arXiv subjects

Shuxia Tao

Publications and source records attributed to Shuxia Tao.

29 records · Page 2Linked to original sources

arXiv2021

The thermodynamic trends of intrinsic defects in primary halide perovskites: A first-principles study

Defects in halide perovskites play an essential role in determining the efficiency and stability of the resulting optoelectronic devices. Here, we present a systematic study of intrinsic point defects in six primary metal halide perovskites, MAPbI$_3$, MAPbBr$_3$, MAPbCl$_3$, FAPbI$_3$, CsPbI$_3$ and MASnI$_3$, using density functional theory calculations with the SCAN+rVV10 functional. We analyse the impact of changing anions and cations on the defect formation energies and the charge state transitions levels and identify the physical origins underlying the observed trends. Dominant defects in the lead-iodide compounds are the A$^+$ cation interstitials (A = Cs, MA, FA), charge-compensated by I$^-$ interstitials or lead $({2-})$ vacancies. In the lead-bromide and -chloride compounds, halide vacancies become relatively more prominent, and for MAPbBr$_3$, the Pb$^{2+}$ interstitial also becomes important. The trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in MASnI$_3$ is strongly dominated by tin $({2-})$ vacancies, promoted by the easy oxidation of the tin perovskite. Intrinsically, all compounds are mildly p-doped, except for MASnI$_3$, which is strongly p-doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, create deep donor traps. Although these traps might hamper the electronic behavior of MAPbBr$_3$ and MAPbCl$_3$, in iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties.

cond-mat.mtrl-sci↗

arXiv2021

Transferable classical force field for pure and mixed metal halide perovskites parameterized from first principles

Many key features in photovoltaic perovskites occur in relatively long time scales and involve mixed compositions. This requires realistic but also numerically simple models. In this work we present a transferable classical force field to describe the mixed hybrid perovskite MA$_x$FA$_{1-x}$Pb(Br$_y$I$_{1-y}$)$_3$ for variable composition ($\forall x,y \in [0,1]$). The model includes Lennard-Jones and Buckingham potentials to describe the interactions between the atoms of the inorganic lattice and the organic molecule, and the AMBER model to describe intramolecular atomic interactions. Most of the parameters of the force field have been obtained by means of a genetic algorithm previously developed to parameterize the CsPb(Br$_x$I$_{1-x}$)$_3$ perovskite. The algorithm finds the best parameter set that simultaneously fits the DFT energies obtained for several crystalline structures with moderate degrees of distortion with respect to the equilibrium configuration. The resulting model reproduces correctly the XRD patterns, the expansion of the lattice upon I/Br substitution and the thermal expansion coefficients. We use the model to run classical molecular dynamics simulations with up to 8600 atoms and simulation times of up to 40~ns. From the simulations we have extracted the ion diffusion coefficient of the pure and mixed perovskites, presenting for the first time these values obtained by a fully dynamical method using a transferable model fitted to first principles calculations. The values here reported can be considered as the theoretical upper limit for ion migration dynamics induced by halide vacancies in photovoltaic perovskite devices under operational conditions.

cond-mat.mtrl-sci↗

arXiv2021

A Reparameterized Density Functional Tight-Binding Method for Engineering phase-stable CsPbX\textsubscript{3} Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications, due to their excellent optoelectronic performance. However, they suffer several dynamical degradation problems, the characterization of which is challenging in experiments. Atomic scale simulations can provide valuable insights, however, the high computational cost of traditional quantum mechanical methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present a re-parameterized GFN1-xTB method for the accurate description of structural and dynamical properties of CsPbBr\textsubscript{3}, CsPbI\textsubscript{3}, and CsPb(I\textsubscript{1-x}Br\textsubscript{x})\textsubscript{3}. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, the directional movement of the Cs cations decreases contact with the surrounding halides, initiating a transition to the non-perovskite phase. However, this loss of contact can be compensated by increased halide displacement, once enough thermal energy is available, resulting in a transition to the tetragonal or cubic phases. Furthermore, we find the mixing of halides increase halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

cond-mat.mtrl-sci↗

arXiv2021

The Role of Solvents in the Formation of Methylammonium Lead Triiodide Perovskite

