42 research outputs found
Assessment of the Ab Initio Bethe-Salpeter Equation Approach for the Low-Lying Excitation Energies of Bacteriochlorophylls and Chlorophylls
Bacteriochlorophyll and Chlorophyll molecules are crucial building blocks of
the photosynthetic apparatus in bacteria, algae and plants. Embedded in
transmembrane protein complexes, they are responsible for the primary processes
of photosynthesis: excitation energy and charge transfer. Here, we use ab
initio many body perturbation theory within the approximation and
Bethe-Salpeter equation (BSE) approach to calculate the electronic structure
and optical excitations of Bacteriochlorophylls , , , and and
Chlorophylls and . We systematically study the effects of structure,
basis set size, partial self-consistency in , and the underlying
exchange-correlation approximation, and compare our calculations with results
from time-dependent density functional theory, multireference RASPT2 and
experimental literature results. We find that optical excitations calculated
with +BSE are in excellent agreement with experimental data, with an
average deviation of less than 100\,meV for the first three bright excitations
of the entire family of (Bacterio)chlorophylls. Contrary to state-of-the-art
TDDFT with an optimally-tuned range-separated hybrid functional, this accuracy
is achieved in a parameter-free approach. Moreover, +BSE predicts the
energy differences between the low-energy excitations correctly, and eliminates
spurious charge transfer states that TDDFT with (semi)local approximations is
known to produce. Our study provides accurate reference results and highlights
the potential of the +BSE approach for the simulation of larger pigment
complexes
Electric field and strain induced Rashba effect in hybrid halide perovskites
Using first principles density functional theory calculations, we show how
Rashba-type energy band splitting in the hybrid organic-inorganic halide
perovskites APbX (A=CHNH, CH(NH), Cs and X=I, Br)
can be tuned and enhanced with electric fields and anisotropic strain. In
particular, we demonstrate that the magnitude of the Rashba splitting of
tetragonal (CHNH)PbI grows with increasing macroscopic alignment of
the organic cations and electric polarization, indicating appreciable
tunability with experimentally-feasible applied fields, even at room
temperature. Further, we quantify the degree to which this effect can be tuned
via chemical substitution at the A and X sites, which alters amplitudes of
different polar distortion patterns of the inorganic PbX cage that directly
impact Rashba splitting. In addition, we predict that polar phases of CsPbI
and (CHNH)PbI with symmetry possessing considerable Rashba
splitting might be accessible at room temperature via anisotropic strain
induced by epitaxy, even in the absence of electric fields
Halogen Vacancy Migration at Surfaces of CsPbBr Perovskites: Insights from Density Functional Theory
Migration of halogen vacancies is one of the primary sources of phase
segregation and material degradation in lead-halide perovskites. Here we use
first principles density functional theory to compare migration energy barriers
and paths of bromine vacancies in the bulk and at a (001) surface of cubic
CsPbBr. Our calculations indicate that surfaces might facilitate bromine
vacancy migration in these perovskites, due to their soft structure that allows
for bond lengths variations larger than in the bulk. We calculate the migration
energy for axial-to-axial bromine vacancy migration at the surface to be only
half of the value in the bulk. Furthermore, we study the effect of modifying
the surface with four different alkali halide monolayers, finding an increase
of the migration barrier to almost the bulk value for the NaCl-passivated
system. Migration barriers are found to be correlated to the lattice mismatch
between the CsPbBr surface and the alkali halide monolayer. Our
calculations suggest that surfaces might play a significant role in mediating
vacancy migration in halide perovskites, a result with relevance for perovskite
nanocrystals with large surface-to-volume ratios. Moreover, we propose viable
ways for suppressing this undesirable process through passivation with alkali
halide salts
Towards predictive band gaps for halide perovskites: Lessons from one-shot and eigenvalue self-consistent GW
Halide perovskites constitute a chemically-diverse class of crystals with
great promise as photovoltaic absorber materials, featuring band gaps between
about 1 and 3.5 eV depending on composition. Their diversity calls for a
general computational approach to predicting their band gaps. However, such an
approach is still lacking. Here, we use density functional theory (DFT) and
many-body perturbation theory within the GW approximation to compute the
quasiparticle or fundamental band gap of a set of ten representative halide
perovskites: CHNHPbI (MAPbI), MAPbBr, CsSnBr,
(MA)BiTlBr, CsTlAgBr, CsTlAgCl, CsBiAgBr,
CsInAgCl, CsSnBr, and CsAuI. Comparing with recent
measurements, we find that a standard generalized gradient exchange-correlation
functional can significantly underestimate the experimental band gaps of these
perovskites, particularly in cases with strong spin-orbit coupling (SOC) and
highly dispersive band edges, to a degree that varies with composition. We show
that these nonsystematic errors are inherited by one-shot GW and
eigenvalue self-consistent GW calculations, demonstrating that semilocal
DFT starting points are insufficient for MAPbI, MAPbBr, CsSnBr,
(MA)BiTlBr, CsTlAgBr, and CsTlAgCl. On the other hand,
we find that DFT with hybrid functionals leads to an improved starting point
and GW results in better agreement with experiment for these perovskites.
Our results suggest that GW with hybrid functional-based starting points
are promising for predicting band gaps of systems with large SOC and dispersive
bands in this technologically important class of semiconducting crystals
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
Mapping Charge-Transfer Excitations in Bacteriochlorophyll Dimers from First Principles
Photoinduced charge-transfer excitations are key to understand the primary
processes of natural photosynthesis and for designing photovoltaic and
photocatalytic devices. In this paper, we use Bacteriochlorophyll dimers
extracted from the light harvesting apparatus and reaction center of a
photosynthetic purple bacterium as model systems to study such excitations
using first-principles numerical simulation methods. We distinguish four
different regimes of intermolecular coupling, ranging from very weakly coupled
to strongly coupled, and identify the factors that determine the energy and
character of charge-transfer excitations in each case. We also construct an
artificial dimer to systematically study the effects of intermolecular distance
and orientation on charge-transfer excitations, as well as the impact of
molecular vibrations on these excitations. Our results provide design rules for
tailoring charge-transfer excitations in Bacteriochloropylls and related
photoactive molecules, and highlight the importance of including
charge-transfer excitations in accurate models of the excited-state structure
and dynamics of Bacteriochlorophyll aggregates