Charge Transfer in Nanocrystalline Semiconductor Electrodes

Nanocrystalline electrodes in liquid junction devices possess a number of unique properties arising from their convoluted structure and the dimensions of their building units. The light-induced charge separation and transport in photoelectrochemical systems using nanocrystalline/nanoporous semiconductor electrodes is discussed here in connection with the basic principles of the (Schottky) barrier theory. Recent models for charge transfer kinetics in normal and unipolar (dye-sensitized) cells are reviewed, and novel concepts and materials are considered. 1. Introduction In electrochemical systems, ionic charge transport in the electrolyte (EL) phase can be coupled to electron transport in the solid electrode via interfacial charge transfer, which may occur in the dark or be stimulated by illumination. Charge transfer characteristics depend mainly on the electrode nature, that is, the particular type and magnitude of conduction in the solid state as determined by bulk and surface properties. Photoelectrochemical cell (PEC) devices use illuminated semiconductor (SC) electrodes for power generation, or electrosynthetic and photocatalytic purposes. The light-induced or light-assisted conversion efficiencies in PECs using compact polycrystalline electrodes are often limited by processes occurring at grain boundaries and are typically inferior to those observed with single crystals, since the individual particles that make up the solid may be small with respect to key properties associated with charge transport (e.g., the minority carrier diffusion length). Fortunately, liquid junctions partly tolerate polycrystallinity, as the active region in which light absorption and separation of photogenerated carriers occurs (i.e., the space charge layer, SCL, with a thickness of the order of ca. 100？nm) is located at the SC surface (for typical front-wall illumination); hence, the crystallites need not exceed some tens of nm in size. By contrast, in solid p/n junction cells, the crystallites should be of micrometric dimensions since the active region is located at a depth of 0.1 to 1.0？μm below the illuminated SC surface, and recombination of photocarriers at grain boundaries will considerably decrease the efficacy of the cell. Requirements for crystallinity in the sense relevant to solid state devices may be relaxed in liquid junctions, when the SC particle size or layer thickness is sufficiently small or a porous morphology is present [1, 2]. A mesoporous/nanostructured electrode comprises a porous electrode built up from interconnected particles of crystallite size