Mitigating Surface Defects in Tin-Based Perovskite Films with α-Tocopherol for Enhanced Photovoltaic Performance.

Surface defects in tin-based perovskite films disrupt the periodic arrangement of atoms in crystals, making surface atoms more susceptible to interactions with water and oxygen molecules in the surrounding environment. The diffusion of oxygen ions into the perovskite interior leads to the formation of severe bulk defects, which compromises the performance of tin-based perovskite solar cells (PSCs). As a result, surface defects are recognized as the primary source of degradation and require special attention. In this study, α-Tocopherol (also known as vitamin E) into tin-based perovskite films is introduced. Experimental results show that because of its larger volume, α-Tocopherol does not enter the perovskite lattice. Instead, it forms van der Waals and hydrogen bond interactions with the formamidine ion (FA+) and the [SnI6]4- octahedron at the perovskite terminals. Through α-Tocopherol passivation, both surface and interior oxidation of the perovskite are significantly suppressed as α-Tocopherol firmly embeds itself on the perovskite surface. Density functional theory analysis confirms the inhibition of I─Sn antisite defects (ISn) and Sn interstitial defects (Sni), which possess deep trap states within the bandgap. Ultimately, it is demonstrated that α-Tocopherol enhances the power conversion efficiency (PCE) from 9.19% to 13.14% and prolongs the lifetime of tin-based PSCs to over 50 days.

Influence of Organic-Cation Defects on Optoelectronic Properties of ASnI3 Perovskites A=HC(NH2 )2 , CH3 NH3.

Tin organic-inorganic halide perovskites (tin OIHPs) possess a desirable band gap and their power conversion efficiency (PCE) has reached 14 %. A commonly held view is that the organic cations in tin OIHPs would have little impact on the optoelectronic properties. Herein, we show that the defective organic cations with randomly dynamic characteristics can have marked effect on optoelectronic properties of the tin OIHPs. Hydrogen vacancies originated from the proton dissociation from FA [HC(NH2 )2 ] in FASnI3 can induce deep transition levels in the band gap but yield relatively small nonradiative recombination coefficients of 10-15  cm3 s-1 , whereas those from MA (CH3 NH3 ) in MASnI3 can yield much larger nonradiative recombination coefficients of 10-11  cm3 s-1 . Additional insight into the "defect tolerance" is gained by disentangling the correlations between dynamic rotation of organic cations and charge-carrier dynamics.

Potassium doping-induced variations in the structures and photoelectric properties of a MAPbI3 perovskite and a MAPbI3/TiO2 junction.

It has been experimentally demonstrated that mixed metallic cation modification could be an effective strategy to enhance the performance and stability of perovskite-based solar cells (PSCs). However, there is limited microscopic understanding at the atomic/molecular level of the behavior of small radius alkali metal cation doping in both perovskite materials and perovskite/TiO2 junctions. Here, we perform a first-principles density functional theory study on the doping-induced variation of the geometric and electronic structures of MAPbI3 (MA = methylammonium) and the MAPbI3/TiO2 junction. The impacts of different doping methods, and different charge states and locations of the given dopants have been investigated. At first, we theoretically confirm that the structures doped by K+ are the most thermally stable compared to the structures doped by the other charge states of K, and that K+ dopants prefer to modify the perovskite lattice interstitially and stay near the MAPbI3/TiO2 interface. Meanwhile, we find that a severe geometric deformation occurs if two doped lattices come into contact directly, indicating that the lattice may rapidly collapse from the interior if the doping concentration is too high. Additionally, we observe that K+ doped interstitially near the MAPbI3/TiO2 interface causes the intensive distortion of the surface Ti-O bonds and severe bond-length fluctuations. Consequently, this results in distorted TiO2 bands of the interfacial layer and a slight decrease of the band offset of conduction bands between two phases. This work complements experiments and provides a better microscopic understanding of the doping modification of the properties of perovskite materials and PSCs.

Zero-Dimensional Organic-Inorganic Perovskite Variant: Transition between Molecular and Solid Crystal.

Low-dimensional organic-inorganic halide perovskites (OIHPs) have attracted intense interest recently for photovoltaic applications, due to their markedly high chemical stability as compared to the widely studied three-dimensional (3D) counterparts. However, low-dimensional OIHPs usually give much lower device performance than the 3D OIHPs. In particular, for the zero-dimensional (0D) OIHPs, it is believed that the strong intrinsic quantum-confinement effects can lead to extremely low carrier motility, which can severely limit the photovoltaic performance. Herein, we predict a new family of 0D perovskite variants that, surprisingly, exhibit outstanding optoelectronic properties. We show that the "atypical" carrier mobility of these new 0D perovskites is attributed to the strong electronic interaction between neighboring octahedrons in the crystal. These findings also suggest a new materials design strategy for resolving the low-performance issue commonly associated with the low-dimensional OIHPs for photovoltaic applications.

Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors.

Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here we show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS2-WS2-MoSe2, electrons excited in MoSe2 transfer to MoS2 in about one picosecond. Surprisingly, these electrons do not populate the middle WS2 layer during this process. Calculations showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS2 to MoSe2 is also found to be efficient and ultrafast. The separation of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics.

Lead-Free Mixed Tin and Germanium Perovskites for Photovoltaic Application.

The power-conversion efficiency (PCE) of lead halide perovskite photovoltaics has reached 22.1% with significantly improved structural stability, thanks to a mixed cation and anion strategy. However, the mixing element strategy has not been widely seen in the design of lead-free perovskites for photovoltaic application. Herein, we report a comprehensive study of a series of lead-free and mixed tin and germanium halide perovskite materials. Most importantly, we predict that RbSn0.5Ge0.5I3 possesses not only a direct bandgap within the optimal range of 0.9-1.6 eV but also a desirable optical absorption spectrum that is comparable to those of the state-of-the-art methylammonium lead iodide perovskites, favorable effective masses for high carrier mobility, as well as a greater resistance to water penetration than the prototypical inorganic-organic lead-containing halide perovskite. If confirmed in the laboratory, this new lead-free inorganic perovskite may offer great promise as an alternative, highly efficient solar absorber material for photovoltaic application.

In- and Ga-based inorganic double perovskites with direct bandgaps for photovoltaic applications.

Double perovskites in the form of A2B'B''X6 (A = Cs, B' = Ag, B'' = Bi) have been reported as potential alternatives to lead-containing organometal trihalide perovskites. However, all double perovskites synthesized to date exhibit indirect bandgaps >1.95 eV, which are undesirable for photovoltaic and optoelectronic applications. Herein, we report a comprehensive computer-aided screening of In- and Ga-based double perovskites for potential photovoltaic applications. To this end, several preconditions are implemented for the screening of optimal candidates, which include structural stability, electronic bandgaps, and optical absorption. Importantly, four In- and Ga-based double perovskites are identified to possess direct bandgaps within the desirable range of 0.9-1.6 eV for photovoltaic applications. Dominant optical absorption of the four double perovskites is found to be in the UV range. The structural and thermal stability of the four double perovskites are examined using both the empirical Goldschmidt ratio and convex-hull calculations. Only Cs2AgInBr6 is predicted to be thermodynamically stable.

Lateral and flexural phonon thermal transport in graphene and stanene bilayers.

Stanene, a low thermal conductivity two-dimensional (2D) sheet composed of group-IV element Sn, is a prototype material with novel properties such as 2D topological insulating behavior and near-room-temperature quantum Hall effects. Monolayer graphene, on the other hand, possesses unusual thermal properties, but has a zero bandgap. By stacking stanene and graphene monolayers vertically into a hetero-bilayer, an indirect bandgap can be obtained, making the hetero-bilayer a good candidate for special applications. In this work, the in-plane thermal conductivity (κ) and out-of-plane interfacial thermal resistance (R) in the hetero-bilayer are systematically investigated using non-equilibrium molecular dynamics and transient pump-probe methods. Effects of dimension, system temperature and van der Waals coupling strength on the thermal properties are examined. The predicted in-plane thermal conductivity of the graphene/stanene hetero-bilayer is 311.1 W m-1 K-1, higher than most 2D materials such as phosphorene, hexagonal boron nitride (h-BN), MoS2 and MoSe2. Phonon power spectra are recorded for graphene and stanene individually to help the explanation of their κ difference. The inter-layer thermal resistance between graphene and stanene hetero-bilayers is predicted to be 2.13 × 10-7 K m2 W-1, which is on the same order of magnitude as several other 2D bilayer structures.

Homogenous Alloys of Formamidinium Lead Triiodide and Cesium Tin Triiodide for Efficient Ideal-Bandgap Perovskite Solar Cells.

The alloying behavior between FAPbI3 and CsSnI3 perovskites is studied carefully for the first time, which has led to the realization of single-phase hybrid perovskites of (FAPbI3 )1-x (CsSnI3 )x (0

The effect of moisture on the structures and properties of lead halide perovskites: a first-principles theoretical investigation.

With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb-I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture.

Thin-Film Transformation of NH4 PbI3 to CH3 NH3 PbI3 Perovskite: A Methylamine-Induced Conversion-Healing Process.

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

A computational view of the change in the geometric and electronic properties of perovskites caused by the partial substitution of Pb by Sn.

