Photocurrent-potential relationship

The flat band potentials of a semiconductor can be determined from the photocurrent-potential relationship for small band bending [equation (4.2.1)], or derived from the intercept of Mott-Schottky plot [equation (4.2.2)] using following equations... 

Several etching procedures were attempted for p-CdTe for the photoelectrochemical reduction of carbon dioxide. Etching with dilute thiosulfite or bromine in methanol did not result in better photocurrent-potential relationship. Hence, it was concluded that etching with aqua regia followed by rinsing with water is the best surface treatment for the photoelectrochemical reduction of carbon dioxide. All the impedance results described below were recorded using this surface in contact with electrolyte. 

Photocurrent-Potential Relationship. The photocurrent-potential curve under monochromatic light (555 nm) in a DMF (5% H O) solution containing O.IM TBAP is shown in Fig. 2. In the absence of CO, the photocurrent starts to increase at -1.4V NHE due to hydrogen evolution. When CO gas is bubbled through the solution, the onset potential for the photocurrent is shifted to less cathode potentials by about 700 mV. The reduction product was found to be carbon monoxide. At low cathodic potentials an anodic current is observed. 

The oxidation of lead in various electrolyte solutions has been studied [736-738]. In a tetraborate solution, the photocurrent spectrum measured with white light showed a marked dependence on the positive (anodic) potential limit of the potential scans. With the most positive limit, the photocurrent-potential relationship displayed in Fig. 5.128 was obtained. The formation of tetragonal PbO by reduction of 0 -PbO2 was monitored. The onset of the photocurrent indicates the appearance of the formation of PbO and the electrode potential coincides with the sudden onset of the photocurrent in the anodic scan. In the positive-going scan, the photocur-... 

FIG. 17 Reversal of the photocurrent sign upon replacing the electron donor DCMFc (a) by the electron acceptor TCNQ (b) in the presence of the porphyrin heterodimer ZnTMPyP-ZnTPPS at the water-DCE interface. The strong back electron-transfer features in the photoreduction of TCNQ were diminished upon addition of an equimolar ratio of ferri/ferrocyanide acting as supersensitizer in the aqueous phase (b). The mechanism of supersensitization is described in Fig. 11. From the potential relationship between these redox couples (Fig. 4), these phenomena can be regarded as interfacial photosynthetic processes as defined in Fig. 3(b). (Reprinted with permission from Ref. 87. Copyright 1999 American Chemical Society and from Ref. 166 with permission from Elsevier Science.)... 

The ratio g/I(0) defines the photocurrent efficiency . In the absence of surface recombination, qg corresponds to the photocurrent density Jphow measured in the external circuit. The Gartner equation has been used successfully to explain the photocurrent-potential characteristics of many semiconductor electrodes under conditions where surface recombination is absent. Plots of ln — O) against dsc (which according to the Mott-Schottky relationship is proportional to (1/ — have... 

The photocurrent onset potential is often taken as the flatband potential, since the measurement of the flatband potential is typically only good to 100 mV and the onset of photocurrent is often observed with less than 100 mV of band bending. This practice is dangerous, however, since the onset potential is actually the potential at which the dark cathodic current and the photoanodic current are equal. Even though in the case of the p-GaP illustration, the observation of an anodic current and a photocathodic current are separated by several hundred millivolts, in many systems these two currents overlap. In those cases, the relationship between the flatband potential and the onset potential becomes unclear. 

If we assume k T = lcms 1 and conditions otherwise are as above, (- c1/2) a 20, corresponding to a bias of 0.5 V. Above this potential, it will appear that the Gaertner relationship is obeyed by the photocurrent and reliable values of Lp and a can be derived examples are given in ref. 145. 

If the density of holes Ps at the surface - or equivalently the quasi-Fermi level Ep p — are equal at the surface of an n- and p-semiconductor electrode, then the same reaction with identical rates, i.e. equal currents, takes place at both types of electrodes (Fig. 15). Since holes are majority carriers in a p-type semiconductor, the position of the quasi-Fermi level Ep,p is identical to the electrode potential (see right side of Fig. 15), and therefore-with respect to the reference electrode - directly measurable. The density of p can easily be calculated, provided that the positions of the energy bands at the surface are known. The measurements of a current-potential curve also yields automatically the relationship between current and quasi-Fermi level of holes. The basic concept implies that the position of the quasi-Fermi level Ep,p at the surface of an n-type semiconductor and the corresponding hole density Ps can be derived for a given photocurrent, since the same relationship between current and the quasi-Fermi level of holes holds. 

IMPS uses modulation of the light intensity to produce an ac photocurrent that is analysed to obtain kinetic information. An alternative approach is to modulate the electrode potential while keeping the illumination intensity constant. This method has been referred to as photoelectrochemical impedance spectroscopy (PEIS), and it has been widely used to study photoelectrochemical reactions at semiconductors [30-35]. In most cases, the impedance response has been fitted using equivalent circuits since this is the usual approach used in electrochemical impedance spectroscopy. The relationship between PEIS and IMPS has been discussed by a number of authors [35, 60, 64]. Vanmaekelbergh et al. [64] have calculated both the IMPS transfer function and the photoelectrochemical impedance from first principles and shown that these methods give the same information about the mechanism and kinetics of recombination. Recombination at CdS and ZnO electrodes has been studied by both methods [62, 77]. Ponomarev and Peter [35] have shown how the equivalent circuit components used to fit impedance data are related to the physical properties of the electrode (e.g. the space charge capacitance) and to the rate constants for photoelectrochemical processes. 

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