42 research outputs found
Tilt-induced charge localisation in phosphide antiperovskite photovoltaics
Antiperovskites are a rich family of compounds with applications in battery
cathodes, superconductors, solid-state lighting, and catalysis. Recently, a
novel series of antimonide phosphide antiperovskites (ASbP, where A = Ca,
Sr, Ba) were proposed as candidate photovoltaic absorbers due to their ideal
band gaps, small effective masses and strong optical absorption. In this work,
we explore this series of compounds in more detail using relativistic hybrid
density functional theory. We reveal that the proposed cubic structures are
dynamically unstable and instead identify a tilted orthorhombic Pnma phase as
the ground state. Tilting is shown to induce charge localisation that widens
the band gap and increases the effective masses. Despite this, we demonstrate
that the predicted maximum photovoltaic efficiencies remain high (24-31% for
200 nm thin films) by bringing the band gaps into the ideal range for a solar
absorber. Finally, we assess the band alignment of the series and suggest hole
and electron contact materials for efficient photovoltaic devices
The temperature-dependence of carrier mobility is not a reliable indicator of the dominant scattering mechanism
The temperature dependence of experimental charge carrier mobility is
commonly used as a predictor of the dominant carrier scattering mechanism in
semiconductors, particularly in thermoelectric applications. In this work, we
critically evaluate whether this practice is well founded. A review of 47
state-of-the-art mobility calculations reveals no correlation between the major
scattering mechanism and the temperature trend of mobility. Instead, we
demonstrate that the phonon frequencies are the prevailing driving forces
behind the temperature dependence and can cause it to vary between to
even for an idealised material. To demonstrate this, we calculate the
mobility of 23,000 materials and review their temperature dependence, including
separating the contributions from deformation, polar, and impurity scattering
mechanisms. We conclusively demonstrate that a temperature dependence of
is not a reliable indicator of deformation potential scattering. Our
work highlights the potential pitfalls of predicting the major scattering type
based on the experimental mobility temperature trend alone
Anion Distribution, Structural Distortion, and Symmetry-Driven Optical Band Gap Bowing in Mixed Halide Cs2SnX6 Vacancy Ordered Double Perovskites.
Mixed anion compounds in the Fm3Ì…m vacancy ordered perovskite structure were synthesized and characterized experimentally and computationally with a focus on compounds where A = Cs+. Pure anion Cs2SnX6 compounds were formed with X = Cl, Br, and I using a room temperature solution phase method. Mixed anion compounds were formed as solid solutions of Cs2SnCl6 and Cs2SnBr6 and a second series from Cs2SnBr6 and Cs2SnI6. Single phase structures formed across the entirety of both composition series with no evidence of long-range anion ordering observed by diffraction. A distortion of the cubic A2BX6 structure was identified in which the spacing of the BX6 octahedra changes to accommodate the A site cation without reduction of overall symmetry. Optical band gap values varied with anion composition between 4.89 eV in Cs2SnCl6 to 1.35 eV in Cs2SnI6 but proved highly nonlinear with changes in composition. In mixed halide compounds, it was found that lower energy optical transitions appeared that were not present in the pure halide compounds, and this was attributed to lowering of the local symmetry within the tin halide octahedra. The electronic structure was characterized by photoemission spectroscopy, and Raman spectroscopy revealed vibrational modes in the mixed halide compounds that could be assigned to particular mixed halide octahedra. This analysis was used to determine the distribution of octahedra types in mixed anion compounds, which was found to be consistent with a near-random distribution of halide anions throughout the structure, although some deviations from random halide distribution were noted in mixed iodide-bromide compounds, where the larger iodide anions preferentially adopted trans configurations
Efficient calculation of carrier scattering rates from first principles
The electronic transport behaviour of materials determines their suitability
for technological applications. We develop an efficient method for calculating
carrier scattering rates of solid-state semiconductors and insulators from
first principles inputs. The present method extends existing polar and
non-polar electron-phonon coupling, ionized impurity, and piezoelectric
scattering mechanisms formulated for isotropic band structures to support
highly anisotropic materials. We test the formalism by calculating the
electronic transport properties of 16 semiconductors and comparing the results
against experimental measurements. The present work is amenable for use in
high-throughput computational workflows and enables accurate screening of
carrier mobilities, lifetimes, and thermoelectric power.Comment: 11 pages, 4 figures (SI: 21 pages, 14 figures
Relativistic electronic structure and band alignment of BiSI and BiSeI:candidate photovoltaic materials
Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Here, we computationally screen BiSI and BiSeI and show they possess electronic structures ideal for solar cell applications.</p
Hybrid Organic-Inorganic Coordination Complexes as Tunable Optical Response Materials.
Novel lead and bismuth dipyrido complexes have been synthesized and characterized by single-crystal X-ray diffraction, which shows their structures to be directed by highly oriented π-stacking of planar fully conjugated organic ligands. Optical band gaps are influenced by the identity of both the organic and inorganic component. Density functional theory calculations show optical excitation leads to exciton separation between inorganic and organic components. Using UV-vis, photoluminescence, and X-ray photoemission spectroscopies, we have determined the materials' frontier energy levels and show their suitability for photovoltaic device fabrication by use of electron- and hole-transport materials such as TiO2 and spiro-OMeTAD respectively. Such organic/inorganic hybrid materials promise greater electronic tunability than the inflexible methylammonium lead iodide structure through variation of both the metal and organic components
The stability of the M2 phase of vanadium dioxide induced by coherent epitaxial strain
Tensile strain along the cR axis in epitaxial VO2 films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the TiO2 substrate and identify the phases of VO2 as a function of temperature in epitaxial VO2/TiO2 thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the cR axis is insufficient for describing the transition pathway of VO2 epitaxial films and that a strain induced increase of electron correlation effects must be considered