Recently, solar cells with hybrid organic-inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches about 17.9%. For practical applications, one has to avoid the toxicology issue of lead, to develop lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step protocol based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1-xI3 with different crystal symmetries. At first, a GGA functional with the inclusion of the van der Waals interaction, vdW-DF3, is used to optimize the geometries and it reproduces closely the unit cell volume. Then, a more accurate hybrid functional PBE0 combined with the spin-orbit coupling effect is used to perform electronic-structure calculations. The calculated results reveal that the band gaps of MAPbxSn1-xI3 are sensitive to the ratio of Sn/Pb, and are proportional to the x component, consistent with the previous reports. Further investigations show that the crystal symmetry can also modify the band gap in an order of Pnma > I4cm > P4mm at x = 0.5. The random rotation of organic cations, which makes the band alignments in the compounds, facilitates the separation and transfer of holes and electrons. Interestingly, the computed binding energies of the unrelaxed exciton have the same trend as band gaps, which decreases with decreasing x, the binding energies of MAPb0.5Sn0.5I3 also decrease as the crystal symmetry decreases, implying a faster exciton dissociation with lower x and lower symmetry at an ambient temperature.

Mechanisms of large Stokes shift and aggregation-enhanced emission of osmapentalyne cations in solution: combined MD simulations and QM/MM calculations.

Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations. The results demonstrate that the large Stokes shift in the gas phase comes from a photoexcitation-induced deformation of the osmium plane, whereas in solution it corresponds to the variation of osmium ring symmetry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 join the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution functions of their mass-central distances display several peaks immersed in a broad envelope due to different aggregation pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation structures do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.

Machine Learning Accelerates Precise Excited-State Potential Energy Surface Calculations on a Quantum Computer.

Electronically excited-state problems represent a crucial research field in quantum chemistry, closely related to numerous practical applications in photophysics and photochemistry. The emerging of quantum computing provides a promising computational paradigm to solve the Schrödinger equation for predicting potential energy surfaces (PESs). Here, we present a deep neural network model to predict parameters of the quantum circuits within the framework of variational quantum deflation and subspace search variational quantum eigensolver, which are two popular excited-state algorithms to implement on a quantum computer. The new machine learning-assisted algorithm is employed to study the excited-state PESs of small molecules, achieving highly accurate predictions. We then apply this algorithm to study the excited-state properties of the ArF system, which is essential to a gas laser. Through this study, we believe that with future advancements in hardware capabilities, quantum computing could be harnessed to solve excited-state problems for a broad range of systems.

Coupled Dynamic Characteristics of Mobile Ions in Halide Perovskites.

Mixed perovskites show immense promise for diverse applications owing to their exceptional compositional flexibility and outstanding optoelectronic performance. Nevertheless, a significant hurdle in their widespread use is their susceptibility to compositional instability. Some mixed perovskites exhibit a tendency to segregate into regions with varying halide content, negatively impacting the material's electronic properties and impeding its overall advancement. This study focuses on investigating the lattice and A-site cation dynamics in mixed-halide perovskites as well as the relationship between the stability and dynamic properties of mixed-halide perovskites. Our findings reveal an intrinsic link between the kinetics of organic molecules and halogen ion migration. The stability of halide ions is linearly positively correlated with the radius, number of H atoms, and moment of inertia of the organic molecules. Organic molecules with lower rotational kinetics effectively suppress the overall cationic kinetic activity, enhancing lattice dynamic stability in mixed perovskite systems. This inhibition further impedes the migration of halogen ions and hinders the halide segregation process. The presence of dominant I/MA vacancies in perovskites accelerates the rotation of MA and the migration of halogen ions. The coupled dynamic behavior of varying vacancy concentrations in A-site cations/X-site anions within the inorganic framework significantly impacts the photovoltaic performance of these halide perovskites.

Intragrain impurity annihilation for highly efficient and stable perovskite solar cells.

Intragrain impurities can impart detrimental effects on the efficiency and stability of perovskite solar cells, but they are indiscernible to conventional characterizations and thus remain unexplored. Using in situ scanning transmission electron microscopy, we reveal that intragrain impurity nano-clusters inherited from either the solution synthesis or post-synthesis storage can revert to perovskites upon irradiation stimuli, leading to the counterintuitive amendment of crystalline grains. In conjunction with computational modelling, we atomically resolve crystallographic transformation modes for the annihilation of intragrain impurity nano-clusters and probe their impacts on optoelectronic properties. Such critical fundamental findings are translated for the device advancement. Adopting a scanning laser stimulus proven to heal intragrain impurity nano-clusters, we simultaneously boost the efficiency and stability of formamidinium-cesium perovskite solar cells, by virtual of improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, inspired and guided by atomic-scale in situ microscopic imaging, presents a new prototype for solar cell advancement.

