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Charge transfer in type-II heterostructures plays important roles in determining device performance for photovoltaic and photocatalytic applications. However, current theoretical studies of charge transfer process don't consider the effects of operating conditions such as illuminations and yield systemically larger interlayer transfer time of hot electrons in MoS2/WS2 compared to experimental results. Here in this work, we propose a general picture that, illumination can induce interfacial dipoles in type-II heterostructures, which can accelerate hot carrier transfer by reducing the energy difference between the electronic states in separate materials and enhancing the nonadiabatic couplings. Using the first-principles calculations and the ab-initio nonadiabatic molecular dynamics, we demonstrate this picture using MoS2/WS2 as a prototype. The calculated characteristic time for the interlayer transfer (60 fs) and the overall relaxation (700 fs) processes of hot electrons is in good agreement with the experiments. We further find that illumination mainly affects the ultrafast interlayer transfer process but has little effects on the relatively slow intralayer relaxation process. Therefore, the overall relaxation process of hot electrons has a saturated time with increased illumination strengths. The illumination-accelerated charge transfer is expected to universally exist in type-II heterostructures.

Illumination has been long known to affect semiconductor defect properties during either growth or operating process. Current theories of studying the illumination effects on defects usually have the assumption of unaffected formation energies of neutral defects as well as defect transition energy levels, and use the quasi-Fermi levels to describe behaviors of excess carriers with conclusions at variance. In this work, we first propose a method to simulate steady illumination conditions, based on which we demonstrate that formation energies of neutral defects and defect transition energy levels are insensitive to illumination. Then, we show that optical and thermal excitation of electrons can be seen equivalent with each other to reach a steady electron distribution in a homogeneous semiconductor. Consequently, the electron distribution can be characterized using just one effective temperature T' and one universal Fermi level E_F' for a homogeneous semiconductor under continuous and steady illuminations, which can be seen as a combination of quasi-equilibrium electron system with T' and a lattice system with T. Using the new concepts, we uncover the universal mechanisms of illumination effects on charged defects by treating the band edge states explicitly in the same footing as the defect states. We find that the formation energies of band edge 'defect' states shift with increased T' of electrons, thus affecting the E_F', changing defect ionic probabilities, and affecting concentrations of charged defects. We apply our theory to study the illumination effects on the doping behaviors in GaN:Mg and CdTe:Sb, obtaining results in accordance with experimental observations. More interesting experimental defect-related phenomena under steady illuminations are expected to be understood from our theory.

Li$_x$CoO$_2$ based batteries have serious capacity degradation and safety issues when cycling at high-delithiation states but full and consistent mechanisms are still poorly understood. Herein, a global neural network potential (GNNP) is developed to provide direct theoretical understandings by performing long-time and large-size atomic simulations. We propose a self-consistent picture as follows: (i) CoO$_2$ layers are easier to glide with longer distances at more highly delithiated states, resulting in structural transitions and structural inhomogeneity; (ii) at regions between different phases with different Li distributions due to gliding, local strains are induced and accumulate during cycling processes; (3) accumulated strains cause the rupture of Li diffusion channels and result in formation of oxygen dimers during cycling especially when Li has inhomogeneous distributions, leading to capacity degradations and safety issues. We find that large tensile strains combined with inhomogeneous distributions of Li ions play critical roles in the formation processes of blocked Li diffusion channels and the oxygen dimers at high-delithiation states, which could be the fundamental origins of capacity degradations and safety issues. Correspondingly, suppressing accumulations of strains by controlling charge and discharge conditions as well as suppressing the gliding will be helpful for improving the performance of lithium-ion batteries (LIBs)., Comment: 17 pages, 5 figures, 1 table

Despite the successes of machine learning methods in physical sciences, prediction of the Hamiltonian, and thus electronic properties, is still unsatisfactory. Here, based on graph neural network architecture, we present an extendable neural network model to determine the Hamiltonian from ab initio data, with only local atomic structures as inputs. Rotational equivariance of the Hamiltonian is achieved by our complete local coordinates. The local coordinates information, encoded using the convolutional neural network and designed to preserve Hermitian symmetry, is used to map hopping parameters onto local structures. We demonstrate the performance of our model using graphene and SiGe random alloys as examples. We show that our neural network model, although trained using small-size systems, can predict the Hamiltonian, as well as electronic properties such as band structures and densities of states (DOS) for large-size systems within the ab initio accuracy, justifying its extensibility. In combination with the high efficiency of our model, which takes only seconds to get the Hamiltonian of a 1728-atom system, present work provides a general framework to predict electronic properties efficiently and accurately, which provides new insights into computational physics and will accelerate the research for large-scale materials., Comment: 21 pages, 7 figures

