Cheng, Wen-HuiWen-HuiChengRichter, Matthias H.Matthias H.RichterMay, Matthias M.Matthias M.MayOhlmann, JensJensOhlmannLackner, DavidDavidLacknerDimroth, FrankFrankDimrothHannappel, ThomasThomasHannappelAtwater, Harry A.Harry A.AtwaterLewerenz, Hans-JoachimHans-JoachimLewerenz2022-03-0525.06.20192018https://publica.fraunhofer.de/handle/publica/25669910.1021/acsenergylett.8b00920Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional crystalline titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water reduction, facilitating electron transport at the cathode-electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradiation, solar-to-hydrogen efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, respectively. The system reaches a value of 0.85 of the theoretical limit for photoelectrochemical water splitting for the energy gap combination employed in the tandem-junction photoelectrode structure.en621697journal article