Quantized nanocrystalline photoelectrodes: A review

Abstract

Abstract Photoelectrocheḿical (PEC) cells are described which use photoelectrodes of nanocrystalline semiconductors (ca. 5 nm crystal size). The photoelectrodes, which can be CdSe or CdS, are prepared as nanocrystalline films by either chemical solution deposition or by electrodepostion. They exhibit strong spectral shifts due to size quantization in the nanocrystals. The behavior of the nanocrystalline photoelectrode can be explained by a model whereby the electrolyte makes contact with individual nanocrystals in the porous film. Charges are photogenerated essentially at the surface of the individual crystallites and separation of the charges occurs due to differing kinetics of electron and hole transfer to the electrolyte. The charge which is less rapidly transferred to the elecrolyte (usually electrons) can be removed at the back contact, and recombination, which is normally a severe problem in films of such small crystallites, is minimized since the opposite charge is rapidly removed. The photoelectrodes can exhibit both n-type and p-type photoresponse, as defined by direction of current flow, in a polysulfide electrolyte. It is shown that, under illumination, this electrolyte interacts with nanocrystalline CdSe films to give a red shift of transmission and photocurrent spectra, i.e., a loss in size quantization.