Enhanced Li-ion dynamics in trivalently doped Lithium Phosphidosilicate Li2SiP2: A candidate material as a solid li electrolyte

Oxide and sulphide solid electrolyte materials have enjoyed significant interest in the solid-state battery community. Phosphide materials however are relatively unexplored despite the potential for being high lithium containing systems. This work reports on the phosphidosilicate system Li2SiP2 , one of many systems in the Li-Si-P phase diagram. The phosphidosilicates display complex structures and very large unit cells, which present challenges for ab-initio simulations. We present the first computational report on the theoretical ionic conductivity and related diffusion mechanisms of the material Li2SiP2 , selected due to it’s unusual supertetrahedral framework which is a recurrent motif amongst the phosphidosilicates. Group 13 dopants have also been introduced into Li2SiP2 showing preference for the silicon site over the lithium site, with Al0 Si doping showing extremely low defect incorporation energies of 0.05 eV, with no increase in defect energy up to concentrations of 10% Al0 Si. Furthermore, clustering of Al0 Si has been found to be unfavourable, in line with trends seen in oxide zeolite structures. Ab-initio molecular dynamics (AIMD) simulations indicate high ionic conductivity in pure Li2SiP2 of up to 3.19 × 10−1 S.cm-1 at 700 K. Doping with 10% Al0 Si and associated Li• i compensating defects leads to higher ionic conductivities at lower temperatures when compared to pure Li2SiP2 . The activation energies to lithium diffusion were found to be low at 0.30 eV and 0.24 eV for pure and 10% Al0 Si doped Li2SiP2 respectively, in line with previous experimental observations of pure Li2SiP2 . Multiple lithium migration pathways have also been extracted, with some mechanisms displaying activation energies as low as 0.05 eV. Furthermore, our calculated intercalation voltages suggest that these materials are stable against lithium metal and therefore could be very attractive in stabilising the electrode/electrolyte interface

Conductivity Limits in CuAlO₂ from Screened-Hybrid Density Functional Theory

CuAlO₂ is a prototypical delafossite <i>p</i>-type transparent conducting oxide (TCO). Despite this, many fundamental questions about its band structure and conductivity remain unanswered. We utilize the screened hybrid exchange functional (HSE06) to investigate defects in CuAlO₂ and find that copper vacancies and copper on aluminum antisites will dominate under Cu-poor/Al-poor conditions. Our calculated transitions levels are deep in the band gap, consistent with experimental findings, and we identify the likely defect levels that are often mistaken as indirect band gaps. Finally, we critically discuss delafossite oxides as TCO materials

Exploring the PbS–Bi₂S₃ Series for Next Generation Energy Conversion Materials

As photovoltaics become an ever more important part of the global energy economy, the search for inexpensive, earth-abundant solar absorbers has grown rapidly. The binary compounds PbS and Bi₂S₃ have both seen success in previous photovoltaic studies; however, bulk PbS has a small band gap, restricting its efficiency, and Bi₂S₃, while strongly absorbing, can be limited by its layered structure. The mixed PbS–Bi₂S₃ series has previously been the focus of mostly structural studies, so in this article, we examine the electronic structure of the known members of this series using hybrid density functional theory. We find that the lead bismuth sulfides are able to retain optimal properties, such as low carrier effective masses and strong absorption, from both parent phases, with band gaps between 0.25 and 1.32 eV. PbBi₂S₄ emerges from our computational screening as a possible earth-abundant solar absorber, with a predicted maximum efficiency of 26% at a film thickness of 0.2 μm and with the retention of the three-dimensional connectivity of lead and bismuth polyhedra

(CH₃NH₃)₂Pb(SCN)₂I₂: A More Stable Structural Motif for Hybrid Halide Photovoltaics?

Hybrid halide perovskites have recently emerged as a highly efficient class of light absorbers; however, there are increasing concerns over their long-term stability. Recently, incorporation of SCN^– has been suggested as a novel route to improving stability without negatively impacting performance. Intriguingly, despite crystallizing in a 2D layered structure, (CH₃NH₃)₂Pb­(SCN)₂I₂ (MAPSI) possesses an ideal band gap of 1.53 eV, close to that of the 3D connected champion hybrid perovskite absorber, CH₃NH₃PbI₃ (MAPI). Here, we identify, using hybrid density functional theory, the origin of the smaller than expected band gap of MAPSI through a detailed comparison with the electronic structure of MAPI. Furthermore, assessment of the MAPSI structure reveals that it is thermodynamically stable with respect to phase separation, a likely source of the increased stability reported in experiment

Defect Engineering of Earth-Abundant Solar Absorbers BiSI and BiSeI

Bismuth-based solar absorbers have recently garnered attention due to their promise as cheap, nontoxic, and efficient photovoltaics. To date, however, most show poor efficiencies far below those seen in commercial technologies. In this work, we investigate two such promising materials, BiSI and BiSeI, using relativistic first-principles methods with the aim of identifying their suitability for photovoltaic applications. Both compounds show excellent optoelectronic properties with ideal band gaps and strong optical absorption, leading to high predicted device performance. Using defect analysis, we reveal the electronic and structural effects that can lead to the presence of deep trap states, which may help explain the prior poor performance of these materials. Crucially, detailed mapping of the range of experimentally accessible synthesis conditions allows us to provide strategies to avoid the formation of killer defects in the future

Tolerance Factor and Cooperative Tilting Effects in Vacancy-Ordered Double Perovskite Halides

