Solar energy conversion systems interfaces

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... 

There is a large volume of contemporary literature dealing with the structure and chemical properties of species adsorbed at the solid-solution interface, making use of various spectroscopic and laser excitation techniques. Much of it is phenomenologically oriented and does not contribute in any clear way to the surface chemistry of the system included are many studies aimed at the eventual achievement of solar energy conversion. What follows here is a summary of a small fraction of this literature, consisting of references which are representative and which also yield some specific information about the adsorbed state. 

The recent experimental developments on dark and photoinduced ET reactions give support to the previous speculations on the relevance of these interfaces in such fields as catalysis and solar energy conversion. These disciplines have been, and still are centered on processes at solid solution interfaces. However, particular applications require molecu-larly defined interfaces, where reactants exhibit different solubility properties. In this section, we shall consider some of these systems and the advances reported so far. 

The organization of molecules at interfaces and the formation of complex assemblies of molecules are the basic procedures for the construction of devices in molecular dimensions. The appropriate components must be adequately arranged in space and energy to achieve the intended function (1). A variety of different types of molecular assemblies have been studied like ion-polyelectrolyte associates (2), monolayers at interfaces (3), lipid monolayer (A) and bilayer (5) membranes, micelles (6,7), vesicles (8) and monolayer assemblies (9) particularly with regard to their suitability as systems for solar energy conversion. 

By using a multichamber system [129], exchange of residual gases between successive depositions will be strongly decreased, and very sharp interfaces can be made. Furthermore, the use of a load-lock system ensures high quality of the background vacuum, and thus low levels of contaminants in the bulk layers. Multichamber reactor systems have been used for the fabrication of solar cells, and considerable improvements in energy conversion efficiency have been achieved [130, 131]. 

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

---