Photoexcitation energy equivalence

The quasi-Fermi level of interfacial holes nearly equals the Fermi level pe , ( pEp,.) in photoexcited p-type electrodes, but the quasi-Fermi level pej. of interfacial holes is lower than the Fermi level aSp,. (> p p,) in photoexcited n-type electrodes as shown in Fig. 10-21. It then follows that the range of electrode potential, where the anodic reaction occurs on the photoexcited n-type electrode, shifts itself, from the range of potential where the same anodic reaction occurs on the dark p-type electrode, toward the caliiodic (more negative) direction by an energy equivalent to (nEp - p p,). 

When the n-type semiconductor anode is photoexcited, as shown in Fig. 10-25(c), the Fermi level of the anode is raised (the potential of the anode is lowered) by an energy equivalent to the photopotential at the same time, the Fermi... 

Discussion of Photoelectron and Photofragment Images. The simplest picture for photoexcitation of a molecular Rydberg state would be that of a vertical transition (Av = 0), producing only O2, X(2Ilg)(t = 2) (direct ionization) in the example case. Here electronic motion (ionization) is assumed to be much faster than nuclear motion (dissociation). 02 is much more complicated, of course, and some of the deviations from the simplistic picture could be due not only to the molecule but also to the unconventional three-photon preparation scheme. It is thus important to consider the differences in one-photon and stepwise (2 + 1) excitation. Even with direct one-photon excitation at the energy equivalent of three laser photons, it is known, [78] for example, that the quantum yield for ionization is only 0.5 the other half of the molecules do, in fact, dissociate. 

Upon illumination, photons having energy higher than the band gap (eg = ec — v) are absorbed in the semiconductor phase and the electron-hole-pairs (e //i+) are generated. This effect can be considered equivalent to the photoexcitation of a molecule (Fig. 5.57) if we formally identify the HOMO with the ec level and LUMO with the v level. The lifetime of excited e //i+ pairs (in the bulk semiconductor) is defined analogously as the lifetime of the excited molecule in terms of a pseudo-first-order relaxation (Eq. 5.10.2). 

The effect of wavelength on the (piantum yield of CO production from the photolysis of CII2CO can be satisfactorily interpreted on the basis of the RE.K model, but unfortunately the lack of data on the precise bond dissociation energy prevents a uiiiciuc assignment of parameters. The wavelength dependence of the fluorescence of photoexcited /3-naphthyl-amine has also been reasonably well interpreted in terms of the rate of spontaneous isomerization to a inetastable state incapable of fluorescence. A model for k E) equivalent to the RRK model was used. 

The equivalent circuit diagram used to model solar cell current-voltage characteristics is shown at the top of Figure 1.1. The schematic energy level diagram of a DSSC at the bottom of Figure 1.1 shows the various charge transfer processes that occur in photoelectrochemical cells and relates these processes to current pathways via components of the model circuit. An illumination current density /l is induced upon photoexcitation of the... 

Figure 1.1 Simple equivalent circuit (top) for modeling solar cell current-voltage characteristics and energy level diagram (bottom) mapping the various charge transfer processes in a DSSC to the current pathways of the model circuit. The dominant mechanisms are described by a current density Jl induced upon photoexcitation and electron injection into the conduction band of the metal oxide semiconductor surface MO, linear (Jsh) and nonlinear (/jj) reverse current densities in parallel with photocurrent source and a series resistance to account for electrode and ionic resistances. In Section 1.2.2 M0 = Ti02, Sn02, X = Br, I.

---