Cathodic photocurrents, short-circuit

Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents in addition to corresponding positive and negative photovoltages at solvent-evaporated films of a Chl-oxidant mixture and a Chl-reductant mixture, respectively, on platinum electrodes. Various redox species were examined, respectively, as a donor or acceptor added in an aqueous electrolyte (69). In a typical experiment (71), NAD and Fe(CN)g, each dissolved in a neutral electrolyte solution, were employed as an acceptor for a photocathode and a donor for a photoanode, respectively, and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)J at a Chl-anthrahydroquinone-coated anode were performed under either short circuit conditions or potentiostatic conditions. The reduction of NAD at the photocathode was demonstrated as a model for the photosynthetic system I. In their studies, the photoactive species was attributed to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism for photocurrent generation at the Chi photocathode (71). 

Another type of Chi interfacial layer employed on a metal electrode was a film consisting of ordered molecules. Villar (79) studied short circuit cathodic photocurrents at multilayers of Chi a and b built up on semi-transparent platinum electrodes in an electrolyte consisting of 96% glycerol and 4% KCl-saturated aqueous solution. Photocurrent quantum efficiencies of multilayers and of amorphous films prepared by solvent evaporation were compared. The highest efficiency (about 10 electrons/ absorbed photon, calculated from the paper) was obtained with Chi a multilayers, and the amorphous films of Chi a proved to be less efficient than Chi b multilayers. 

Under short-circuit conditions the cathodic photocurrent was observed for the photoelectrochemical cell the quantum yield was ca 0.5 % [309, 318, 332]. The photocurrent intensity in the free-base porphyrin-Ceo system was enhanced fivefold compared with that in the corresponding free-base porphyrin monomer system. Such enhancement of the photocurrent demonstrates that C6o acts an effective mediator in a sequential electron-transfer process. The photocurrent intensity in the free-base porphyrin-Ceo cell was ca one order of magnitude larger than that in the zinc porphyrin-Cfio cell [309, 318, 332]. Two different electron-transport mecha-... 

The author with co-workers [39] has investigated systems with the structures Al/Cso/PANI+CdS/ITO and Al/Cgo/PPV-l-CdInS/ITO. Most worthy of note are the dependencies of the short circuit current and the open circuit voltage on the nanoparticles concentration (Fig. 21). Since the fullerene molecule acts as a strong electron acceptor, excitons generated both in the polymer matrix and in the CdS particles are decomposed, electrons are accepted by the Ceo layer and the holes are transported to the anode through the polymer. Once the concentration of the nanoparticles exceeds the percolation threshold value, the system becomes short contacted, since the electrons can pass from the anode to the cathode through the barrier-free connected network of CdS clusters this latter fact leads to the disappearance of the photovoltaic effect, as illustrated in Fig. 21b. The increase in the photocurrent and the open circuit... 

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