Photocurrents transients

More recently, a series of papers based on photocurrent responses involving water soluble porphyrin species has allowed to address the various aspects involved in the mechanism of Fig. 11 [50,73,83]. Photocurrent transients at the water-DCE interface in the presence of zinc tetrakis(carboxyphenyl) porphyrin (ZnTPPC" ) and Fc under monochromatic light are shown in Fig. 15 at various Galvani potential differences [50]. The... 

The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

Fig. 6 Electron time-of-flight photocurrent transients of solution-cast film of EHO-OPPE (L=8 pm), measured at 295 K and an electric field of 2.5-10 V cm in (a) linear and (b) double logarithmic plots. Reproduced with permission from [61]...

Typical photocurrent transients are shown in Fig. 6 for electrons and in Fig. 7 for holes. The shape of these curves is representative for all transients observed in the study and is characteristic of dispersive transport [64-68]. The carrier mobility p was determined from the inflection point in the double logarithmic plots (cf. Fig. 6b and Fig. 7b) [74]. TOF measurements were performed as a function of carrier type, applied field, and film thickness (Fig. 8). As can be seen from Fig. 8, the drift mobility is independent of L, demonstrating that the photocurrents are not range-limited but indeed reflect the drift of the carrier sheet across the entire sample. Both the independence of the mobility from L, and the fact that the slopes of the tangents used to determine the mobility (Fig. 6 and Fig. 7) do not add to -2 as predicted by the Scher-Montroll theory, indicate that the Scher-Montroll picture of dispersive transients does not adequately describe the transport in amorphous EHO-OPPE [69]. The dispersive nature of the transient is due to the high degree of disorder in the sample and its impact on car-... 

As expected, the coordination of Pt markedly influences the photophysical characteristics of the PPE. The photoluminescence is efficiently quenched, and the absorption maximum in the visible regime experiences a hypsochromic shift. The charge-carrier mobility of different EHO-OPPE-Pt samples was determined by TOE measurements as described above for the neat EHO-OPPE. The shape of the photocurrent transients of all EHO-OPPE-Pt samples was similar to those shown in Figs. 6 and 7 for the neat EHO-OPPE. This indicates that these organometallic conjugated polymers networks are also characterized... 

The limiting role of the diffusion in case of Co(II) electrolytes could be further investigated by recording photocurrent transients under strong illumination (ca. 

Figure 17.34 Photocurrent transients recorded on (a) Co(DTB)32 +-mediated DSSC. Electrolyte composition Co(II) — 0.15 M, Li+ -0.5M, and Tbpy = 0.1M. (h) Lil/I2 0.3/0.03 M 0.1 M Tbpy in acetonitrile. From Bignozzi et al., unpublished results.

Figure 17.35 Photocurrent transients obtained with (a) spacer equipped cells and (b) with-...

Fig. 33 Photocurrent (transient maxima) versus light intensity for PTS-polydiacetylene single crystals. (After Reimer and Bassler, 1975)...

Salvador, P. 1985. Kinetic approach to the photocurrent transients in water photoelectrolysis at n-Ti02 electrodes. 1. 

The photocurrent responses of nanocrystalline electrodes to stepped or pulsed illumination exhibit features on rather slow timescales. This is illustrated, for example, by Fig. 8.26, which is a set of photocurrent transients reported by Solbrand et al. [78] for band-band excitation at 308 nm of nanocrystalline Ti02 films of differing thicknesses permeated by 0.7 mol dm-3 LiC104 in ethanol. The 30 ns excimer laser pulse was incident from the solution side, and since the penetration depth of the light was much smaller than the film thickness, electron-hole pairs were effectively... 

Fig. 8.26. Experimental photocurrent transients for pulsed excimer laser excitation of nanocrystalline Ti02 electrodes of differing thicknesses taken from Ref. [78], Illumination from the electrolyte side (200 mJ, 30 ns, A 308 nm). Electrolyte 0.7 mol dm- 1 LiCI04 in ethanol. The insert shows that time rpcuk at which the current peak occurs depends on the square of the film thickness (VV), as expected for diffusion controlled electron transport.

Figure 8.27 illustrates the theoretical electron density profiles and photocurrent transients calculated by Solbrand et al. The transients exhibit a maximum at a time fpeak = d2/6D. The inset in Fig. 8.26 shows that a plot of fpeak VS. d2 is linear as predicted (the authors use W rather than d to denote the film thickness), and the slope of the plot gives a value of 1.5 x 10-scm-2s-1 for the electron diffusion coefficient. 

It is clear that the observed photocurrent transient is the composite of two effects diffusion of the particles to the electrode and the pseudo-first order decay of the photo-generated electrons. In a subsequent communication, this technique will be taken one step further and used to obtain rate constants for dynamic and static electron transfer from the particles to a redox active species deliberately introduced into the system. 

Figure 94 (a) The SCL transient currents for various normalized trapping times (R = Ttrap/t0) as calculated from theory (see Ref. 26) R = oo denotes the trap-free case is the steady-state current without trapping, (b) t trap-free SCL transient current injected from ITO under a positive step voltage applied to an IT0/PPV/TPD PC/A1 device jScl corresponds to in part (a). Bottom TOF photocurrent transient for holes generated by a light pulse at the A1/(TPD PC) interface (the negative polarity applied to ITO). (From Ref. 428). 

In addition to a.c. techniques as described above, photocurrent transients have also been investigated extensively as a means of providing additional evidence in favour of models of the sort discussed above. The most typical type of behaviour, observed at relatively long time constants ( 100 ms),... 

Fig. 79. Photocurrent transients with white light for n-MnTi03 in 1 MNaOH. Flat-band potential is ca. - 1.0 V/SSE and the dark-current density is shown. It can be seen that the transients become essentially square for V - VjJ 1 V.

Inverse photocurrent transients are also of interest. Clearly, in cases where a surface state or surface-adsorbed species has been charged during illumination, a transient current of sign opposite to the photocurrent is expected when the light is switched off. The time constant for such as transient will not contain the light intensity as a parameter thus, for the redox case discussed above, we have... 

Under these circumstances, the height of the transient will depend on the light intensity and the decay constant again only on the electron concentration at the surface. As an example of this behaviour, consider the photocurrent transients for depopulation of a set of surface states on p-GaP, which are shown in Fig. 80 [152]. The statistics are poor but show an approximate linear dependence of the reverse transient on light intensity, 0, and also... 

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