscale and condensed phases, this Account considers how the phenomena important on the nanoscale can be incorporated into NA-MD and what approximations can be made to increase its efficiency with complex systems and processes.

Donor-Acceptor Competition via Halide Vacancy Filling for Oxygen Detection of High Sensitivity and Stability by All-Inorganic Perovskite Films

Oxygen detection by organic-inorganic halide perovskites (OIHPs) has demonstrated advantages in operating temperature, response time, and reversibility over traditional materials. However, OIHPs can only sense O₂ in light and the unavoidable O₂ exposure during detection easily induces the degradation of OIHPs. The trade-off between sensitivity and stability makes the OIHP-based oxygen sensors impractical. By replacing organic groups with Cs, the compact films of all-inorganic halide perovskites (AIHPs) that can adsorb O₂ at grain boundaries in dark are developed. AIHPs show conductance increase of 1875.5% from 1 × 10^-5 to 700 Torr of O₂ pressure, associated with full reversibility and long-term stability. Combining experiments and modeling, this work reveals the donor-acceptor competition via halide vacancy filling leading to the modulation of carrier concentration and mobility. This work offers understandings on oxygen sensing by perovskite materials and paves the way for further optimization of AIHPs as promising oxygen sensors with high sensitivity and stability.

Spin-Orbit Coupling Is the Key to Promote Asynchronous Photoinduced Charge Transfer of Two-Dimensional Perovskites

Two-dimensional (2D) perovskites are emerging as promising candidates for diverse optoelectronic applications because of low cost and excellent stability. In this work, we explore the electronic structures and interfacial properties of (4Tm)₂PbI₄ with both the collinear and noncollinear DFT (PBE and HSE06) methods. The results evidently manifest that explicitly considering the spin-orbit coupling (SOC) effects is necessary to attain correct band alignment of (4Tm)₂PbI₄ that agrees with recent experiments (Nat. Chem.2019, 11, 1151; Nature2020, 580, 614). The subsequent time-domain noncollinear DFT-based nonadiabatic carrier dynamics simulations with the SOC effects reveal that the photoinduced electron and hole transfer processes are asymmetric and associated with different rates. The differences are mainly ascribed to considerably different nonadiabatic couplings in charge of the electron and hole transfer processes. Shortly, our current work sheds important light on the mechanism of the interfacial charge carrier transfer processes of (4Tm)₂PbI₄. The importance of the SOC effects on correctly aligning the band states of (4Tm)₂PbI₄ may be generalized to similar organic-inorganic hybrid 2D perovskites having heavy Pb atoms.

Thermal-Driven Dynamic Shape Change of Bimetallic Nanoparticles Extends Hot Electron Lifetime of Pt/MoS2 Catalysts

Using a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we demonstrate that the replacement of noble Pt with cheap Sn in the Pt nanoparticles sensitized MoS₂ greatly retards the photoexcited "hot" electron relaxation. The simulations show that Sn substitution causes significant geometry distortion associated with the Sn dopant detaching from the Pt nanoparticle base, which decreases the NA coupling and creates an isolated trap state distant from the electron donor state. Generally, smaller NA coupling delays "hot" electron relaxation. At the same time, the photoexcited electron on MoS₂ first populates the nanoparticles state and then slowly goes to the trap state, following relaxation to the nanoparticle acceptor state over 1 ps. As a result, the "hot" electron lives over 3.5 times longer than that in pristine Pt/MoS₂ system. The long-lived "hot" electron associated with the reduced cost establishes a novel concept for developing high-efficient and cost-effective photocatalysts and photovoltaics.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Water Splitting with a Single-Atom Cu/TiO2 Photocatalyst: Atomistic Origin of High Efficiency and Proposed Enhancement by Spin Selection

Anatase TiO2 is an intensely investigated photocatalytic material due to its abundance and chemical stability. However, it suffers from weak light harvesting and low photocatalytic efficiency. Experiments show that light absorption and photocatalytic properties can be enhanced simultaneously by TiO2 doping with well-dispersed Cu atoms, forming a single-atom catalyst (Cu/TiO2) that can be used for solar water splitting and other applications. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrate that Cu/TiO2 is inactive before light irradiation due to rapid electron-hole recombination via both shallow and deep traps. Surprisingly, the shallow trap is more detrimental to the Cu/TiO2 performance than the deep trap because it couples better to free carriers. After light irradiation, leading to electron transfer and Cu/TiO2 protonation, the shallow trap is eliminated, and a local distortion around the Cu atom stabilizes the deep trap state on the Cu d-orbital, decoupling it from free charges and giving rise to high photocatalytic hydrogen generation activity. We further demonstrate that the photocatalytic performance of Cu/TiO2 can be enhanced by spin selection, achievable experimentally via optical intersite spin transfer or chiral semiconductor coating. Both H adsorption and spin selection enhance charge carrier lifetimes by an order of magnitude. The spin selection mechanism does not require formation of the H species, which necessitates concurrent sources of electrons and protons and which is intrinsically unstable because water splitting involves frequent proton shuffling. Our results rationalize the experimental observations at the atomistic level, provide mechanistic insights into operation of single atom photocatalysis, and demonstrate that spin selection can be used to develop advanced and efficient systems for solar energy conversion.

