Photoelectrochemical Semiconductor Electrode Systems

On the basis of our theoretical considerations and preliminary experimental work, it is hoped that fast processes of charge carriers will become directly measurable in functioning photoelectrochemical cells, Typical semiconductor electrodes are not the only systems accessible to potential-dependent microwave transient measurements. This technique may also be applied to the interfacial processes of semimetals (metals with energy gaps) or thin oxide or sulfide layers on ordinary metal electrodes. 

A thorough insight into the comparative photoelectrochemical-photocorrosion behavior of CdX crystals has been motivated by the study of an unusual phenomenon consisting of oscillation of photocurrent with a period of about 1 Hz, which was observed at an n-type CdTe semiconductor electrode in a cesium sulfide solution [83], The oscillating behavior lasted for about 2 h and could be explained by the existence of a Te layer of variable width. The dependence of the oscillation features on potential, temperature, and light intensity was reported. Most striking was the non-linear behavior of the system as a function of light intensity. A comparison of CdTe to other related systems (CdS, CdSe) and solution compositions was performed. 

Surface orientation and imperfections of the surface or the bulk expose very drastical influences on the photoelectrochemical behavior of semiconductor electrodes. This is very important for all applications of such systems, for example for the conversion... 

Moderately doped diamond demonstrates almost ideal semiconductor behavior in inert background electrolytes (linear Mott -Schottky plots, photoelectrochemical properties (see below), etc.), which provides evidence for band edge pinning at the semiconductor surface. By comparison in redox electrolytes, a metal-like behavior is observed with the band edges unpinned at the surface. This phenomenon, although not yet fully understood, has been observed with numerous semiconductor electrodes (e.g. silicon, gallium arsenide, and others) [113], It must be associated with chemical interaction between semiconductor material and redox system, which results in a large and variable Helmholtz potential drop. 

A photoelectrochemical (photoelectrolysis) system can be constructed using a n-type semiconductor electrode, a p type semiconductor, or even mating n- and p-type semiconductor photoelectrodes as illustrated in Figs. 2a c respectively. In the device in Fig. 2a, OER occurs on the semiconductor photoanode while the HER proceeds at a catalytic counterelectrode (e.g., Pt black). Indeed, the classical n-Ti02 photocell alluded to earlier,53 57 belongs to this category. Alternately, the HER can be photo-driven on a p type semiconductor while the OER occurs on a "dark" anode. 

Interfacial energetics in two-photosystem cells combining n- and p-type semiconductor electrodes respectively (Fig. 2c) have been discussed.67 Stability issues in photoelectrochemical energy conversion systems have been reviewed.318... 

Figure 6. Schematic diagram of a photoelectrochemical cell based on a sensitized semiconductor electrode. M = metal-polypyridine sensitizer, R = reversible electron-relay system, e.g. h/lj -...

Modification of semiconductor electrode response with adsorbed or attached dye molecules is an attractive alternative to other photoelectrochemical systems (7-13). Metal oxides which are stable or have very low corrosion rates but are transparent to visible wavelength light can be used in light-assisted electrochemical reactions when modified with monolayers and multilayers of a wide variety of chromophores interposed between the electrode and electrolyte. With one exception, the initial reports of energy conversion efficiencies of electrodes with adsorbed dyes was disappointingly low. Recently however,... 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Such protecting systems are oxidized or reduced, thereby stabilizing semiconductor photoelectrodes, and this effect is used in photoelectrochemical cells for solar energy conversion. The solvent (e.g., water) can also act as a protector if it is oxidized (reduced) easier than the semiconductor electrode material. Situations like this arise with many oxide electrodes (Ti02, SrTi03,... 

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