Sensitization Processes at Semiconductor Surfaces Modified by Dye Monolayers

4 Sensitization Processes at Semiconductor Surfaces Modified by Dye Monolayers 

10 Electron Transfer Processes - Excited Molectdes and Semiconductor Electrodes 

A very interesting example was published by Arden and Fromherz [34] who studied the performance of a multilayer carbocyanine dye structure. They used two types of cyanine dyes, A and D, incorporated into the multilayer structure as shown schematically in the upper part of Fig. 10.14. Dye A absorbs at around 420 nm and dye D around 370 nm. The photocurrent spectra presented in Fig. 10.14 exhibited the following features. If only one dye was used (curves I and II), the typical photocurrent was observed. In the case of curve I, the photocurrent was relatively low because the dye was separated from the electrode by an inert layer of arachidate molecules which inhibited the electron transfer process. If both dyes were used, in a structure given by III, the corresponding photocurrent exhibited a spectrum which was determined by both dyes. It should be noted that the spectrum of dye D was then much more pronounced compared with curve I. This effect was interpreted as energy transfer from D to A. The complete reaction scheme is then given by 

In contrast to cyanine dyes and Ru complexes, fairly concentrated chlorophyl monolayers could be deposited on Sn02- In the latter case, the quantum yield of the photocurrent was determined as a function of the molar ration of chlorophyl and stearic acid (concerning the definition of quantum yield see Section 10.2.5). An optimum was obtained for a 1 1 ratio, whereas the quantum yield dropped by a factor of nearly 3 for a pure chlorophyl layer [35]. This effect was interpreted as concentration quenching which cannot be further discussed here. The same authors have also investigated chlorophyl multilayers, all of which were deposited using the Langmuir- 

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