Photoinjection

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... 

Charge mobilities determined by the time of flight (TOF) method show that values of 1-10 cm s are not rare for MOMs (Karl et al, 1999). In TOF experiments the transit time r for a sheet of photoinjected carriers to move across a sample of thickness L is monitored. The sheet of carriers is usually generated... 

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 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. 

The final value of the sample voltage measured after photoexcitation. This is known in the literature as the residual voltage and is due to deeply trapped photoinjected carriers when they transit across the sample. If the sample is repeatedly charged-discharged, the residual voltage builds up with the number of cycles and saturates. In this case, one can use the saturated residual voltage and extract information about the deep traps. 

Reducing agents, such as hydroquinone, Br , I, SCN- ions, and others, are used as supersensitizers for electron photoinjection reactions. Figure 28 illustrates how an admixture of hydroquinone affects photocurrent in the system CdS—rhodamine B. In certain cases the solvent, water, can also act as a supersensitizer. For example, if the bipyridyl complex of Ru(II) is a sensitizer, the oxidized form, the complex of Ru(III), can oxidize water to oxygen... 

In the conventional DSSC, this reaction is extremely rapid, occurring in the subpicosecond regime [19-23] and often exhibits near-unit efficiency. Note that the electron and the hole (D + at first, and then R +) are created on opposite sides of the interface by the photoinjection process They never coexist in the same phase and are already separated from each other upon creation. This fundamental... 

The photoinjection reaction is followed by regeneration of the dye by the redox species, R ... 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

The photoconductivity of poly-p-xylylenes were investigated under various conditions [218-221]. The fundamental edge absorption of the polymers is at 300 nm. The photocurrents in the visual range of the spectra depended on the electrode nature. So it was interpreted as photoinjection of the charges from electrodes and separation of them at a Schottky type barrier. Suppression of hole injection for the plasma-treated polymer is related to the existence of an oxidized surface layer. 

It was pointed out earlier that photosensitivity may be realized not only by the inclusion of dyes in a polymer matrix, but also by means of multilayered system production. For TPA dispersed in a polycarbonate, sensitized by a thin layer of vacuum-deposited amorphous selenium, the quantum yield was equal to 0.7 at the electric field strength 6 x 10s V cm-1 [308]. The photoinjection efficiency of holes into polymer was equal to the efficiency in pure selenium. The spectra of the quantum efficiency is shown in Fig. 54. 

Fig. 54. Spectrum of the photoinjection efficiency from selenium layer into polycarbonate film with 35% content of triphenylamine [308]...

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. 

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