Illuminated semiconductor electrodes

Under light illumination, semiconductor electrodes absorb the energy of photons to produce excited electrons and holes in the conduction and valence bands. Compared with photoelectrons in metals, photoexcited electrons and holes in semiconductors are relatively stable so that the photo-effect on electrode reactions manifests itself more distinctly with semiconductor electrodes than with metal electrodes. 

Let us note in conclusion that the thermodynamic approach has widely been used to describe the kinetics of electrochemical reactions at an illuminated semiconductor electrode (see, for example, Gerischer, 1977c Dog-onadze and Kuznetsov, 1975). Clearness and simplicity are an unqualified advantage of this approach, but the use of the quasilevel concept is not justified in all the cases. In particular, conditions (48) alone appear to be insufficient to substantiate the applicability of the quasilevel concept to the description of the processes of electron transfer across the interface (for greater details, see Pleskov and Gurevich, 1983 Nozik, 1978). Obviously, if photogeneration of the carriers occurs mainly near the surface, at which a... 

This description of the photoelectrochemical event was originally based on the use of an illuminated semiconductor electrode in a standard three electrode cell configuration. The theory can easily be extended, however, for practical applications to short-circuited cells prepared by deposition of inert metal with low overvoltage characteristics on a powdered semiconductor Such a metallized powder is shown in... 

Assuming that we have such a situation favorable for charge separation, we have to consider what factors influencing the efficiency of charge separation in an illuminated semiconductor electrode are affected by crystal orientation or crystal imperfections. Five such factors are listed in the following table ... 

The current-voltage characteristics of an illuminated semiconductor electrode in contact with a redox electrolyte can be obtained by simply adding together the majority and minority current components. The majority carrier component is given by the diode equation (Eq. 17) while the minority carrier current (iph) is directly proportional to the photon flux (see, e.g., Eq. 24). Thus, the net current is given by... 

P. C. Searson, D. D. Macdonald andL. M. Peter, Frequency domain analysis of photoprocesses at illuminated semiconductor electrodes by transient transformation, J. Electrochem. Soc. 139, 2538, 1992. 

The electrochemistry of semiconductors has played a major part in the development of modern electrochemistry, especially in recent years with regard to photoelectrochemical energy conversion using illuminated semiconductor electrodes. Drs. Pleskov and Gurevich of the Institute of Electrochemistry, Moscow, contribute an important chapter (3) on New Problems and Prospects in this field. Readers will find that their chapter gives a thorough account of the current directions of development in this field, as well as some of the difficulties and new areas of research in this subject. 

It is well known that pyrrole molecules are oxidized on a platinum electrode at potentials more positive than +0.6 V vs. SCE in aqueous solutions and the resultant oxidized molecules react to form polypyrrole on the electrode surface. The oxidation of pyrrole, however, takes place with less bias on an illuminated semiconductor electrode owing to the photosensitized electrolysis... 

The Gartner equation provides a useful theoretical basis for the analysis of the photocurrent response of illuminated semiconductor electrodes. It can be rewritten in the form... 

The simple models of electron transfer at semiconductor interfaces, which have been used until recently, are now being extended and improved, and Wilson has provided an authoritative review of the theory, which includes some discussion of solar photoelectrochemical cells. Albery et have explored the transport and kinetics of minority carriers at illuminated semiconductor electrodes. The exact analytical solution of the problem is obtained in terms of confluent... 

The theoretical treatment of the transport and kinetics of minority carriers at illuminated semiconductor electrodes by Albery et al. is likely to be applied widely. 

W. J. Albery, P. N. Bartlett, A. Hamnett, and M. P. Dare-Edwards, The transport and kinetics of minority carriers in illuminated semiconductor electrodes, J. Electrochem. Soc. 128 (1981) 1492-1501. 

D. Laser, Modes of charge transfer at an illuminated semiconductor electrode A digital simulation, J. Electrochem. Soc. 126 (1979) 1011-1014. 

For the description of the effects of illuminated semiconductor electrodes the concept of the quasi Fermi level was developed. For stationary illumination An photoelectrons and Ap photoholes are generated in the surface region with the result that there is no longer equilibrium between the conduction and valence bands. One can define individual electrochemical potentials for the photoelectrons (quasi Fermi level of electrons) and the photoholes (quasi Fermi level of holes). 

Goossens, A., Schoonman, J. The impedance of surface recombinatirai at illuminated semiconductor electrodes - a nonequiUbrium approach. J. Electroanal. Chem. 289, 11-27 (1990)... 

In principle, there exist various possibilities to improve the efficiency by using devices with two illuminated semiconductor electrodes or by using two threshold absorbers with different band gaps in series. Such attempts for application in electrochemical cells will be discussed in the series of lectures given by A. Nozik. 

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

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