Controlling Charge Carrier Trapping and Recombination in BiVO4 with the Oxygen Vacancy Oxidation State

The lack of an in-depth understanding of the intrinsic oxygen vacancy (O_V) defect properties in the photoanode BiVO₄ limits the further improvement of its photoelectrochemical water splitting performance. To address this issue, nonadiabatic molecular dynamics simulations are performed to study the impact of O_V on charge carrier lifetimes in BiVO₄. The simulations show that a neutral O_V gives rise to local structural distortions due to the formation of V-O-V bonds, forcing the electrons trapped on the nearer of the two V atoms to form two deep polaron-like V⁴⁺ hole traps. These localized midgap states greatly accelerate nonradiative electron-hole recombination compared to that of pristine BiVO₄, reaching a time scale of several nanoseconds in good agreement with experiments. The ionized O_V state restores the bandgap to its value in pristine BiVO₄ and restores the charge carrier lifetimes due to the fast loss of coherence time. Our study reveals the mechanism of the detrimental role of O_V in BiVO₄ and provides valuable insights for improving the performance of the BiVO₄ photoanode by ionizing the oxygen vacancy.

Rapid Charge Separation Boosts Solar Hydrogen Generation at the Graphene-MoS2 Junction: Time-Domain Ab Initio Analysis

Transition metal dichalcogenides and graphene hybrids hold great promise for photovoltaics and photocatalysts. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we investigate the interplay between forward and backward electron transfer (ET), as well as energy relaxation in a van der Waals graphene-MoS₂ heterojunction. We demonstrated that built-in potential formed at the polarized interface produces charge separation upon photoexcitation. The electron left on graphene is injected into MoS₂ on an ultrafast time scale, which is notably faster than energy losses to heat regardless of the initial state energy. Once the electron is relaxed to the conduction band edge state of MoS₂, it transfers back and recombines with the hole remaining on graphene on ultrafast time scales by considering quantum transitions among multiple k points. The obtained time scales for ET, back-ET, and energy relaxation agree well with experimental data. The study reveals that ET that is faster than energy loss makes the graphene-MoS₂ heterojunction efficient for optoelectronic applications.

Elucidating the Influence of Sulfur Vacancies on Nonradiative Recombination Dynamics in Cu2ZnSnS4 Solar Absorbers

We report a time-domain ab initio simulation of charge carrier trapping and relaxation dynamics in pristine and defect-containing kesterite Cu₂ZnSnS₄ (CZTS) structures. Our simulations show that introduction of a neutral sulfur vacancy in the CZTS system leads to a decrease of the charge recombination rate by a factor of ∼4, and the doubly positively charged sulfur vacancy results in a minor decrease of carrier lifetime, as compared to the pristine CZTS system. The neutral sulfur vacancy weakens the nonadiabatic (NA) electron-phonon coupling by moderately localizing charge density and accelerates the pure dephasing process, extending charge carrier lifetime. Therefore, the neutral sulfur vacancy is electrically benign. The doubly positively charged sulfur vacancy introduces a subgap state which is hardly populated, and recombination of the electron and hole bypassing the trap state dominates. As a result, the recombination rate decreases in the doubly charged sulfur vacancy structure. The reported results identified the key role of the sulfur-related vacancy on charge carrier trapping and relaxation of CZTS materials, carrying important implications for further optimization of CZTS and other thin-film solar cell materials.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective

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.

Intramolecular Band Alignment and Spin-Orbit Coupling in Two-Dimensional Halide Perovskites

In two-dimensional (2D) halide perovskites, four distinct types of intramolecular band alignment (I_a, I_b, II_a, and II_b) can be formed between the organic and inorganic components. Molecular design to achieve desirable band alignments is of crucial importance to the applications of 2D perovskites and their heterostructures. In this work, by means of first-principles calculations, we have developed molecular design strategies that lead to the discovery of 2D halide perovskites with favorable band alignments toward light-emitting and photovoltaic applications. The same design strategies can be extended to vertical and lateral heterostructures of 2D perovskites with selective light emissions from the organic and/or inorganic layer of constituent 2D perovskites. For each intramolecular band alignment, the charge density and binding energy of the lowest energy exciton are examined. The effect of spin-orbit coupling (SOC) on the band structures is assessed. While SOC significantly lowers the band gaps in type-I_a and type-II_a alignments, it has a negligible effect in type-I_b and type-II_b alignments.