Two-dimensional materials with tunable in-plane anisotropic infrared response promise versatile applications in polarized photodetectors and field-effect transistors. Black phosphorus is a prominent example. However, it suffers from poor ambient stability. Here, we report the strain-tunable anisotropic infrared response of a layered material Nb2SiTe4, whose lattice structure is similar to the 2H-phase transition metal dichalcogenides (TMDCs) with three different kinds of building units. Strikingly, some of the strain-tunable optical transitions are crystallographic axis-dependent, even showing opposite shift when uniaxial strain is applied along two in-plane principal axes. Moreover, G0W0-BSE calculations show good agreement with the anisotropic extinction spectra. The optical selection rules are obtained via group theory analysis, and the strain induced unusual shift trends are well explained by the orbital coupling analysis. Our comprehensive study suggests that Nb2SiTe4 is a good candidate for tunable polarization-sensitive optoelectronic devices.

Ideal two-dimensional (2D) semiconductors with high mobility comparable to three-dimensional (3D) Si or GaAs are still lacking, hindering the development of high-performance 2D devices. Here in this work, using first-principles calculations and considering all the electron-phonon couplings, we show that monolayer BX (X = P, As, Sb) with honeycomb lattices have intrinsic phonon-limited carrier mobility reaching record-high values of 1200-14000 $cm^2V^{-1}s^{-1}$ at room temperature. Despite being polar and the band edges located at the K point with multiple valleys, these three systems unusually have small carrier scattering rates. Detailed analysis shows that, both the intravalley scattering and the intervalley scattering between two equivalent K points are weak, which can be understood from the large mismatch between the electron bands and phonon spectrum and suppressed electron-phonon coupling strength. Furthermore, we reveal the general trend of mobility increase from BP to BAs and to BSb and conclude that: smaller effective masses, larger sound velocities, higher optical phonon energies, heavy atomic masses, and out-of-plane orbitals tend to result in small match between the electron and phonon bands, small electron-phonon coupling strengths, and thus high mobility. Our work demonstrates that 2D semiconductors can achieve comparable carrier mobility to 3D GaAs, thus opening doors to 2D high-performance electronic devices., Comment: 25 pages, 8 figures

It is known that carrier mobility in layered semiconductors generally increases from two-dimension (2D) to three-dimension due to suppressed scattering channels resulting from decreased densities of electron and phonon states. In this work, we find an abnormal decrease of electron mobility from monolayer to bulk MoS2. By carefully analyzing the scattering mechanisms, we can attribute such abnormality to the stronger intravalley scattering in the monolayer but weaker intervalley scattering caused by less intervalley scattering channels and weaker corresponding electron-phonon couplings compared to the bulk case. We show that, it is the matching between electronic band structure and phonon spectrum rather than their densities of electronic and phonon states that determines scattering channels. We propose, for the first time, the phonon-energy-resolved matching function to identify the intra- and inter-valley scattering channels. Furthermore, we show that multiple valleys do not necessarily lead to strong intervalley scattering if: (1) the scattering channels, which can be explicitly captured by the distribution of the matching function, are few due to the small matching between the corresponding electron and phonon bands; and/or (2) the multiple valleys are far apart in the reciprocal space and composed of out-of-plane orbitals so that the corresponding electron-phonon coupling strengths are weak. Consequently, the searching scope of high-mobility 2D materials can be reasonably enlarged using the matching function as useful guidance with the help of band edge orbital analysis., Comment: 20 pages, 10 figures

Determination of defect ionization energy in low-dimensional semiconductors has been a long-standing unsolved problem in first-principles defect calculations because the commonly used methods based on jellium model introduce an unphysical charge density uniformly distributed in the material and vacuum regions, causing the well-known divergence issue of charged defect formation energies. Here in this work, by considering the physical process of defect ionization, we propose a charge correction method based on jellium model to replace the unphysical jellium background charge density with the band edge charge density to deal with charged defects. We demonstrate that, our method is physically meaningful, quantitatively accurate and technically simple to determine the defect ionization energies, thus solving the long-standing problem in defect calculations. Our proposed method can be applied to any dimensional semiconductors.

