In dye sensitized solar cells, three structurally similar dyes are commonly employed to sensitize anatase nano-crystals, namely the cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II) dye (N3), its twice deprotonated (N719) and completely deprotonated (N712) form. Using density functional theory, several possible binding geometries of these dyes are identified on the anatase(101) surface. Computed relative energies show that protonation of the surface can strongly influence the relative stabilities of these configurations, and could induce a conformational transition from double bidentate-bridged binding to mixed bidentate/monodentate binding. Attenuated total reflection (ATR)-IR experiments and computed vibrational spectra provide additional support for a protonation dependent equilibrium between two different configurations. Furthermore, self-assembly in chains of hydrogen bonded dye molecules seems structurally favorable on the anatase(101) surface, for enantiopure dyes a packing density of 0.744/nm 2 could be achieved.

The synchronous combination of time resolved (energy dispersive) EXAFS, diffuse reflectance infrared spectroscopy (DRIFTS), and mass spectrometry (MS), applied within the general framework of concentration modulation spectroscopy, is demonstrated for in situ and time-resolved study of the behaviour of Rh/Al(2)O(3) and Pd/Al(2)O(3) catalysts during CO/NO redox cycling at 573 K. We show that by applying a phase sensitive detection technique the quality of information arising from the experiment is significantly improved. Moreover, in the case of the dispersive EXAFS the acuity of this technique is greatly enhanced and a surface sensitivity in the normally bulk sensitive EXAFS measurement can be induced. Lastly we apply this approach to a system (0.3 wt% Rh/Al(2)O(3)) that cannot normally be studied in any meaningful way with transmission based and highly time resolved EXAFS, and show that this method may provide a novel experimental window through which it is possible to restore highly time resolved structural-kinetic information from previously intractable systems.

A highly efficient mechanism for the regeneration of the cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium (II) sensitizing dye (N3) by I − in acetonitrile has been identified by using molecular dynamics simulation based on density functional theory. Barrier-free complex formation of the oxidized dye with both I − and I − 2 , and facile dissociation of I − 2 and I − 3 from the reduced dye are key steps in this process. In situ vibrational spectroscopy confirms the reversible binding of I 2 to the thiocyanate group. Additionally, simulations of the electrolyte near the interface suggest that acetonitrile is able to cover the (101) surface of anatase with a passivating layer that inhibits direct contact of the redox mediator with the oxide, and that the solvent structure specifically enhances the concentration of I − at a distance which further favors rapid dye regeneration.density functional theory | molecular dynamics simulations | photovoltaics | solid/liquid interfaces | statistical mechanics T he basic design of today's high performance dye sensitized solar cells (DSSC) was developed in the early 1990's by Grätzel et al. (1,2). The photoactive part of these devices consists of a wide band gap semiconductor covered by a monolayer of sensitizing dye. The semiconductor is directly supported by a transparent electrode on one side, while the dye is connected to the back electrode via a liquid electrolyte or a solid hole conducting material. The initial step of the photovoltaic process is a light induced electron injection from the dye into the semiconductor material. This process yields an oxidized dye and an energetic electron. Rapid regeneration (reduction) of the dye by the electrolyte prevents back transfer of the electron or degradation of the photo-oxidized dye (3). Meanwhile, the energetic electron diffuses away from the dye, passing through the electrode and an external load, finally reaching the counter electrode where it regenerates the electrolyte.The class of devices with the highest light to current conversion efficiency (above 11%) (4, 5) is based on sintered nanocrystalline anatase as the semiconducting oxide, Ruthenium polypyridyl dyes as sensitizer, and the iodide/triidiode redox couple dissolved in a nitrile containing organic solvent as electrolyte. DSSC using organic dyes, solid state electrolytes, different semiconductors, or redox couples do not match this performance but expose other desirable properties for the commercial use of this technology. Optimizing the various components of the oxide/dye/ electrolyte interface, the efficiency and stability of the systems can be improved and its cost reduced. A detailed knowledge of the interfacial structure and key reaction mechanisms in high performance DSSC is, therefore, essential to guide rational design of improved devices. Numerous experimental and theoretical studies have already led to a deeper understanding of this system, but several basic questions are still the subject of active research. In this study, we propose a...