Semiconductor Electrode Stability

The photo-generated holes and electrons in semiconductor electrodes are generally characterized by strong oxidizing and reducing potentials, respectively. Instead of being injected into the elec- 

Flat-band potential locates the semiconductor bands with respect to the redox couples of the electrolyte. These relationships are shown for several semiconductors. Those with the valence band above H / H2 operate with zero external bias. For electrochemical photovoltaic cells, the flat-band potential determines the open circuit voltage. 

In cases where the redox potentials of the electrode decomposition reactions are more thermodynamically favoured than the electrolyte redox reactions (oxidative decomposition potential more negative, reductive decomposition potential more positive, than the corresponding electrolyte redox reactions), the products of the electrolyte redox reactions have sufficient potential to drive the electrode decomposition reactions. Hence, this situation usually results in electrode instability, assuming that the electrode decomposition reaction is not kinetically inhibited. This is the case with ZnO, Cu20, and CdS in simple aqueous electrolytes, and these semiconductors are indeed unstable under these conditions. 

It appears that the more thermodynamically favoured redox reactions also become kinetically favoured, so that these reactions predominate. This effect has been used to stabilize semiconductor electrodes by establishing a redox couple in the electrolyte with a redox potential more negative than the oxidative decomposition potential (or more positive than the reductive decomposition potential), such that this electrolyte redox reaction occurs preferentially compared to the decomposition reaction, and scavenges the photo-generated minority carriers. However, this stabilization technique can only be used for electrochemical photovoltaic cells, and it is discussed later in further detail. 

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Cardon, F., Gomes, W.P., Vanden Kerchove, F., Vanmaekelbergh, D, and Van Overmeire, F. 1980. On the kinetics of semiconductor electrode stabilization. Faraday Disc., 70, 153-164. 

The liquid-junction semiconductor electrodes stabilized by polymer-coating can be used for photochemical conversion systems. The stabilized n-CdS coated with PP which incorporates RUO2 as catalyst was used for visible-light-induced water cleavage Photochemical diodes were fabricated by coating CdS with PP and polystyrene films, the latter containing metal dispersions such as Pt, Rh and RUO2 as a catalyst (Fig. 35). 

Park SM, Barber ME (1979) Thermodynamic stabilities of semiconductor electrodes. Electroanal Chem 99 67-75... 

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. 

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Finally cells containing a p-type semiconductor electrode should be mentioned. In principle the application of p-type electrodes would be even more favorable because electrons created by light excitation are transferred from the conduction band to the redox system. Stability problems are less severe because most semiconductors do not show cathodic decomposition (see e.g. earlier review article. However, there is only one system, p-InP/(V " /V ), with which a reasonable efficiency was obtained (Table 1) . There are mainly two reasons why p-electrodes were not widely used (i) not many materials are available from which p-type electrodes can be made (ii)... 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

Electrode Stability and Photoelectrolysis Using Appropriate Bandgap Semiconductors... 

Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

The stability of semiconductor electrodes, their resistance to photocorrosion, become an especially urgent problem in connection with ever-extending photoelectrochemical applications of semiconductors. This refers, first of all, to electrodes of photoelectrochemical cells for solar energy conversion. 

The stabilization of illuminated n-type III-V semiconductor electrodes through competing hole capture by reducing agents added to the aqueous solution has been studied as a function of concentration and of the light intensity. The main result concerns the observed light-intensity dependence. From a kinetic analysis of the stabilization process, it follows that two types of reaction mechanisms can be held responsible for the observed kinetics. 

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