Photoanodic current, semiconductor

Figure 25. Schematic illustration of the transient profiles that would be seen under interrupted illumination as the potential is slowly scanned in the reverse bias regime. The situation as shown is for an n-type semiconductor-electrolyte interface. is the potential for the onset of photoanodic current flow.

Ey is the photovoltage obtained for the derivatized n-type semiconductor photoanodes. We assume E° to be the values given in brackets and Ey is the extent to which the peak of the photoanodic current is more negative than E° under >Eg illumination. Data are from references given in (a). cWe assume E° to be the same on the n-type semiconductors as on metallic electrodes but these values have not been measured, since the n-type semiconductors generally are not reversible. 

Figure 9.20 i-v curve for semiconductors/iedox junction in dark (dotted line) and on iUuniination (hard line) for (a) n-type semicondnctor, which on iUnmination leads to photoanodic current, and (b) p-type semiconductor, which on iUnmination leads to photocathodic current. The onset for photocurrents in both the diagrams suggests the flat-band potential, fl>-... 

This chapter considers photoanodes comprised of metal oxide semiconductors, which are of relatively low cost and relatively greater stability than their non-oxide counterparts. In 1972 Fujishima and Honda [1] first used a crystal wafer of n-type Ti02 (rutile) as a photoanode. A photoelectrochemical cell was constructed for the decomposition of water in which the Ti02 photoanode was connected with a Ft cathode through an external circuit. With illumination of the Ti02 current flowed from the Ft electrode to the... 

To compare quantitatively the current-voltage characteristic of an illuminated electrode, given by formula (31), with experimental data, Butler (1977) and Wilson (1977) measured the photocurrent, which arises in a cell with an n-type semiconductor photoanode ( 2, W03) when irradiated with monochromatic light at a frequency satisfying the condition ha>> Eg. In this case a light-stimulated electrochemical reaction of water oxidation with oxygen evolution... 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

However, the last few years have also seen a growing awareness of the problems inherent in using the semiconductor-electrolyte interface as a means of solar-energy conversion. Very long-term stability may not be possible in aqueous electrolytes and no oxide material has been identified that has properties suitable for use as a photoanode in a photoelectrolysis cell. Highly efficient photovoltaic cells are known, both in aqueous and non-aqueous solutions, but it is far from clear that the additional engineering complexity, over and above that required for the dry p-n junction photovoltaic device, will ever allow the "wet photovoltaic cells to be competitive. These, and other problems, have led to something of a pause in the flood of papers on semiconductor electrochemistry in the last two years and the current review is therefore timely. I have tried to indicate what is, and is not, known at present and where future lines of development may lie. Individual semiconductors are not treated in detail, but it is hoped that most of the theoretical strands apparent in the last few years are discussed. 

---