photoelectrodes

Photoelectrodes are electrodes that absorb light and participate directly in electrochemical reactions. In photoelectrochemical cells, they serve as the light-activated site where charge carriers are generated and subsequently drive redox processes. Upon illumination, a semiconductor photoelectrode absorbs photons with energy exceeding its bandgap, creating electron–hole pairs. If the material forms a proper band alignment with the electrolyte, photoexcited electrons are injected into the external circuit or into a reducible species at the surface, while the holes oxidize a species at the electrolyte–semiconductor interface. Depending on the polarity and material, photoelectrodes function as photoanodes (oxidation at the surface, typically n-type semiconductors like TiO2, Fe2O3, WO3) or as photocathodes (reduction at the surface, typically p-type or suitably doped materials).

Common materials include wide-bandgap oxides such as TiO2, Fe2O3, and WO3 for robust photoanodes; narrower bandgap materials (CdS, CdSe, BiVO4, sulfides, and perovskites) or dye-sensitized configurations for improved light harvesting. Strategies to improve performance include doping, nanostructuring for longer charge-carrier lifetimes, surface catalysts for faster surface kinetics, and protective overlayers to prevent photocorrosion. Interfaces and band alignment relative to the redox potentials of the electrolyte are critical; proper alignment allows efficient electron transfer and minimizes recombination.

Applications of photoelectrodes include solar water splitting, generating hydrogen and oxygen, as well as photocatalytic reduction of CO2 or pollutants. Photoelectrodes are central to dye-sensitized photoelectrochemical cells and tandem devices that couple a photoanode with a photocathode. Limitations include limited charge separation efficiency, recombination losses, stability under illumination, and reliance on rare or expensive materials for high-performance systems.