In spite of great appeal of two-dimensional (2D) semiconductors for electronics and optoelectronics, to achieve required charge carrier concentrations by means of chemical doping remains a challenge, due to large defect ionization energies (IEs). Here by decomposing the defect IEs into the neutral single-electron defect level, the structural relaxation energy gain, and the electronic relaxation energy cost, we propose a conceptual picture that the large defect IEs are caused by two effects of reduced dimensionality. While the quantum confinement effect (QCE) makes the neutral single-electron point defect levels deep, the reduced screening effect (RE) leads to high energy cost for the electronic relaxation. The first-principles calculations for monolayer, few-layer, and bulk black phosphorus (BP), MoS2, and ReS2 with strong, medium, and weak interlayer interactions, respectively, as examples, do demonstrate the general trend. Based on the gained insight into defect behaviors, strategies can be envisaged for reducing defect IEs and improving charge carrier doping. Using BP monolayer either embedded into dielectric continuum or encapsulated between two h-BN layers, as practical examples, we demonstrate the feasibility of increasing the screening to reduce the defect IEs and boost carrier concentration. Our analysis is expected to help achieving effective carrier doping and thus to open ways towards more extensive applications of 2D semiconductors., Comment: 29 pages, 7 figures

Common substitutional isovalent semiconductor alloys usually form disordered metastable phases with positive excess formation enthalpies ({\Delta}H). In contrast, monolayer alloys of transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se) always have negative {\Delta}H, suggesting atomic ordering, which is, however, not yet experimentally observed. Using first-principles calculations, we find that the negative {\Delta}H of cation-mixed TMD alloys results from the charge transfer from weak Mo-X to nearest strong W-X bonds and the negative {\Delta}H of anion-mixed TMD alloys comes from the larger energy gain due to the charge transfer from Se to nearest S atoms than the energy cost due to the lattice mismatch. Consequently, cation-mixed and anion-mixed alloys should energetically prefer to have Mo-X-W and S-M-Se ordering, respectively. The atomic ordering, however, is only locally ordered but disordered in the long range due to the symmetry of TMD monolayers, as demonstrated by many energetically degenerate structures for given alloy compositions. Besides, the local ordering and disordering effects on the macroscopic properties such as bandgaps and optical absorptions are negligible, making the experimental observation of locally ordered TMD alloys challenging. We propose to take the advantage of microscopic properties such as defects which strongly depend on local atomic configurations for experiments to identify the disordering and local ordering in TMD alloys. Finally, quaternary TMD alloys by mixing both cations and anions are studied to have a wide range of bandgaps for optoelectronic applications. Our work is expected to help the formation and utilization of TMD alloys., Comment: 25 pages, 6 figures

Hybrid organic-inorganic halide perovskites with the prototype material of CH$_{3}$NH$_{3}$PbI$_{3}$ have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 20% achieved by their solar cells, two key issues -- the poor device stabilities associated with their intrinsic material instability and the toxicity due to water soluble Pb$^{2+}$ -- need to be resolved before large-scale commercialization. Here, we address these issues by exploiting the strategy of cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells. The idea is to convert two divalent Pb$^{2+}$ ions into one monovalent M$^{+}$ and one trivalent M$^{3+}$ ions, forming a rich class of quaternary halides in double-perovskite structure. We find through first-principles calculations this class of materials have good phase stability against decomposition and wide-range tunable optoelectronic properties. With photovoltaic-functionality-directed materials screening, we identify eleven optimal materials with intrinsic thermodynamic stability, suitable band gaps, small carrier effective masses, and low excitons binding energies as promising candidates to replace Pb-based photovoltaic absorbers in perovskite solar cells. The chemical trends of phase stabilities and electronic properties are also established for this class of materials, offering useful guidance for the development of perovskite solar cells fabricated with them., Comment: pages 19, 4 figures in main text

Defects play important roles in semiconductors for optoelectronic applications. Common intuition is that defects with shallow levels act as carrier providers and defects with deep levels are carrier killers. Here, taking the Cu defects in CdTe as an example, we show that shallow defects can play both roles. Using first-principles calculation methods combined with thermodynamic simulations, we study the dialectic effects of Cu-related defects on hole density and lifetime in bulk CdTe over a wide range of Cu incorporation conditions. Because CuCd can form a relatively shallow acceptor in CdTe, we find that increased Cu incorporation into CdTe indeed can help achieve high hole density; however, too much Cu can cause significant non-radiative recombination. We discuss two strategies to balance the contradictory effects of Cu defects based on the calculated impact of Cd chemical potential, copper defect concentrations, and incorporation temperature on lifetime and hole density. The results indicates that to optimize the Cu doping in CdTe, it is important to control the total amount of Cu incorporated into CdTe and optimize the match between the Cu incorporation temperature and the Cd chemical potential. These findings can help understand the roles of Cu defects in CdTe and the potential complex defect behaviors of relatively shallow defect states in semiconductors for optoelectronic applications.

