Polaron formation and hopping in tantalate perovskite oxides: NaTaO3 and KTaO3

Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in NaTaO3 and KTaO3. We perform Perdew-Burke-Ernzerhof hybrid density functional PBE0(alpha) calculations, in which we define the fraction alpha of the Fock exchange by enforcing the Koopmans\u27 condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta-O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature similar to 2.94 x 10-6 cm2/Vs and similar to 1.87 x 10-4 cm2/Vs for KTaO3 and NaTaO3, respectively

Ab Initio Insights into Charge Localization in Bismuth Oxyhalides BiOX (X = F, Cl, Br, I)

Developing efficient photocatalysts for clean energy generation is crucial to achieving net-zero emissions. To this end, we investigate the behavior of photoexcited charges in bismuth oxyhalides BiOX (X = F, Cl, Br, and I), a family of inexpensive and promising photocatalysts. To model the localization of excess electrons and holes, we use hybrid density functional theory PBE0(α). Our results indicate that electron polarons are unstable in these materials. Concurrently, we find that hole localization is favorable, and we identify two different possible configurations in which polarons are formed. One consists of two dimerized halogen atoms (VK center) and is preferentially formed in BiOBr and BiOI with binding energies that amount to-0.26 and-0.21 eV, respectively. The other corresponds to localization on a single Bi site and the surrounding oxygen and halogen atoms (BiXO). This form of polaron is favorable in BiOF and BiOCl with binding energies that amount to-0.35 and-0.23 eV, respectively. These findings highlight the behavior of photogenerated carriers and may open up avenues for future investigations on carrier transport in bismuth oxyhalides

Two-dimensional MoTe2/SnSe2 van der Waals heterostructures for tunnel-FET applications

Two-dimensional (2D) van der Waals heterostructures (vdWHs) are attractive candidates for realizing tunnel field-effect transistors (TFETs) for low-power applications. In this work, using first-principles calculations based on density functional theory (DFT), we explore heterostructures composed of 2D MoTe2 and SnSe2. Our calculations reveal that upon forming the heterostructures, the valence band top of MoTe2 and the conduction band bottom of SnSe2 are almost aligned, forming the nearly broken-gap or type-III band alignment which is highly promising for TFETs. Interestingly, we find that the band alignment can be tuned by applying external electric fields. For positive electric fields, MoTe2 (SnSe2) band-edge positions are shifted upward (downward) with respect to the Fermi level, and more electrons are expected to tunnel from MoTe2 to SnSe2. Overall, our simulations provide fundamental insights into the electronic properties of MoTe2/SnSe2 stacks, and pave the way for the design and fabrication of future MoTe2/SnSe2-based TFETs

Influence of Oxygen Vacancies on the Structure of BiVO4

We study oxygen vacancies in the tetragonal scheelite phase of bismuth vanadate and identify stable oxygen-deficient structures. Upon subjecting these to variable-cell optimization, we find that oxygen vacancies give rise to significant structural distortions, the degree of which exhibits a vacancy concentration dependence. Furthermore, we show that these distortions give rise to splitting of powder X-ray diffraction peaks, yielding patterns similar to that of the monoclinic scheelite phase, and that these effects are also present at finite temperatures. Our results highlight the need for characterization methods beyond X-ray diffraction for identifying the phase of synthesized bismuth vanadate samples and the importance of oxygen partial pressure control during synthesis

Strong electron-phonon coupling and carrier self-trapping in Sb2_2S3_3

Antimony sulphide (Sb$_2$S$_3$) is an Earth-abundant and non-toxic material that is under investigation for solar energy conversion applications. However, it still suffers from poor power conversion efficiency and a large open circuit voltage loss that have usually been attributed to point or interfacial defects and trap states. More recently, a self-trapped exciton has been suggested as the microscopic origin for the performance loss. By using first-principles methods, we demonstrate that Sb$_2$S$_3$ exhibits strong electron-phonon coupling, which results in a large renormalization of 200 meV of the absorption edge when temperature increases from 10K to 300K, and in a quasi-1D electron polaron that is delocalized in the ribbon direction of the crystal structure, but localized in the inter-ribbon directions. The calculated polaron formation energy of 67 meV agrees well with experimental measurements, suggesting that self-trapped excitons are likely to form with the mediation of an electron polaron. Our results demonstrate the importance of systematically investigating electron-phonon coupling and polaron formation in the antimony chalcogenide family of semiconductors for optoelectronic applications.Comment: 4 figure

Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials

The atomic scale dynamics of halide perovskites havea direct impactnot only on their thermal stability but also on their optoelectronicproperties. Progress in machine-learned potentials has only recentlyenabled modeling the finite temperature behavior of these materialsusing fully atomistic methods with near first-principles accuracy.Here, we systematically analyze the impact of heating and coolingrate, simulation size, model uncertainty, and the role of the underlyingexchange-correlation functional on the phase behavior of CsPbX3 with X = Cl, Br, and I, including both the perovskite andthe & delta;-phases. We show that rates below approximately 60 K/nsand system sizes of at least a few tens of thousands of atoms shouldbe used to achieve convergence with regard to these parameters. Bycontrolling these factors and constructing models that are specificfor different exchange-correlation functionals, we then assess thebehavior of seven widely used semilocal functionals (LDA, vdW-DF-cx,SCAN, SCAN+rVV10, PBEsol, PBE, and PBE+D3). The models based on LDA,vdW-DF-cx, and SCAN+rVV10 agree well with experimental data for thetetragonal-to-cubic-perovskite transition temperature in CsPbI3 and also achieve reasonable agreement for the perovskite-to-deltaphase transition temperature. They systematically underestimate, however,the orthorhombic-to-tetragonal transition temperature. All other models,including those for CsPbBr3 and CsPbCl3, predicttransition temperatures below the experimentally observed values forall transitions considered here. Among the considered functionals,vdW-DF-cx and SCAN+rVV10 yield the closest agreement with experiment,followed by LDA, SCAN, PBEsol, PBE, and PBE+D3. Our work providesguidelines for the systematic analysis of dynamics and phase transitionsin inorganic halide perovskites and similar systems. It also servesas a benchmark for the further development of machine-learned potentialsas well as exchange-correlation functionals

Charge Localization in Defective BiVO4

We study the native defects in bismuth vanadate using hybrid density functional theory. We pay special attention to where excess charges localize by considering different polaronic distortions and find that charge localization has a profound effect on the local chemical environment around certain defects. In particular, oxygen dimerization may occur in the presence of acceptor defects. On the basis of Fermi level pinning due to compensation between donors and acceptors we additionally find that intrinsic p-type conductivity is difficult to achieve in BiVO4, in good agreement with experimental observations. Our results give new insights into the defect chemistry of bismuth vanadate and act as a guide for future studies on defects in complex metal oxides

Quantifying Dynamic Tilting in Halide Perovskites: Chemical Trends and Local Correlations

Halide perovskiteshave emerged as one of the most interestingmaterials for optoelectronic applications due to their favorable properties,such as defect tolerance and long charge carrier lifetimes, whichare attributed to their dynamic softness. However, this softness hasled to apparent disagreements between the local instantaneous andglobal average structures of these materials. In this study, we rationalizethis situation through an assessment of the local tilt angles of octahedrain the perovskite structure using large-scale molecular dynamics simulationsbased on machine-learned potentials trained using density functionaltheory calculations. We compare structural properties given by differentdensity functionals [local density approximation, PBE, PBE + D3, PBEsol,strongly constrained and appropriately normed (SCAN), SCAN + rVV10,and van der Waals density functional with consistent exchange] andestablish trends across a family of CsMX3 perovskites withM = Sn or Pb and X = Cl, Br or I. Notably, we demonstrate strong short-rangeordering in the cubic phase of halide perovskites. This ordering isreminiscent of the tetragonal phase and provides the bridge betweenthe disordered local structure and the global cubic arrangement. Ourresults provide a deeper understanding of the structural propertiesof halide perovskites and their local distortions, which is crucialfor further understanding their optoelectronic properties

Charge Localization in Acene Crystals from Ab Initio Electronic Structure

The performance of Koopmans-compliant hybrid functionals in reproducing the electronic structure of organic crystals is tested for a series of acene crystals. The calculated band gaps are found to be consistent with those achieved with the GW method at a fraction of the computational cost and in excellent accord with the experimental results at room temperature, when including the thermal renormalization. The energetics of excess holes and electrons reveals a struggle between polaronic localization and band-like delocalization. The consequences of these results on the transport properties of acene crystals are discussed