Potential Dependence of Sensitization Currents

In the saturation range, all injected electrons are forced to move into the bulk and measured as a current. The current was found to be proportional to the light intensity. It should be emphasized that we have here a photocurrent caused by the injection of majority carriers, whereas an excitation within the semiconductor itself always leads to the transfer of minority carriers across a corresponding interface. The same results have been obtained with other semiconductor electrodes such as ZnO [28], Ti02, and Sn 2 [21, 22]. 

Sensitization Processes at Semiconductor Surfaces Modified by Dye Monolayers 

As already discussed in the previous sections, sometimes problems arise because it is difficult to distinguish between electron transfer reactions between the semiconductor electrode and adsorbed dye molecules or dye molecules in the solution. Therefore, various investigators have studied sensitization effects with 

In contrast to cyanine dyes and Ru complexes, fairly concentrated chlorophyll monolayers could be deposited on SnOj. In the latter case, the quantum yield of the photocurrent was determined as a function of the molar ration of chlorophyll 

10 Electron Transfer Processes - Excited Molecules and Semiconductor Electrodes 

Similar results were also obtained with p-type semiconductors, such as p-GaP (Fig. 10.12a) [20]. In this case, a cathodic sensitization process was observed which corresponds to a reduction of the excited dye via the valence band (see also excitation spec- 

As already shown by the spectrum in Fig. 10.8, a cathodic photocurrent was also observed with n-GaP, its potential dependence being shown in Fig. 10.12b. Since the UfY, of n-GaP was found at a rather negative electrode potential, the energy bands are bent upwards at potentials positive of Ufh. Accordingly, the hole injected from the excited dye, cannot move into the bulk of the n-electrode (see insert of Fig. 10.12b) and are transferred back to the reduced dye molecule. The cathodic photocurrent shows a maximum, i.e. it decreases again at high cathodic polarization. This decrease is due to a reduction of the dye in the dark. The sensitization of the n-GaP electrode is, of course, a minority carrier process. The injected holes recombine with the electrons in the bulk and the current is carried by electrons. Many investigations have shown, however, that more reliable results could be obtained with majority carrier systems. 

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