Current thermoelectric (TE) materials often have low performance or contain less abundant and/or toxic elements, thus limiting their large-scale applications. Therefore, new TE materials with high efficiency and low cost are strongly desirable. Here we demonstrate that, SiS and SiSe monolayers made from non-toxic and earth-abundant elements intrinsically have low thermal conductivities arising from their low-frequency optical phonon branches with large overlaps with acoustic phonon modes, which is similar to the state-of-the-art experimentally demonstrated material SnSe with a layered structure. Together with high thermal power factors due to their two-dimensional nature, they show promising TE performances with large figure of merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. We establish some basic understanding of identifying layered materials with low thermal conductivities, which can guide and stimulate the search and study of other layered materials for TE applications., Comment: 18 pages, 3 figures

The electronic properties of ABX3 (A = Cs, CH3NH3, NH2CHNH2; B = Sn, Pb; X = Cl, Br, I) type compounds in the cubic phase are systematically studied using the first-principles calculations. We find that these compounds have direct band gaps at R point where the valance band maximum is an anti-bonding state of B s-X p coupling, while the conduction band minimum is a non-bonding state with B p characters. The chemical trend of their properties as A or B or X varies is fully investigated, which is of great importance to understand and optimize this kind of solar cell materials. We find that: (i) as the size of A increases, the band gap of ABX3 will increase; (ii) as B varies from Sn to Pb, the band gap of ABX3 will increase; and (iii) as X ranges from Cl to Br to I, the band gap will decrease. We explained these trends by analyzing their band structures. Furthermore, optical properties of the ABX3 compounds are investigated. Our calculations show that taking into account the spin-orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH3NH3SnBr3 is a promising material for solar cells absorber with a perfect band gap and good optical absorption.

We have developed a new global optimization method for the determination of interface structure based on the differential evolution algorithm. Here, we applied this method to search for the ground state atomic structures of the grain boundary between the armchair and zigzag oriented graphene. We find two new grain boundary structures with considerably lower formation energy of about 1 eV/nm than that of the previously widely used structural models. These newly predicted structures show better mechanical property under external uniaxial strain, and distinguishable scanning tunneling microscope features, compared with the previous structural models. Our results provide important new information for the determination of grain boundary structures and henceforth the electronic properties of defected graphene.

The formation of (TiO2)x(Cu2O)y solid-solutions are investigated using a global optimization evolutionary algorithm. First-principles calculations based on density functional theory are then used to gain insight into the electronic properties of these alloys. We find that: (i) Ti and Cu in (TiO2)x(Cu2O)y alloys have similar local environments as in bulk TiO2 and Cu2O except for (TiO2)(Cu2O) which has some trigonal-planar Cu ions. (ii) The predicted optical band gaps are around 2.1 eV (590 nm), thus having much better performance for the absorption of visible light compared with both binary oxides. (iii) (TiO2)2(Cu2O) has the lowest formation energy amongst all studied alloys and the positions of its band edges are found to be suitable for solar-driven water splitting applications., Comment: 9 pages, 6 figures

ASnO3 (A=Ba, Sr) are unique perovskite oxides in that they have superior electron conductivity despite their wide optical band gaps. Using first-principles band structure calculations, we show that the small electron effective masses, thus, good electron conductivity of ASnO3 can be attributed to the large size of Sn in this system that gives the conduction band edge with antibonding Sn and O s characters. Moreover, we show that ASnO3 can be easily doped by La with shallow LaA(+/0) donor level. Our results, therefore, explained why the perovskite BaSnO3, SrSnO3, and their alloys are promising candidates for transparent conducting oxides., Comment: 12 pages, 4 figures, 2 tables

First-principles calculations are performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc. 2011, 133, 16212.] We find that the ordered CC-Si3AlP with a basic unit of one P atom surrounded by three Si atoms and one Al atom is the most stable one within the experimentally observed unit cell.1 Si3AlP has a larger fundamental band gap and a smaller direct band gap than Si, thus it has much higher absorption in the visible light region. The calculated properties of Si3AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys.