Photoinjection of electrons

Excess electrons can be introduced into liquids by absorption of high-energy radiation, by photoionization, or by photoinjection from metal surfaces. The electron s chemical and physical properties can then be measured, but this requires that the electrons remain free. That is, the liquid must be sufficiently free of electron attaching impurities for these studies. The drift mobility as well as other transport properties of the electron are discussed here as well as electron reactions, free-ion yields, and energy levels. 

The lifetime T and diffusion coefficient D of photoinjected electrons in DSC measured over five orders of magnitude of illumination intensity using IMVS and IMPS.56) fis proportional to the r m, indicating that the back reaction of electrons with I3 tnay be second order in electron density. On the other hand, D varied with C0 68, attributed to an exponential trap density distribution of the form Nt(E) <=< exp[ P(E - Ec)l(kBT) with 0.6. Since T and D vary with intensity in opposite senses, the calculated electron diffusion length L = (JD-z)m does not change linearly with the irradiance. 

For Class 2 dyes, absorption of a photon causes photoinjection of a hole into the valence band by transfer of an electron from the valence band to the now vacant Sq level of the excited molecule. The dye molecule retains the electron that has been excited to the level and is in effect a dye radical with an excess electron. In the absence of oxygen or other agent that could react with the radical, a thermally assisted transfer of the electron to the conduction band can occur. The time frame during which this transfer could occur is not limited by the normal lifetime of the excited state, as it is in the direct transfer of an electron in the Class 1 dyes. The time available could be much longer, limited only by the occurrence of some other reaction of the dye radical. 

The assumption that some dyes can spectrally sensitize latent image formation in silver halides by direct electron transfer from the excited dye to the conduction band and other dyes by indirect electron transfer from the dye radical formed by photoinjection of a hole into the valence band is in good accord with experiment. The locations of the highest filled and lowest vacant energy levels of the dye relative to the valence and conduction bands of the silver halide determine which mode of sensitization will occur, or whether both can occur. 

There are numerous mechanisms of charge photogeneration. The most common are the direct production of electron-hole pairs, exciton dissociation, and photoinjection from electrodes. 

Shafirovich et al. [71] have also investigated the migration of electrons photo-injected into DNA by two-photon excitation (335 nm) of pyrene derivatives covalently bound to ss and ds DNA. They find that in ds DNA the photoinjected electrons migrate to the acceptor methyl viologen (MV + Figure 6) which is boimd to the DNA within the c. 7-ns time resolution of the laser apparatus. From the dependence of the MV+ yield upon the MV + concentration, and the assumption of a random distribution of the pyrene donor and acceptor, they conclude that... 

Photochemically generated radicals undergo a subsequent photoinduced or thermal oxidation to the corresponding carbocation. Faria and Steenken produced the triphenylmethyl radical through two-photon ionization and subsequent decarboxylation of the triphenylacetate ion. Laser excitation of the triphenylmethyl radical resulted in photoinjection of an electron to give the triphenylmethyl cation and a solvated electron [Eq. (8)] [91]. 

The conversion of solar energy into electricity has been accomplished primarily by semiconductor photovoltaic devices. However, liquid junction photovoltaic cells have been developed.Dye-sensitized Ti02 nanoparticles and films are particularly important because of their potential application in liquid junction photovoltaic cells (Figure 22). In these the dye is bound to the surface of a semiconductor nanoparticle. The high surface area of the nanoparticle makes possible the adsorption of sufficient dye for efficient light collection. Dye sensitization can involve direct photoinjection of an electron from the dye to the nanoparticle... 

ZnO instead of T1O2 because ZnO provides a 220 times higher mobility for photoinjected electrons, which would allow reduction of the exciting laser intensity. The slow PMC decay of TiOrbased nanostructured sensitization solar cells (the Ru complex as sensitizer), which cannot be matched by a single exponential curve and is influenced by a bias illumination, is strongly affected by the concentration of iodide in the electrolyte (Fig. 38). On the basis of PMC transients and their dependence on the iodide concentration, a kinetic mechanism for the reaction of photoinjected electrons could be elaborated.40... 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

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