Optimisation of photoelectrochemical storage

In addition to a variety of etching procedures, several other surface treatments have been used to improve photoelectrode performance. Examples include a Ga ion dip for CdSe (Tomkiewicz et al, 1982), a ZnCl2 dip for thin-film CdSe (Hodes et al, 1980 Reichman and Russak, 1982), and the deposition of Ru on GaAs (Parkinson, et al, 1979) and on InP (Heller, 1982), and Cu on CdSe (Flaisher et al, 1984). Reasons advanced for the effectiveness of these treatments range from a suppression of dark current to electrocatalysis by surface-deposited metal atoms. 

The solution-phase chemistry of the electrolyte is an important feature of the cell that can dramatically influence photoeffects. The redox potential of the photoelectrode couple determines equilibrium band bending. As noted above, a photoanode system allied with a more positive potential redox couple exhibits greater band bending (assuming no shift in band edges—this assumption is often valid for redox species that do not specifically interact with the semiconductor surface), which in turn leads to a higher photovoltage and more efficient carrier separation under normal experimental conditions. 

Semiconductor photoeffects in a complex redox electrolyte are greatly affected by such solution properties as solution redox level and stability, interfacial kinetics (adsorption), conductivity, viscosity, ionic activity, and transparency within a crucial wavelength region. Suitable redox electrolytes are known to inhibit unfavourable phenomena such as surface recombination and trapping (McEvoy et al, 1985). In 

Chemical composition of the electrolyte is a particularly important parameter in PEC systems based on complex electrolytes snch as polysnlphide or ferro/ ferricyanide. In the latter redox conple, as shown in Fig. 10.8, replacement of one of the hexacyano ligands strongly changes the photoelectrochemical response of illnminated n-CdSe dne to a combination of electrochemical and spectroscopic effects (Licht, 1995), and addition of the KCN to the electrolyte can increase n-CdSe and n-CdTe photovoltages by 200 mV (Licht and Peramnnage, 1990). 

An efficient photoelecnochemical conversion and storage system requires not only efficient performance of the separate cell components, but also system compatibility. In the combined photoelecnochemical storage system, simultaneous parameters to be optimised include  

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