Photopotentials and Photocurrents

In the present chapter, the nature of the photopotential and the efficiency of photocurrents will be treated in more detail. 

An and Ap are the concentration changes at the edge between the bulk and space charge regions. To conserve electroneutrality, An = Ap. Since the total 

Large photovoltages of the order of the band gap can be obtained for heavily doped semiconductors (No or Na 10 cm ) for a large degree of band bending. The above equation can be approximated by 

They are valid for exhaustion or inversion layers as long as t/phl Ulc - 0.1 V. Above this value, t/ph saturates. According to Eq. (50), large photovoltages are already obtained at rather low intensities because the carrier density created by light excitation can easily exceed the minority carrier density po(n) in n-type or o(p) in p-type material. 

According to these results, it is in principle possible to determine the flatband potential (Section 2) by measuring the electrode potential of an Illuminated electrode under open circuit conditions. This method, as will be shown later, can lead, however, to rather inaccurate results. 

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Tien and co-workers [100, 238] observed a photopotential and photocurrent arising in the planar BLM containing bridging molecules with a porphyrin bound covalently to a quinone (see System 44 of Table 1). The size of this porphyrin-quinone complex was not large enough to span across the whole of the membrane, therefore the mechanism of the arising photoeffect is most likely to be similar to those discussed in Sect. 2.4 for other BLMs doped with photosensitizers. 

Photoexcited charge-transfer reactions for which the back charge transfer is rather slow, shifting the equilibrium of the system to the product side under illumination. This type of reaction is called a photoredox reaction [89,90], and a typical example is the reaction of the photoexcited state of thionine (dye) with Fe as an electron acceptor shifting the equilibrium to the product side (the color of the aqueous solution is violet in the dark and colorless under illumination, and this color change is reversible). For this kind of reaction couple, when two electrodes are dipped in the solution of the reaction couple, a photopotential and photocurrent can be induced by introducing an asymmetric factor in the cell. Such an as)mimetric factor is, for instance, two different electrodes or different illumination of the electrodes (e.g. illumination at one electrode while the other electrode is kept in the dark). 

J. van de Lagemaat, N. G. Park, A. J. Frank, I nfluence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline Ti02 solar cells a study by electrical impedance and optical modulation techniques, J. Phys. Chem. B 2000, 104(9), 2044-2052. 

Finally, in terms of pH of the PG system, it has been found that photopotential and photocurrent increase with increase in pH up to a certain maximum, after which it decreases at a similar rate. Several research articles, including Gangotri and Gangotri [15], Genwa and Chouhan [32], and Dube et al. [29], have observed that the pH for the optimum condition is related to the pXa of the reductant, with the pH being equal to or slightly higher than the pXa of the reductant. The aforementioned authors cite a possible reason for this as the availability of the reductant in its neutral or anionic form - a superior electron donor. 

De Armond et a/. have studied systems involving the quenching of excited [Ru(bpy)3] in the organic phase by methylviologen in water. However, because in these systems the photoproducts are more hydrophobic or hydro-philic than their respective reactants, the quantitative interpretation of the observed photopotentials and photocurrents was difficult. Furthermore, these studies were carried out at the water-nitrobenzene interface, and the possibility of nitrobenzene reduction by [Ru(bpy)3] was never considered in the analysis. 

When CdSe was in contact with the solid electrolyte containing iodine redox species, negative photopotentials and anodic photocurrents were detected. This is analogous to the behavior of n--type semiconductor in liquid-junction solar cells. In the case of solid... 

It can be seen from Tables 6 and 7 that, in general, PG systems containing a surfactant produced higher relative values for photocurrent, photopotential, and conversion efficiency than systems studied in which a surfactant was absent. Fendler and Fendler [39] and Atwood and Florence [40] have attributed this to the ability of a surfactant to solubilize certain molecules (i.e., the photosensitizing dye) and the catalytic effect that carefully chosen surfactants induce on particular chemical reactions. Furthermore, Rohatgi-Mukherjee et al. theorized that addition of a surfactant into a PG system increases conversion efficiency via facilitating the separation of photogenerated products by hydrophobic-hydrophilic interaction of the products with the surfactant interface [27]. 

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

The experiments were performed with single crystal (111) p-Si electrodes with a resistivity of about 5.5 ohm cm non-aqueous electrolytes were used consisting of absolute methanol containing tetramethylammonium chloride (TMAC) or acetonitrile containing tetraethyl ammonium perchlorate (TEAP). The flat-band potentials or p-Si in the two electrolytes were determined from Mott-Schottky plots (in the dark) in the depletion range of the p-Si electrode, from open-circuit photopotential measurements, and from the values of electrode potential at which anodic photocurrent is first observed in n-type Si electrodes. These three methods all yielded consistent flat-band potential values for p-Si of + 0.05V (vs SCE)... 

In photoemission experiments one measures either the photocurrent or the photopotential when an electrode is placed in a solution and is illuminated by light that the solution does not absorb. The photocurrent of emitted electrons depends on the every ho) and the potential

[Pg.335]

Below, we discuss one more (photoelectrochemical) method for determination of the flat-band potential [40, 172]. The flat-band potential can be determined (i) as the photocurrent onset potential Eomsi (ii) from the dependence of electrode open-circuit photopotential Eocph on light intensity J, as the limiting value of Eoc at a sufficiently high J and (iii) by extrapolating, to zero photocurrent, the potential dependence of the photocurrent jvh squared (see Section 7). These methods are based on the concept... 

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

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