MAI Termination Favors Efficient Hole Extraction and Slow Charge Recombination at the MAPbI3/CuSCN Heterojunction

Photoinduced charge separation is the key step determining the efficiency of photon-to-electron conversion in solar cells, while charge carrier lifetimes govern the overall solar cell performance. Experiments report that copper(I) thiocyanate (CuSCN) is a very promising hole extraction layer for perovskite solar cells. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we show that termination of CH₃NH₃PbI₃ (MAPbI₃) at MAPbI₃/CuSCN heterojunctions has a strong influence on both charge separation and recombination. Both processes are favored by MAI termination, compared to PbI₂ termination. Because the MAPbI₃ valence band originates from iodine orbitals while the conduction band arises from Pb orbitals, MAI termination places holes close to CuSCN, favoring extraction, and creates an MAI barrier for recombination of electrons in MAPbI₃ and holes in CuSCN. The opposite is true for PbI₂ termination. The origin of these effects is attributed solely to the properties of the MAPbI₃ surfaces, and therefore, the conclusions should apply to other hole-transporting materials and can be generalized to other perovskites. Importantly, the simulations show that the injected hole remains hot for several hundreds of femtoseconds, allowing it to escape the interfacial region and prevent formation of bound excitons. This study suggests that metal halide perovskites should be treated with an organic precursor, such as MAI, prior to the formation of their interfaces with hole-transporting materials. The reported results advance the fundamental understanding of the highly unusual properties of metal halide perovskites and provide specific guidelines for optimizing the performance of perovskite solar cells and other devices.

Simulating the Coupled Structural-Electronic Dynamics of Photoexcited Lead Iodide Perovskites

Motivated by the optoelectronic applications of lead halide perovskites (LHPs), researchers have paid considerable attention to their photoexcited-state dynamics, where the coupling between the electronic and nuclear dynamics is pronounced. Here, we present simulations of the photoexcited-state dynamics of representative lead iodide perovskites, CsPbI₃ and MAPbI₃ (MA = CH₃NH₃), by adopting nonadiabatic molecular dynamics combined with the linear-response time-dependent density-functional tight-binding (LR-TD-DFTB) method, an efficient excited-state calculation framework. In the calculations, the electronic wave function and the nuclear coordinates were propagated in a mutually dependent manner. The results suggest that the excited LHPs undergo exciton dissociation, hot carrier cooling, and polaron formation on similar time scales. In particular, the decay of the carrier energy is attributed to not only the relaxation toward the band edge but also the change in orbital energy originating from the structural deformation, highlighting the importance of coupling between the electronic and nuclear degrees of freedom.

Ferroelastic domains drive charge separation and suppress electron-hole recombination in all-inorganic halide perovskites: time-domain ab initio analysis

All-inorganic perovskites have great potential in photovoltaic applications and their performance is subject to phonon-assisted charge recombination dynamics. Local microstructures, such as ferroelastic domains, are considered to have a significant influence on the charge carrier lifetime in the CsPbBr₃ perovskite. Employing a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics simulations, we demonstrate that the formation of ferroelastic domains weakens the NA coupling and suppresses the non-radiative electron-hole recombination. This effect originates from the ferroelastic domains separating electron and hole wave functions spatially and decreasing the NA coupling by a factor of 2.4 compared to pristine CsPbBr₃, delaying electron-hole recombination by a factor of 4.2. We also show that symmetry breaking enhances electronic-vibrational interactions, activating more phonon modes and accelerating quantum decoherence by 1 fs or so, which further slows recombination. Both factors compete successfully with the slightly reduced bandgap of about 0.2 eV and prolong the charge carrier lifetime to several nanoseconds. Our study advances the understanding of the atomistic mechanism for inhibited recombination in the CsPbBr₃ perovskite in the presence of ferroelastic domains, providing an effective route to design high-performance all-inorganic halide perovskites.

Control of Charge Carrier Dynamics in Plasmonic Au Films by TiOx Substrate Stoichiometry

Plasmonic excitations in noble metals have many fascinating properties and give rise to a broad range of applications. We demonstrate, using nonadiabatic molecular dynamics combined with time-domain density functional theory, that the chemical composition and stoichiometry of substrates can have a strong influence on charge dynamics. By changing oxygen content in TiO₂, including stoichiometric, oxygen rich, and oxygen poor phases, and Ti metal, one can alter lifetimes of charge carriers in Au by a factor of 5 and control the ratio of electron-to-hole relaxation rates by a factor of 10. Remarkably, a thin TiO_x substrate greatly alters charge carrier properties in much thicker Au films. Such large variations stem from the fact that the Ti and O atoms are much lighter than Au, and their vibrations are much faster at dissipating the energy. The control over a particular charge carrier and an energy range depends on the Au and TiO_x level alignment, and the interfacial interaction strength. These factors are easily influenced by the TiO_x stoichiometry. In particular, oxygen rich and poor TiO₂ can be used to control holes and electrons, respectively, while metallic Ti affects both charge carriers. The detailed atomistic analysis of the interfacial and electron-vibrational interactions generates the fundamental understanding of the properties of plasmonic materials needed to design photovoltaic, photocatalytic, optoelectronic, sensing, nanomedical, and other devices.

