Non-unity photogeneration yield of mobile charge carriers in transition metal-oxides

Photoelectrochemical water splitting as means of producing clean hydrogen fuel has attracted the attention of researchers for nearly 50 years. Since photoelectrochemical cells must operate in a highly corrosive environment, significant efforts have been directed at the development of high efficiency photoelectrodes comprised of transition metal oxides that can operate stably in these conditions. However, to this point, there are very few demonstrations of materials that show a current density that reaches the limits determined by their band gap and the solar spectrum. In this perspective, we show that unlike the common notion that low photoanode performance is due to a minority carrier low diffusion length, growing evidence indicates that a significant portion of the photons absorbed in these materials do not generate mobile charge carriers. We start by surveying the evidence for non-unity photogeneration yield (PGY). Next, characterization methods which allow quantifying wavelength dependent PGY are proposed. Finally, we discuss the implications of these findings and suggest future research directions to characterize them.