Cell components- Electrodes /Semiconductor electrodes

The basic concepts concerning formation and operation of a junction between a semiconductor and a solution are presented in this entry. The behavior of semiconductor electrodes in the dark and under illumination, different from that of metallic electrodes, is explained with reference to the peculiar properties of semiconductors such as very low concentration of mobile electrons; presence of a gap of forbidden energies for electronic states; control by electrode potential, under appropriate conditions, of the surface concentration of carriers; and the ability of illumination to change drastically the population of minority carriers. The dependence of photoeffects on material properties and light intensity is discussed, considering also the role and effects of surface states. The relevance of interface energetics for the performances and stability of a photoelectrode in contact with different redox couples is pointed out. Some semiconducting materials like oxides and calchogenides of transition metals, possess the ability of intercalating guest charged species (either in the dark or under illumination). Such a possibility is analyzed for consideration of their employment as electrodes of secondary battery cells. The analysis takes into account the various components of the free energy which determine the cell voltage in case of a lithium-ion battery. Moreover, considerations on the electronic and crystal structures are exposed to define the relevant requirements intercalation semiconductors should satisfy in the context of the technology of lithium-ion battery. Some oxides of transition metals manifest also the capability of transporting the electrical current simultaneously via electronic and ionic charge carriers. Such a characteristic of mixed conduction renders possible the practical utilization of some semiconducting oxides either as electrodes or solid-state electrolytes of solid oxide fuel cells (SOFCs). Some significative examples of semiconducting materials for SOFCs are reported underlining with particular emphasis their ability of modulating the conduction properties or switching the type of electrical conduction under the operative conditions of a fuel cell in dependence of the nature of interfaces the semiconductors create in a SOFC. Finally, the impact the advancements in nanotechnologies and nanoparticles synthesis has had in the development of semiconducting materials for photoelectrochemical cells is outlined in the particular case of the quantum dots that have been employed as sensitizers in dye-sensitized solar cells. In this entry organic semiconductors have not been included.