Photocorrosion

There are difficulties in making such cells practical. High-band-gap semiconductors do not respond to visible light, while low-band-gap ones are prone to photocorrosion [182, 185]. In addition, both photochemical and entropy or thermodynamic factors limit the ideal efficiency with which sunlight can be converted to electrical energy [186]. 

Meissner et al. [76] proposed a detailed reaction mechanism for the photocorrosion of n-CdS, based on experimental data in indifferent solutions (i.e., without intentionally added redox couples) of KCl, Na2S04, or NaC104 and in accordance with results reported in the literature. They suggested that illumination of... 

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. 

The strong photocorrosion effect on an electrodeposited CdSe film treated near short-circuit conditions (positive to the flat band potential) in a polysulfide media under intense illumination is shown in Fig. 5.5, as manifested by the formation of numerous, regularly arranged pinholes often reaching the substrate surface [99],... 

Fig. 5.5 SEM surface view and cross section of an electrodeposited, ca. 1 p.m thick, CdSe/li film subjected to accelerated photocorrosion by the apphcation of -0.1 V vs. Pt bias in polysulfide solution under a focused, high-power (1 W cm ) solar illumination for 30 min. The coherence of the as-deposited film morphology is evident. The authors emphasize that, even in this situation, the liquid junction nature prevents the flow of high leakage currents during the process (as it might be the case with a solid junction). (Reprinted from [99], Copyright 2009, with permission from Elsevier)...

Mishra et al. [198] discussed in an exemplary way the dark and photocorrosion behavior of their SnS-electrodeposited polycrystalline films on the basis of Pourbaix diagrams, by performing photoelectrochemical studies in aqueous electrolytes with various redox couples. Polarization curves for the SnS samples in a Fe(CN) redox electrolyte revealed partial rectification for cathodic current flow in the dark, establishing the SnS as p-type. The incomplete rectification was... 

Meissner D, Memming R, Kastening B (1988) Photoelectrochemistry of cadmium sulfide. 1. Reanalysis of photocorrosion and flat-band potential. J Phys Chem 92 3476-3483... 

Vucemilovic MI, Vukelic N, Rajh T (1988) Solubihty and photocorrosion of small CdS particles. J Photochem Photobiol 42 157-167... 

The direct photoelectrolysis of water requires that the v level be below the 02/H20 level and the ec level be above the H+/H2 level. This condition is satisfied, e.g. for CdS, GaP, and several large-band gap semiconductors, such as SrTi03, KTa03, Nb205 and Zr02 (cf. also Fig. 5.59). From the practical points of view, these materials show, however, other specific problems, e.g. low electrocatalytic activity, sensitivity to photocorrosion (CdS, GaP), and inconvenient absorption spectrum (oxides). 

The band-gap excitation of semiconductor electrodes brings two practical problems for photoelectrochemical solar energy conversion (1) Most of the useful semiconductors have relatively wide band gaps, hence they can be excited only by ultraviolet radiation, whose proportion in the solar spectrum is rather low. (2) the photogenerated minority charge carriers in these semiconductors possess a high oxidative or reductive power to cause a rapid photocorrosion. 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

Cu-CuO% nanoparticles (with a content of about 10 wt.%) on titania are effective for the production of hydrogen under sacrificial conditions [176-178], A fairly low concentration of Cu (2.5 wt.%) was sufficient to allow promising H2 production from ethanol-water and glycerol-water mixtures in the case of CuO% nanoparticles encapsulated into porous titania [179]. A key limitation of this system is photocorrosion under oxidizing conditions (oxygen and carboxylic adds as by-products of partial oxidation of the sacrificial agent). However, in the presence of UV irradiation, Cu photodeposition can occur, preventing loss of Cu [179]. 

Generation of hydrogen from H2S using cadmium sulfide with waste products as sacrificial agents to avoid photocorrosion of the semiconductor is expected to grow as a niche application. 

Suppression of Photocorrosion of Photoanodes and Manipulation of Kinetics for Anodic Processes... 

Several groups have recently shown (36,42,43,44) that photoanode materials can be protected from pRotoano3ic corrosion by an anodically formed film of "polypyrrole".(45) The work has been extended (46) to photoanode surfaces first"Treated with reagent that covalently anchors initiation sites for the formation of polypyrrole. The result is a more adherent polypyrrole film that better protects n-type Si from photocorrosion. Unlike the material derived from polymerization of I, the anodically formed polypyrrole 1s an electronic conductor.(45) This may prove ultimately important in that the rate of ionTransport of redox polymers may prove to be too slow... 

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