Simulating Dye-Sensitized TiO2 Heterointerfaces in Explicit Solvent: Absorption Spectra, Energy Levels, and Dye Desorption

Dye-sensitized solar cells (DSCs) represent a valuable, efficient, and low-cost alternative to conventional semiconductor photovoltaic devices. A deeper understanding of the dye/semiconductor heterointerface and of the dye-sensitized semiconductor/ electrolyte interactions are fundamental for further progress in DSC technology. Here we report an ab initio molecular dynamics simulation of a dye-sensitized TiO2 heterointerface “immersed” in an explicit water environment for an efficient organic dye, followed by TDDFT excited state calculations of the coupled dye/semiconductor/ solvent system. This new computational protocol and the extended model system allows us to gain unprecedented insight into the excited state changes occurring for the solvated dye-sensitized heterointerface at room temperature, and to provide an atomistic picture of water-mediated dye desorption

A well-scaling natural orbital theory

We introduce an energy functional for ground-state electronic structure calculations. Its variables are the natural spin-orbitals of singlet many-body wave functions and their joint occupation probabilities deriving from controlled approximations to the two-particle density matrix that yield algebraic scaling in general, and Hartree-Fock scaling in its seniority-zero version. Results from the latter version for small molecular systems are compared with those of highly accurate quantum-chemical computations. The energies lie above full configuration interaction calculations, close to doubly occupied configuration interaction calculations. Their accuracy is considerably greater than that obtained from current density-functional theory approximations and from current functionals of the one-particle density matrix.Comment: http://www.pnas.org/cgi/doi/10.1073/pnas.1615729113. arXiv admin note: text overlap with arXiv:1309.392

Why are MoS2 monolayers not a good catalyst for the oxygen evolution reaction?

We use density functional theory based calculations to study the energetics of the oxygen evolution reaction on a monolayer of MoS2. This material, a prototypical example of a layered transition metal dichalcogenide, is intensely studied in the context of many important catalytical applications, in particular for the hydrogen evolution reaction. The second half-reaction of the water-splitting process, the oxygen evolution reaction, is almost never considered on this material, due to its low activity. Based on our calculations, we explain this experimentally observed poor catalytic activity for the oxygen evolution by the weak binding of two key reaction intermediates (hydroxyl and hydroperoxyl) to the substrate. We explore substitutional doping with oxygen and phosphorous as means to facilitate the oxygen evolution on MoS2 layers. The oxygen substitution slightly increases the reaction´s overpotential, but does not significantly change the energetics. The doping with phosphorous, on the other hand, is not a promising way to promote the oxygen evolution on MoS2 layers. We also explore the role of the edges of MoS2 layers. We find that while the adsorption energies of reaction intermediates are strongly influenced by the presence of an edge, the final reaction overpotential remains nearly the same as on a pristine monolayer, meaning that the presence of edges is not favoring the OER.Fil: German, Estefania. Universidad de Valladolid; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Gebauer, Ralph. The Abdus Salam. International Centre for Theoretical Physics; Itali

A combined molecular dynamics and computational spectroscopy study of a dye-sensitized solar cell

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Density-functional perturbation theory goes time-dependent

The scope of time-dependent density-functional theory (TDDFT) is limited to the lowest portion of the spectrum of rather small systems (a few tens of atoms at most). In the static regime, density-functional perturbation theory (DFPT) allows one to calculate response functions of systems as large as currently dealt with in ground-state simulations. In this paper we present an effective way of combining DFPT with TDDFT. The dynamical polarizability is first expressed as an off-diagonal matrix element of the resolvent of the Kohn-Sham Liouvillian super-operator. A DFPT representation of response functions allows one to avoid the calculation of unoccupied Kohn-Sham orbitals. The resolvent of the Liouvillian is finally conveniently evaluated using a newly developed non-symmetric Lanczos technique, which allows for the calculation of the entire spectrum with a single Lanczos recursion chain. Each step of the chain essentially requires twice as many operations as a single step of the iterative diagonalization of the unperturbed Kohn-Sham Hamiltonian or, for that matter, as a single time step of a Car-Parrinello molecular dynamics run. The method will be illustrated with a few case molecular applications

Solution of the Bethe-Salpeter equation without empty electronic states: Application to the absorption spectra of bulk systems

An approach recently developed to solve the Bethe-Salpeter equation within density matrix perturbation theory is extended to the calculation of optical spectra of periodic systems. This generalization requires numerical integrations within the first Brillouin zone that are efficiently performed by exploiting point group symmetries. The technique is applied to the calculation of the optical spectra of bulk Si, diamond C, and cubic SiC. Numerical convergence and the accuracy of the Tamm-Dancoff approximation are discussed in detail

Turbo charging time-dependent density-functional theory with Lanczos chains

We introduce a new implementation of time-dependent density-functional theory which allows the \emph{entire} spectrum of a molecule or extended system to be computed with a numerical effort comparable to that of a \emph{single} standard ground-state calculation. This method is particularly well suited for large systems and/or large basis sets, such as plane waves or real-space grids. By using a super-operator formulation of linearized time-dependent density-functional theory, we first represent the dynamical polarizability of an interacting-electron system as an off-diagonal matrix element of the resolvent of the Liouvillian super-operator. One-electron operators and density matrices are treated using a representation borrowed from time-independent density-functional perturbation theory, which permits to avoid the calculation of unoccupied Kohn-Sham orbitals. The resolvent of the Liouvillian is evaluated through a newly developed algorithm based on the non-symmetric Lanczos method. Each step of the Lanczos recursion essentially requires twice as many operations as a single step of the iterative diagonalization of the unperturbed Kohn-Sham Hamiltonian. Suitable extrapolation of the Lanczos coefficients allows for a dramatic reduction of the number of Lanczos steps necessary to obtain well converged spectra, bringing such number down to hundreds (or a few thousands, at worst) in typical plane-wave pseudopotential applications. The resulting numerical workload is only a few times larger than that needed by a ground-state Kohn-Sham calculation for a same system. Our method is demonstrated with the calculation of the spectra of benzene, C$_{60}$ fullerene, and of chlorofyll a.Comment: 15 pages, 7 figures, to be pdflatex + bibte