Metal halide perovskites (MHPs) are gaining increasing attention as low-cost, high-performance semiconductors for optoelectronics. In particular, their solution processing is compatible with the large-scale manufacturing of thin-film devices, including solar cells and light-emitting diodes. Understanding the coordination chemistry in precursor-solvent solution and atomistic mechanisms of film formation is of great importance for optimizing the optoelectronic properties of the final films. Using the methylammonium lead triiodide (MAPbI3) as an example, we study the complex evolution of the molecular species from the solution to the initial stage of the crystallization by using a combination of density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. We focus on the widely employed solvents DMSO and DMF, analyze the structures and energies of the iodoplumbate complexes in the form of simple complex and polymeric iodoplumbates. Based on the calculated formation enthalpies, we propose reaction schemes of MAPbI3 formation in DMSO, DMF and DMSO-DMF binary solvent and explain the advantages of the binary solvent. We highlight the important role of NH-O hydrogen bonds in the formation of iodoplumbates monomers. Our calculations indicate unbalanced reaction energies at several elementary reaction steps in either DMF or DMSO. Mixing a small amount of DMSO in DMF gives rise to a better balance in the energies and, therefore, potentially better equilibria in the overall crystallization process and better quality of the final perovskite films.

physics.chem-ph↗

arXiv2021

A ReaxFF molecular dynamics study of hydrogen diffusion in ruthenium -- the role of grain boundaries

Ruthenium thin films can serve as protective caps for multi-layer extreme ultraviolet mirrors exposed to atomic hydrogen. Hydrogen permeation through ruthenium is problematic as it leads to blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, and rapidly diffuses in and out of Ru. Therefore, the underlying mechanisms of the blistering effect remains unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behaviour of H. For the Ru/H system, a ReaxFF force field was parametrised which reproduces structures and energies obtained from quantum-mechanical calculations. Molecular dynamics simulations have been performed with the newly-developed force field, to study the effect of tilt and twist grain boundaries on the overall diffusion behaviour of H in Ru. Our simulations show the tilt and twist grain boundaries provide energetically favourable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes, and favour diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on the hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation.

cond-mat.mtrl-sci↗

arXiv2021

First-principles calculations of defects in metal halide perovskites: a performance comparison of density functionals

Metal halide perovskite semiconductors have outstanding optoelectronic properties. Although these perovskites are defect-tolerant electronically, defects hamper their long-term stability and cause degradation. Density functional theory (DFT) calculations are an important tool to unravel the microscopic structures of defects, but results suffer from the different approximations used in the DFT functionals. In the case of metal halide perovskites, qualitatively different results have been reported with different functionals, either predicting vacancy or interstitial point defects to be most dominant. Here, we conduct a comprehensive comparison of a wide range of functionals for calculating the equilibrium defect formation energies and concentrations of point defects in the archetype metal halide perovskite, MAPbI$_3$. We find that it is essential to include long-range Van der Waals interactions in the functional, and that it is vital to self-consistently optimize structure and volume of all compounds involved in the defect formation. For calculating equilibrium formation energies of point defects in MAPbI$_3$ and similar metal halide perovskites, we argue that the exact values of the chemical potentials of the species involved, or of the intrinsic Fermi level, are not important. In contrast to the simple Schottky or Frenkel pictures, we find that the dominant defects are MA and I interstitials, and Pb vacancies.

cond-mat.mtrl-sci↗

arXiv2021

Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study

Halide perovskites make efficient solar cells due to their exceptional optoelectronic properties, but suffer from several stability issues. The characterization of the degradation processes is challenging because of the limitations in the spatio-temporal resolution in experiments and the absence of efficient computational methods to study the reactive processes. Here, we present the first effort in developing reactive force fields for large scale molecular dynamics simulations of the phase instability and the defect-induced degradation reactions in inorganic CsPbI$_{3}$. We find that the phase transitions are driven by a combination of the anharmonicity of the perovskite lattice with the thermal entropy. At relatively low temperatures, the Cs cations tend to move away from the preferential positions with good contacts with the surrounding metal halide framework, potentially causing its conversion to a non-perovskite phase. Our simulations of defective structures reveal that, although both iodine vacancies and interstitials are very mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work puts ReaxFF forward as an effective computational framework to study reactive processes in halide perovskites.

cond-mat.mtrl-sci↗

arXiv2020

Tuning the Electronic Levels of NiO with Alkali Halides Surface Modifiers for Perovskite Solar Cells

Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface, is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been extensively used in organic optoelectronics, however, rarely studied in perovskite solar cells. Using density functional theory (DFT) calculations, we investigate the effect of single layer adsorption of twenty different alkali halides on the electronic levels of NiO. Our results show that alkali halides can shift the position of the valence band maximum (VBM) of NiO to a surprisingly large extend in both directions, from -3:10 eV to +1:59 eV. We interpret the direction and magnitude of the shift in terms of the surface dipoles, formed by the adsorbed cations and anions, where the magnitude of the VBM shift is a monotonic function of the surface coverage. Our results indicate that with alkali halide surface modifiers, the electronic levels of NiO can be tuned robustly and potentially match those of many perovskite compositions in perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2020

Efficient Modelling of Ion Structure and Dynamics in Inorganic Metal Halide Perovskites

Metal halide perovskites (MHPs) are nowadays one of the most studied semiconductors due to their exceptional performance as active layers in solar cells. Although MHPs are excellent solid-state semiconductors, they are also ionic compounds, where ion migration plays a decisive role in their formation, their photovoltaic performance and their long-term stability. Given the above-mentioned complexity, molecular dynamics simulations based on classical force fields are especially suited to study MHP properties, such as lattice dynamics and ion migration. In particular, the possibility to model mixed compositions is important since they are the most relevant to optimize the optical band gap and the stability. With this intention, we employ DFT calculations and a genetic algorithm to develop a fully transferable classical force field valid for the benchmark inorganic perovskite compositional set CsPb(Br_xI_(1-x))_3 (x = 0,1/3,2/3,1). The resulting force field reproduces correctly, with a common set of parameters valid for all compositions, the experimental lattice parameter as a function of bromide/iodide ratio, the ion-ion distances and the XRD spectra of the pure and mixed structures. The simulated thermal conductivities and ion migration activation energies of the pure compounds are also in good agreement with experimental trends. Our molecular dynamics simulations make it possible to predict the compositional dependence of the ionic diffusion coefficient on bromide/iodide ratio and vacancy concentration. For vacancy concentrations of around 9 10^21 cm^-3, we obtained ionic diffusion coefficients at ambient temperature of 10^-11 and 10^-13 cm2/s for CsPbBr3 and CsPbI3, respectively. Interestingly, in comparison with the pure compounds, we predict a significantly lower activation energy for vacancy migration and faster diffusion for the mixed perovskites.

cond-mat.mtrl-sci↗

arXiv2019

Absolute energy level positions in tin and lead based halide perovskites

Metal-halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for wide variety of compounds, the physical origins of these changes, namely the variations in valence band and conduction band positions, are not well characterized. Knowledge of these band positions is of importance for optimizing the energy level alignment with charge extraction layers in optoelectronic devices. Here, we determine ionization energy and electron affinity values of all primary tin and lead based perovskites using photoelectron spectroscopy data, supported by first-principles calculations. Through analysis of the chemical bonding, we characterize the key energy levels and elucidate their trends via a tight-binding analysis. We demonstrate that energy level variations in perovskites are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions. Secondary changes in the perovskite energy levels result from the cation-anion interaction strength, which depends on the volume and structural distortions of the perovskite lattices. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

cond-mat.mtrl-sci↗

arXiv2017

Accurate and efficient band gap predictions of metal halide perovskites using the DFT-1/2 method: GW accuracy with DFT expense

The outstanding optoelectronics and photovoltaic properties of metal halide perovskites, including high carrier motilities, low carrier recombination rates, and the tunable spectral absorption range are attributed to the unique electronic properties of these materials. While DFT provides reliable structures and stabilities of perovskites, it performs poorly in electronic structure prediction. The relativistic GW approximation has been demonstrated to be able to capture electronic structure accurately, but at an extremely high computational cost. Here we report efficient and accurate band gap calculations of halide metal perovskites by using the approximate quasiparticle DFT-1/2 method. Using AMX3 (A = CH3NH3, CH2NHCH2, Cs; M = Pb, Sn, X=I, Br, Cl) as demonstration, the influence of the crystal structure (cubic, tetragonal or orthorhombic), variation of ions (different A, M and X) and relativistic effects on the electronic structure are systematically studied and compared with experimental results. Our results show that the DFT-1/2 method yields accurate band gaps with the precision of the GW method with no more computational cost than standard DFT. This opens the possibility of accurate electronic structure prediction of sophisticated halide perovskite structures and new materials design for lead-free materials.

cond-mat.mtrl-sci↗