Modeling nonadiabatic dynamics in condensed matter materials: some recent advances and applications

This review focuses on recent developments in the field of nonadiabatic molecular dynamics (NA-MD), with particular attention given to condensed-matter systems. NA-MD simulations for small molecular systems can be performed using high-level electronic structure (ES) calculations, methods accounting for the quantization of nuclear motion, and using fewer approximations in the dynamical methodology itself. Modeling condensed-matter systems imposes many limitations on various aspects of NA-MD computations, requiring approximations at various levels of theory-from the ES, to the ways in which the coupling of electrons and nuclei are accounted for. Nonetheless, the approximate treatment of NA-MD in condensed-phase materials has gained a spin lately in many applied studies. A number of advancements of the methodology and computational tools have been undertaken, including general-purpose methods, as well as those tailored to nanoscale and condensed matter systems. This review summarizes such methodological and software developments, puts them into the broader context of existing approaches, and highlights some of the challenges that remain to be solved.

Passivating Detrimental DX Centers in CH3 NH3 PbI3 for Reducing Nonradiative Recombination and Elongating Carrier Lifetime

After a period of rapid, unprecedented development, the growth in the efficiency of perovskite solar cells has recently slowed. Further improvement of cell efficiency will rely on the in-depth understanding and delicate control of defect passivation. Here, the formation mechanism of iodine vacancies (V_I ), a typical deep defect in CH₃ NH₃ PbI₃ (MAPbI₃ ), is elucidated. The structural and electronic behaviors of V_I are like those of a DX center, a kind of detrimental defect formed by large atomic displacement. Aided by the passivation mechanism of DX centers in tetrahedral semiconductors, it is found that the introduction of Br strengthens chemical bonds and prevents large atomic displacements during defect charging. It therefore reduces the defect states and diminishes electron-phonon coupling. Using time-domain density functional theory (DFT) combined with nonadiabatic molecular dynamics, it is found that the carrier lifetime can be enhanced from 3.2 ns in defective MAPbI₃ to 19 ns in CH₃ NH₃ Pb(I_0.96 Br_0.04 )₃ . This work advances our understanding of how a small amount of Br doping improves the carrier dynamics and cell performance of MAPbI₃ . It may also provide a route to enhance the carrier lifetimes and efficiencies of perovskite solar cells by defect passivation.

Synergy between Ion Migration and Charge Carrier Recombination in Metal-Halide Perovskites

Charge carrier recombination plays a vital role in the CH₃NH₃PbI₃ perovskite solar cell. By investigating a possible synergy between ion migration and charge carrier recombination, we demonstrate that the nonradiative recombination accelerates by an order of magnitude during iodide migration. The migration induces lattice distortion that brings electrons and holes close to each other and increases their electrostatic interactions. The wave function localization in the same spatial region, and the enhanced lattice and iodide movements increase the nonadiabatic coupling. At the same time, quantum coherence lasts longer, because electron and hole energy levels become correlated. All these factors greatly increase the recombination rate. Moreover, the energy level of the iodide vacancy created during the migration moves from inside the conduction band in the equilibrated structure into the band gap, acting as a typical efficient nonradiative charge recombination center. Our work shows that the different dynamic processes are strongly correlated in halide perovskites and demonstrates that defects, considered to be benign, can become very detrimental under non-equilibrium conditions. The reported results strongly suggest that ion migration should be avoided in halide perovskites, both for its own reasons, such as the large current-voltage hysteresis, and because it greatly accelerates charge carrier losses.

Covalent Functionalized Black Phosphorus Greatly Inhibits Nonradiative Charge Recombination: A Time Domain Ab Initio Study

Mono- or few-layer black phosphorus (BP) has emerged as a promising photovoltaic and optoelectronic material with realistic applications subjected to instability and short charge carrier lifetime. Experiments show that covalent functionalization can improve the stability, but the underlying mechanism for the prolonged lifetime remains elusive. By performing spin-polarized time domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we demonstrate that BP passivated with both phenyl and nitrophenyl can suppress the nonradiative electron-hole recombination by a factor of 2 and 3, respectively, relative to the pristine system. The slow recombination is due to the interplay between energy gap, NA coupling, and decoherence time, which happens either through a hole-trap-assisted process or in a direct way between a free electron and hole in the spin-up channel. The observations hold in the spin-down channel. The study suggests that the passivating strategy should work for BP and other two-dimensional materials.