Resonant Lifetime of Core-Excited Organic Adsorbates from First Principles

We investigate by first-principles simulations the resonant electron-transfer lifetime from the excited state of an organic adsorbate to a semiconductor surface, namely isonicotinic acid on rutile TiO$_2$(110). The molecule-substrate interaction is described using density functional theory, while the effect of a truly semi-infinite substrate is taken into account by Green's function techniques. Excitonic effects due to the presence of core-excited atoms in the molecule are shown to be instrumental to understand the electron-transfer times measured using the so-called core-hole-clock technique. In particular, for the isonicotinic acid on TiO$_2$(110), we find that the charge injection from the LUMO is quenched since this state lies within the substrate band gap. We compute the resonant charge-transfer times from LUMO+1 and LUMO+2, and systematically investigate the dependence of the elastic lifetimes of these states on the alignment among adsorbate and substrate states.Comment: 24 pages, 6 figures, to appear in Journal of Physical Chemistry

Hackmanite-The Natural Glow-in-the-Dark Material

"Glow-in-the-dark" materials are known to practically everyone who has ever traveled by airplane or cruise ship, since they are commonly used for self-lit emergency exit signs. The green afterglow, persistent luminescence (PeL), is obtained from divalent europium doped to a synthetic strontium aluminate, but there are also some natural minerals capable of afterglow. One such mineral is hackmanite, the afterglow of which has never been thoroughly investigated, even if its synthetic versions can compete with some of the best commercially available synthetic PeL materials. Here we combine experimental and computational data to show that the white PeL of natural hackmanite is generated and controlled by a very delicate interplay between the natural impurities present. The results obtained shed light on the PeL phenomenon itself thus giving insight into improving the performance of synthetic materials

Optimisation des cellules solaires à colorants à base de ZnO par une approche combinée théorie/expérience

During the last decade, it has been proven that dye-sensitized solar cells have been an economically viable photovoltaic technology. Despite numbers of studies realized in that field, we have to admit that photoconversion efficiencies of such devices are still under 12% on that type of systems. In this context, all works performed along this thesis was thus focused on the optimization of the performances of dyes-sensitized solar cells. To reach this objective, an approach joining theory and experiments was developed. From calculations based on Density Functional Theory (DFT), a new family of dyes, characterized by the presence of a pyridinium fragment, was studied in order to define and identify molecules best adapted to generate a significant photocurrent. Combining, a molecular and a periodic approach, calculations allowed to understand the different mechanisms involved in the solar cell working principle permitting the optimization of some constituents of the cell such as the electrolyte composition and the dye molecules. An experimental protocol for the production and characterization of solar cells based on ZnO as semiconductor in a laboratory scale was also developed during this thesis. Analogously, the new dyes studied at theoretical level were also experimentally synthesized and characterized. Overall, the experimental conception and characterization of solar cells using new dyes allowed to validate the theoretical protocol and paves the route for an ab initio optimization of dye-sensitized solar cells.Cette dernière décennie a montré que les cellules solaires à colorants étaient une technologie photovoltaïque économiquement viable. Malgré les nombreuses études réalisées dans ce domaine, force est de constater que les rendements de photoconversion n'ont toujours pas dépassé 12% avec ce type de cellule. Les travaux réalisés au cours de cette thèse s'inscrivent dans une optique d'optimisation des cellules solaires à colorants. Pour y parvenir, une approche joignant la théorie et l'expérience a été développée. Par des calculs basés sur la Théorie de la Fonctionnelle de la Densité (DFT), une nouvelle famille de colorants, caractérisée par la présence d'un groupement pyridinium, a été étudiée afin d'en choisir les membres les plus aptes à générer un photocourant. En combinant une approche moléculaire et périodique, les calculs ont permis de comprendre différents mécanismes intervenant dans le fonctionnement de la cellule solaire conduisant à une optimisation théorique de certains constituants de la cellule comme la composition de l'électrolyte ou le colorant. Parallèlement aux calculs, une méthodologie de construction et de caractérisation des cellules basée sur l'utilisation de ZnO comme semiconducteur a été mise en place au sein du laboratoire. La synthèse de ces nouveaux colorants a aussi été réalisée au cours de ce travail de doctorat. La conception et la caractérisation expérimentales de cellules utilisant ces colorants a permis de valider le protocole théorique développé ouvrant la voie à une optimisation ab initio des cellules solaires à colorants

