1,811 research outputs found
GW quasiparticle band structures of stibnite, antimonselite, bismuthinite, and guanajuatite
We present first-principles calculations of the quasiparticle band structures
of four isostructural semiconducting metal chalcogenides AB (with A =
Sb, Bi and B = S, Se) of the stibnite family within the GW approach. We
perform extensive convergence tests and identify a sensitivity of the
quasiparticle corrections to the structural parameters and to the semicore
electrons. Our calculations indicate that all four chalcogenides exhibit direct
band gaps, if we exclude some indirect transitions marginally below the direct
gap. Relativistic spin-orbit effects are evaluated for the Kohn-Sham band
structures, and included as scissor corrections in the quasiparticle band gaps.
Our calculated band gaps are 1.5 eV (SbS), 1.3 eV (SbSe), 1.4
eV (BiS) and 0.9 eV (BiSe). By comparing our calculated gaps
with the ideal Shockley-Queisser value we find that all four chalcogenides are
promising as light sensitizers for nanostructured photovoltaics.Comment: 11 pages, 5 figures. Revised manuscript - includes spin-orbit
interactio
Steric engineering of metal-halide perovskites with tunable optical band gaps
Owing to their high energy-conversion efficiency and inexpensive fabrication
routes, solar cells based on metal-organic halide perovskites have rapidly
gained prominence as a disruptive technology. An attractive feature of
perovskite absorbers is the possibility of tailoring their properties by
changing the elemental composition through the chemical precursors. In this
context, rational in silico design represents a powerful tool for mapping the
vast materials landscape and accelerating discovery. Here we show that the
optical band gap of metal-halide perovskites, a key design parameter for solar
cells, strongly correlates with a simple structural feature, the largest
metal-halide-metal bond angle. Using this descriptor we suggest continuous
tunability of the optical gap from the mid-infrared to the visible. Precise
band gap engineering is achieved by controlling the bond angles through the
steric size of the molecular cation. Based on these design principles we
predict novel low-gap perovskites for optimum photovoltaic efficiency, and we
demonstrate the concept of band gap modulation by synthesising and
characterising novel mixed-cation perovskites.Comment: This manuscript was submitted for publication on March 6th, 2014.
Many of the results presented in this manuscript were presented at the
International Conference on Solution processed Semiconductor Solar Cells,
held in Oxford, UK, on 10-12 September 2014. The manuscript is 37 pages long
and contains 8 figure
Phonon Screening of Excitons in Semiconductors: Halide Perovskites and Beyond
The ab initio Bethe-Salpeter equation (BSE) approach, an established method
for the study of excitons in materials, is typically solved in a limit where
only static screening from electrons is captured. Here, we generalize this
framework to also include dynamical screening from phonons at lowest order in
the electron-phonon interaction. We apply this generalized BSE approach to a
series of inorganic lead halide perovskites, CsPbX3, with X = Cl, Br, and I. We
find that inclusion of screening from phonons significantly reduces the
computed exciton binding energies of these systems. By deriving a simple
expression for phonon screening effects, we reveal general trends for the
importance of phonon screening effects in semiconductors and insulators, based
on a hydrogenic exciton model. We demonstrate that the magnitude of the phonon
screening correction in isotropic materials can be reliably predicted using
four material specific parameters: the reduced effective mass, the static and
optical dielectric constants, and the phonon frequency of the most strongly
coupled LO phonon mode. This framework helps to elucidate the importance of
phonon screening and its relation to excitonic properties in a broad class of
semiconductors
Chemical Mapping of Excitons in Halide Double Perovskites
Halide double perovskites are an emerging class of semiconductors with
tremendous chemical and electronic diversity. While their bandstructure
features can be understood from frontier-orbital models, chemical intuition for
optical excitations remains incomplete. Here, we use \textit{ab initio}
many-body perturbation theory within the and the Bethe-Salpeter Equation
approach to calculate excited-state properties of a representative range of
CsBBCl double perovskites. Our calculations reveal that double
perovskites with different combinations of B and B cations display a broad
variety of electronic bandstructures and dielectric properties, and form
excitons with binding energies ranging over several orders of magnitude. We
correlate these properties with the orbital-induced anisotropy of
charge-carrier effective masses and the long-range behavior of the dielectric
function, by comparing with the canonical conditions of the Wannier-Mott model.
Furthermore, we derive chemically intuitive rules for predicting the nature of
excitons in halide double perovskites using electronic structure information
obtained from computationally inexpensive DFT calculations
Band gaps of crystalline solids from Wannier-localization based optimal tuning of a screened range-separated hybrid functional
Accurate prediction of fundamental band gaps of crystalline solid state
systems entirely within density functional theory is a long standing challenge.
Here, we present a simple and inexpensive method that achieves this by means of
non-empirical optimal tuning of the parameters of a screened range-separated
hybrid functional. The tuning involves the enforcement of an ansatz that
generalizes the ionization potential theorem to the removal of an electron in
an occupied state described by a localized Wannier function in a modestly sized
supercell calculation. The method is benchmarked against experiment for a set
of systems ranging from narrow band gap semiconductors to large band gap
insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 eV and
is found to yield quantitative accuracy across the board, with a mean absolute
error of 0.1 eV and a maximal error of 0.2 eV.Comment: 10 pages, 2 figure
Optical absorption spectra of metal oxides from time-dependent density functional theory and many-body perturbation theory based on optimally-tuned hybrid functionals
Using both time-dependent density functional theory (TDDFT) and the
``single-shot" plus Bethe-Salpeter equation (-BSE) approach, we
compute optical band gaps and optical absorption spectra from first principles
for eight common binary and ternary closed-shell metal oxides (MgO,
AlO, CaO, TiO, CuO, ZnO, BaSnO, and BiVO), based on the
non-empirical Wannier-localized optimally-tuned screened range-separated hybrid
functional. Overall, we find excellent agreement between our TDDFT and -BSE
results and experiment, with a mean absolute error less than 0.4 eV, including
for CuO and ZnO, traditionally considered to be challenging for both
methods
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