Photocurrent effects

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

A role is also played by the temperature and frequency dependence of the photocurrent, the variable surface sensitivity at various parts of the cathode and the vector effect of polarized radiation [40]. All the detectors discussed below are electronic components whose electrical properties vary on irradiation. The effects depend on external (photocells, photomultipliers) or internal photo effects (photoelements, photodiodes). 

These Schottky energy barriers are measured in the presence of an electric field in the structure which is necessary to be able to collect the photocurrent. The photocurrent thresholds are not the zero electric field Schottky barriers because of the electric field in the polymer and the image chaise potential created when the electron leaves the metal. This effect results in a lowering of the Schottky energy barrier given by [34]... 

Figure 15. Effect of interfacial rate constants on PMC behavior and on the photocurrent (/0 = 1 cm-2), (a) Fast interfacial charge-transferrate, and (b) low interfacial charge-transfer rate.

Figure 25. Effect of corrosion and prepolarization on (a) PMC voltage and (b) photocurrent voltage dependence. Left n-Si (covered with Pt particles) in contact with a 5 M HBr/0.05 M Br2 aqueous solution. A comparison is made of the PMC peak during the first and the third potential sweeps. Right n-WSe2 in contact with an aqueous 0.05 M Fe2+/3+ solution. The effect of cathodic prepolarization on position and height of the PMC peak is shown.

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

Figure 15.6 Effects of magnetic processing on (a) DPV curves (b) potential dependences of photocurrents of QqN " -MePH clusters on ITO electrodes.

Konno, A., Mogi, I. and Watanabe, K. (2001) Effect of strong magnetic fields on the photocurrent of a poly(N-methylpyrrole) modified electrode. 

Yonemura, H Yoshida, M Mitake, S. and Yamada, S. (1999) Magnetic field effects on photocurrent responses from modified electrodes with CdS nanopartides. Electrochemistry, 67, 1209-1210. 

In conclusion it should be mentioned that the same type of effects are possible for p-type electrodes. In this case an anodic dark current occurs whereas the photocurrent corresponds to an electron transfer via the conduction band (cathodic plEiotocurrent). 

Frequently it has been observed with n-type as well as with p-type electrodes in aqueous solutions that the onset potential of the pure photocurrent differs considerably from the flatband potential. The latter can be determined by capacity measurements in the dark as illustrated by the dashed line in the ij — Ub curve in Fig. 8 a. This effect is usually explained by recombination and trapping of minority carriers created by light excitation at the surface. It is obvious that these effects have a negative effect... 

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