Assessing the Use of BiCuOS for Photovoltaic Application: From DFT to Macroscopic Simulation

International audienceThe objectives of the work presented in this article are two folds. First it presents computed properties of the semiconductor BiCuOS at the HSE06+spin–orbit coupling level and these properties are interpreted from the composition of the material point of view and are analyzed from a photovoltaic perspective. The calculated properties (Eg = 1.22 eV, εr = 36.2, me* = 0.42, mh* = 0.33, Eb = 2 meV) illustrate that BiCuOS is a promising material for photovoltaic application. The second objective is to presents a multiscale approach whose objective is to simulate photovoltaic macroscopic characteristics (Jsc, Voc, FF, ...) from microscopic properties computed at the DFT level. The approach is first tested in the CuInS2 solar cell, that has several similarities with BiCuOS, allowing to determine the strengths and limits of this approach. Then, this protocol is applied to BiCuOS solar cells in order to determine the best n-type semiconductor to put in contact with BiCuOS to achieve high photoconversion efficiencies. The results allow to dress the list of the drawbacks that must be overcome to use this material for photovoltaic application. Beyond BiCuOS, this protocol can be used by the community wanting to use modeling to design and characterize new semiconductors beyond the bandgap calculation

Effect of solvent and additives on the open-circuit voltage of ZnO-based dye-sensitized solar cells: a combined theoretical and experimental study

International audienceWe have investigated the role of electrolyte composition, in terms of solvent and additive, on the open-circuit voltage (Voc) of ZnO-based dye-sensitized solar cells (DSSCs) using a combined experimental and theoretical approach. Calculations based on density functional theory (DFT) have been performed in order to describe the geometries and adsorption energies of various adsorbed solvents (nitromethane, acetonitrile and dimethylformamide) and p-tert-butylpyridine (TBP) (modeled by methylpyridine) on the ZnO (100) surface using a periodic approach. The densities of states (DOS) have been calculated and the energy position of the conduction band edge (CBE) has been evaluated for the different molecules adsorbed. The effect of the electrolyte composition on the standard redox potential of the iodide/triiodide redox couple has been experimentally determined. These two data values (CBE and standard redox potential) allowed us to determine the dependence of Voc on the electrolyte composition. The variations determined using this method were in good agreement with the measured Voc for cells made of electrodeposited ZnO films sensitized using D149 (indoline) dye. As in the case of TiO2-based cells, a correlation of Voc with the donor number of the adsorbed species was found. The present study clearly points out that both the CBE energy and the redox potential variation are important for explaining the experimentally observed changes in the Voc of DSSCs

Kinetics of surface dissolution: A coupled thermodynamics-climatic approach for Titan and the Earth

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The nature of vertical excited states of dyes containing metals for DSSC applications: insights from TD-DFT and density based indexes

International audienceTransition metal complexes, typically Ru-based complexes, are the most efficient dyes used in dye-sensitized solar cells. The absorption spectra of these molecules generally involve numerous electronic transitions, which are not equivalent for the conversion of the light into electricity. In the present manuscript, an analysis of each electronic transition of selected inorganic complexes is performed based on the variation of the electronic density upon light absorption. To this end, a series of indices recently proposed in the literature is applied. The main conclusions of this work are twofold: from a methodological point of view, global hybrid functionals confirm their robustness for studying the electronic transitions of these compounds and from an application oriented point of view it is clear that the most intense transitions are not necessarily the most efficient ones for the light conversion

Karstification processes on Titan and on Earth: Denudation rates and timescales

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