Abstract
Photoelectrochemical (PEC) cells directly convert sunlight into oxygen and hydrogen, thus combining energy capture, conversion and storage into a single system.[1] In a typical single junction PEC cell, a Schottky barrier formed at the photoanode/electrolyte interface separates photogenerated carriers such that holes diffuse to the surface and react with adsorbed hydroxide ions to form oxygen, while electrons flow through a back contact to a counter metal electrode where they reduce water to hydrogen and hydroxide ions. In a similar manner, single junction photocathode/electrolyte cells electrons diffuse to the surface to produce hydrogen while holes flow to the counter electrode. A variety of binary, tertiary, and quaternary transition metal oxides and oxinitrides (TiO2, Fe2O3, WO3, etc) have been explored as photoanodes, motivated by their relative chemical stability, suitable conduction and valence band edge energies, and potentially very low fabrication costs. However, all of these materials suffer from low solar-to-hydrogen efficiency due to low charge carrier mobility, fast carrier recombination times, low optical absorption in the visible, and slow water oxidation kinetics. Strategies to improve water splitting efficiency include doping the oxides with mid-gap color centers, nanostructuring the grain morphology to increase the electrode/electrolyte interface area and reduce the distance the carrier need to traverse, co-loading or surface treatment with catalysts that promote water oxidation and potentially reduce surface recombination, and most recently, concentrating solar radiation using plasmonic nanoparticles. Despite these efforts, PEC efficiencies remain well below the ~10 percent levels necessary for this technology to be economically viable.
© 2013 Optical Society of America
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