Charge transfer state mediated

The Charge Transfer State Mediated Sensitisation Process... 

In Refs. [50-53], two levels of analysis were successively addressed (i) a two-state XT-CT model which is able to capture the basic features of the phonon-mediated exciton dissociation process (ii) a three-state XT-IS-CT model which also comprises an intermediate state (IS), i.e., an additional charge transfer state whose presence can have a significant influence on the dynamics, see Fig. 6. In the latter case, comparative calculations for several interface configurations were carried out, leading to a realistic, molecular-level picture of the photophysical events at the heterojunction. In the following, we start with a summary of the findings reported in Refs. [50,51], where the two-state model was explored (Sec. 5.1). Following this, we address in more detail the analysis of Refs. [52,53] for the three-state model (Sec. 5.2). 

A further aspect, not yet treated in detail for organic solar cells, is the role of intermediate charge-transfer states, such as bound polaron pairs, in mediating charge separation and recombination rates (Morteani et al, 2004). 

Mediators may allow for coulometry to be carried out when an analyte is not itself electroactive. Such mediators are often aromatic organic molecules which can interconvert reversibly between several oxidation states. The mediator is itself electromodiiied at an electrode, and then effects a chemical redox charge to the analyte in the same solution. Coulometry is then possible, provided that the transfer of charge from the mediator to the analyte is 100% efficient. 

Surface states can form due to abrupt distortion of the semiconductor crystal lattice. Charge transfer processes between surface states and the electrolyte have been analyzed in relation to water photoelectrolysis application [96]. Electron transfer mediated through surface states for an n-type semiconductor under dynamic equilibrium is shown in Fig. 3.13(d). 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

UPS spectra of clean Ar+ sputtered and in Vacuo carbon-contaminated surfaces are shown in Figure 4. On the clean, sputtered surface a filled state due to Ti3+ lies 0.6 eV below the conduction band.(22) Carbon-induced filled states lie in a broad peak with considerable intensity between the valence band edge and the Ti + peak. Frank et al.lQ) reported evidence that a state lying about 1.2 eV above the valence band mediates electron transfer from Ti02 electrodes. Although these carbon states are as of now poorly defined and have not been directly implicated in any aqueous photochemistry, their nearly ubiquitous presence should be considered in discussions of charge transfer at real oxide surfaces. 

---