Semiconductor/liquid electrolyte problems

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

In any case, it is perceived from the above discussion that the problem of longterm chemical stability of polycrystalline semiconductor liquid junction solar cells is far from being solved. Still, as already pointed out in the early research, any practical photovoltaic and PEC device would have to be based on polycrystalline photoelectrodes. Novel approaches mostly involving specially designed PEC systems with alternative solid or gel electrolytes and, most importantly, hybrid/sensitized electrodes with properties dictated by nanophase structuring - to be discussed at the end of this chapter - promise new advances in the field. 

So-called wet solar cells show promise, particularly because of their relative ease of fabrication. In this type of photovoltaic cell, the junction is formed, between a semiconductor and a liquid electrolyte. No doping is required because a junction forms spontaneously when a suitable semiconductor, such as GaAs, is contacted with a suitable electrolyte, Three knotty problems (accelerated oxidation of surface of semiconductor exchange of ions between semiconductor and electrolyte forming a blocking layer and deposition of ions of impurities on the surface of the semiconductor) all have been solved and thus the concept now appears technically viable,... 

A group in Switzerland has developed a photovoltaic cell that can function as a window for a building.376 In one example, a ruthenium pyridine complex photosensitizer is attached to the titanium dioxide semiconductor by a phos-phonate. An iodine-based electrolyte (Kl3 dissolved in 50 50 ethylene carbonate/propylene carbonate) is between the panes. All the films are so thin that they are transparent. The efficiency is 10-11%. Variants on such cells have included fullerenes377 and polypyrrole.378 The use of solid electrolytes will avoid problems that might occur if a seal on a liquid electrolyte leaked. 

These types of sensors are represented by semiconductor devices and devices incorporating solid electrolytes. These have certain advantages over sensors utilising liquid electrolytes. These are the inherent, robust nature of such sensors, the lack of problems due to evaporation of solvent or corrosion due to spillage, the possibilities of miniturisation and the possibility of mass production. Sensors based on solid electrolytes usually operate in the potentiometric or amperometric mode but the semiconductor devices usually operate by measuring changes in conductivity. 

In a typical experiment with semiconductor-liquid junctions, one of the most important experimental problems is the differentiation between reactions that involve chemical changes at the semiconductor electrode (corrosion with insoluble products) and chemical changes in the electrolyte that might be subject to mass transfer limitations. The technique of Rotating Ring Disc Electrode (RRDE) (17-19) provides an opportunity to differentiate between these two types of reactions under controlled hydrodynamic conditions. In its simplestform, the metallic ring is isolated... 

Marcus has recently returned to this problem [133] and, by analogy with the problem of donor-acceptor electron transfer at the interface between two immiscible liquids, has derived the following expression for ka, the heterogeneous rate constant for electron transfer from a semiconductor to a species in a contacting electrolyte ... 

The problem of theoretically describing the elementary charge-transfer act across interfaces between two condensed media has a long history. The earliest studies of interfacial electrochemical phenomena focused on metal/electrolyte or semiconductor/electro-lyte interfaces. Considerable progress has been made recently in extending the theory to liquid/liquid interfaces [6,7,127,162-173]. 

The application of liquid-junction technology to photovoltaic power conversion is limited by problems associated with the semiconductor-electrolyte interface. Primary among these problems is corrosion. Efficient conversion of solar energy requires a band gap between 1.0 and 1.5 eV, and most semiconductors near this band gap corrode readily under illumination. Semiconductors with large band gaps (4-5 eV) tend to be more stable but cannot convert most of the solar spectrum. 

---