A measurement probe could lead to improvements in how photoelectrodes are designed, making them more efficient and stable. Photoelectrochemical cells can be used to convert solar energy into transportable chemical fuels. The photoconversion process employs a semiconductor photoelectrode where photoexcited electrons move to the surface of the electrode to drive chemical reactions, such as reduction of water to produce hydrogen. As the charges move, fields are formed within the photoelectrode. Now a team from the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) has developed a spectroscopic probe that allows direct monitoring of the formation and decay of these fields. The NREL researchers used the probe to better understand the photophysics of photoelectrodes made from gallium indium phosphide (GaInP2). Previous work at NREL used GaInP2 as part of a high-efficiency water splitting system to generate hydrogen from sunlight. Using the newly developed probe, the scientists uncovered the role that a titanium dioxide (TiO2)/GaInP2 interface plays in the photoconversion process. The research was published in Science (doi: 10.1126/science.aad3459).