Lattice dynamics and structural instabilities are strongly implicated in dictating the electronic properties of perovskite halide semiconductors. We present a study of the vacancy-ordered double perovskite Rb₂SnI₆ and correlate dynamic and cooperative octahedral tilting with changes in electronic behavior compared to those of Cs₂SnI₆. Though both compounds exhibit native <i>n</i>-type semiconductivity, Rb₂SnI₆ exhibits carrier mobilities that are reduced by a factor of ∼50 relative to Cs₂SnI₆. From synchrotron powder X-ray diffraction, we find that Rb₂SnI₆ adopts the tetragonal vacancy-ordered double perovskite structure at room temperature and undergoes a phase transition to a lower-symmetry monoclinic structure upon cooling, characterized by cooperative octahedral tilting of the [SnI₆] octahedra. X-ray and neutron pair distribution function analyses reveal that the local coordination environment of Rb₂SnI₆ is consistent with the monoclinic structure at all temperatures; we attribute this observation to dynamic octahedral rotations that become frozen in to yield the low-temperature monoclinic structure. In contrast, Cs₂SnI₆ adopts the cubic vacancy-ordered double perovskite structure at all temperatures. Density functional calculations show that static octahedral tilting in Rb₂SnI₆ results in marginally increased carrier effective masses, which alone are insufficient to account for the experimental electronic behavior. Rather, the larger number of low-frequency phonons introduced by the lower symmetry of the Rb₂SnI₆ structure yield stronger electron–phonon coupling interactions that produce larger electron effective masses and reduced carrier mobilities relative to Cs₂SnI₆. Further, we discuss the results for Rb₂SnI₆ in the context of other vacancy-ordered double perovskite semiconductors, in order to demonstrate that the electron–phonon coupling characteristics can be predicted using the geometric perovskite tolerance factor. This study represents an important step in designing perovskite halide semiconductors with desired charge transport properties for optoelectronic applications

Tolerance Factor and Cooperative Tilting Effects in Vacancy-Ordered Double Perovskite Halides

Lattice dynamics and structural instabilities are strongly implicated in dictating the electronic properties of perovskite halide semiconductors. We present a study of the vacancy-ordered double perovskite Rb₂SnI₆ and correlate dynamic and cooperative octahedral tilting with changes in electronic behavior compared to those of Cs₂SnI₆. Though both compounds exhibit native <i>n</i>-type semiconductivity, Rb₂SnI₆ exhibits carrier mobilities that are reduced by a factor of ∼50 relative to Cs₂SnI₆. From synchrotron powder X-ray diffraction, we find that Rb₂SnI₆ adopts the tetragonal vacancy-ordered double perovskite structure at room temperature and undergoes a phase transition to a lower-symmetry monoclinic structure upon cooling, characterized by cooperative octahedral tilting of the [SnI₆] octahedra. X-ray and neutron pair distribution function analyses reveal that the local coordination environment of Rb₂SnI₆ is consistent with the monoclinic structure at all temperatures; we attribute this observation to dynamic octahedral rotations that become frozen in to yield the low-temperature monoclinic structure. In contrast, Cs₂SnI₆ adopts the cubic vacancy-ordered double perovskite structure at all temperatures. Density functional calculations show that static octahedral tilting in Rb₂SnI₆ results in marginally increased carrier effective masses, which alone are insufficient to account for the experimental electronic behavior. Rather, the larger number of low-frequency phonons introduced by the lower symmetry of the Rb₂SnI₆ structure yield stronger electron–phonon coupling interactions that produce larger electron effective masses and reduced carrier mobilities relative to Cs₂SnI₆. Further, we discuss the results for Rb₂SnI₆ in the context of other vacancy-ordered double perovskite semiconductors, in order to demonstrate that the electron–phonon coupling characteristics can be predicted using the geometric perovskite tolerance factor. This study represents an important step in designing perovskite halide semiconductors with desired charge transport properties for optoelectronic applications

Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs₂SnI₆ and Cs₂TeI₆

Vacancy-ordered double perovskites of the general formula <i>A</i>₂<i>BX</i>₆ are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [<i>BX</i>₆] units with <i>A</i>-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs₂SnI₆ has recently been explored for applications in photovoltaic devices. To elucidate the structure–property relationships of these materials, we have synthesized solid-solution Cs₂Sn_1–<i>x</i>Te_<i>x</i>I₆. However, even though tellurium substitution increases electronic dispersion via closer I–I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs₂SnI₆ reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te–I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs₂TeI₆ is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the <i>B</i>- and <i>X</i>-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure–property relationships in functional perovskite halide

Computational and Experimental Study of Ta₂O₅ Thin Films

This paper reports the novel synthesis of amorphous Ta₂O₅ and the subsequent isolation of the orthorhombic (β) crystallographic phase, using aerosol-assisted chemical vapor deposition. Hybrid density functional theory was used to obtain the calculated optical band gap (3.83 eV) for the first time, which closely matches our experimental findings (3.85 eV). The films were highly transparent in the visible and near-IR region of the electromagnetic spectrum. The refractive indexes, calculated using the Swanepoel method, showed good agreement with literature findings. The photocatalytic properties of the films, determined through the photominerilization of stearic acid under 254 nm radiation showed the amorphous sample to be an order of magnitude superior over crystalline β-Ta₂O₅

Single Step Solution Processed GaAs Thin Films from GaMe₃ and ^<i>t</i>BuAsH₂ under Ambient Pressure

This article reports on the possibility of low-cost GaAs formed under ambient pressure via a single step solution processed route from only readily available precursors, ^<i>t</i>BuAsH₂ and GaMe₃. The thin films of GaAs on glass substrates were found to have good crystallinity with crystallites as large as 150 nm and low contamination with experimental results matching well with theoretical density of states calculations. These results open up a route to efficient and cost-effective scale up of GaAs thin films with high material properties for widespread industrial use. Confirmation of film quality was determined using XRD, Raman, EDX mapping, SEM, HRTEM, XPS, and SIMS