Universal machine learning aided synthesis approach of two-dimensional perovskites in a typical laboratory.

The past decade has witnessed the significant efforts in novel material discovery in the use of data-driven techniques, in particular, machine learning (ML). However, since it needs to consider the precursors, experimental conditions, and availability of reactants, material synthesis is generally much more complex than property and structure prediction, and very few computational predictions are experimentally realized. To solve these challenges, a universal framework that integrates high-throughput experiments, a priori knowledge of chemistry, and ML techniques such as subgroup discovery and support vector machine is proposed to guide the experimental synthesis of materials, which is capable of disclosing structure-property relationship hidden in high-throughput experiments and rapidly screening out materials with high synthesis feasibility from vast chemical space. Through application of our approach to challenging and consequential synthesis problem of 2D silver/bismuth organic-inorganic hybrid perovskites, we have increased the success rate of the synthesis feasibility by a factor of four relative to traditional approaches. This study provides a practical route for solving multidimensional chemical acceleration problems with small dataset from typical laboratory with limited experimental resources available.

Disorder on Mixed Cation Halide Perovskite for Photovoltaic Applications.

With reduced toxicity and tunable optoelectronic properties, mixed cation halide perovskites (MCHPs) featuring partially substituted Pb with Sn and Ge have emerged as promising candidates for photovoltaic applications. However, the introduction of the disorder through large-scale preparation and alloying strategies leads to a significant challenge in comprehending the disorder's microscopic-level impact. Here, we found that, in addition to compositional variation, a synergy of disorder and cation radii ratio significantly affects optoelectronic properties. For Pb-Ge/Ge-Sn MCHPs, severe octahedral distortion with increasing degree of disorder adjusted their bandgaps in a wide range, giving rise to large effective masses, exciton binding energies, and weak visible absorption coefficients. The synergy of disorder and distortion transforms the Wannier excitons into localized characteristics, whereas the optoelectronic properties of Pb-Sn MCHPs are modulated by the disorder. Our work highlights the role of disorder in the tunability of optoelectronic properties, providing a novel strategy for designing photovoltaic materials.

Eco-friendly inorganic molecular novel antiperovskites for light-emitting application.

The development of perovskite light-emitting diodes (PeLEDs) has progressed rapidly over the past several years, with high external quantum efficiencies exceeding 20%. However, the deployment of PeLEDs in commercial devices still faces severe challenges, such as environmental pollution, instability and low photoluminescence quantum yields (PLQYs). In this work, we perform high-throughput calculations to exhaustively search the unexplored and eco-friendly novel antiperovskite space (formula: X3B[MN4], with octahedron [BX6] and tetrahedron [MN4]). The novel antiperovskites have a unique structure whereby a tetrahedron can be embedded into an octahedral skeleton as a light-emitting center causing a space confinement effect, leading to the characteristics of a low-dimensional electronic structure, which then makes these materials potential light-emitting material candidates with a high PLQY and light-emitting stability. Under the guidance of newly derived tolerance, octahedral, and tetrahedral factors, 266 stable candidates are successfully screened out from 6320 compounds. Moreover, the antiperovskite materials Ba3I0.5F0.5(SbS4), Ca3O(SnO4), Ba3F0.5I0.5(InSe4), Ba3O0.5S0.5(ZrS4), Ca3O(TiO4), and Rb3Cl0.5I0.5(ZnI4) possess an appropriate bandgap, thermodynamic and kinetic stability, and excellent electronic and optical properties, making them promising light-emitting materials.

Flattening Grain-Boundary Grooves for Perovskite Solar Cells with High Optomechanical Reliability.

Optomechanical reliability has emerged as an important criterion for evaluating the performance and commercialization potential of perovskite solar cells (PSCs) due to the mechanical-property mismatch of metal halide perovskites with other device layer. In this work, grain-boundary grooves, a rarely discussed film microstructural characteristic, are found to impart significant effects on the optomechanical reliability of perovskite-substrate heterointerfaces and thus PSC performance. By pre-burying iso-butylammonium chloride additive in the electron-transport layer (ETL), GB grooves (GBGs) are flattened and an optomechanically reliable perovskite heterointerface that resists photothermal fatigue is created. The improved mechanical integrity of the ETL-perovskite heterointerfaces also benefits the charge transport and chemical stability by facilitating carrier injection and reducing moisture or solvent trapping, respectively. Accordingly, high-performance PSCs which exhibit efficiency retentions of 94.8% under 440 h damp heat test (85% RH and 85 °C), and 93.0% under 2000 h continuous light soaking are achieved.