Wen-Hui Cheng1 Matthias Richter1 Matthias May2 Sisir Yalamanchili1 Phillip Jahelka1 Jens Ohlmann3 David Lackner3 Frank Dimroth3 Thomas Hannappel4 Hans-Joachim Lewerenz1 Harry Atwater1

1, California Inst of Technology, Pasadena, California, United States
2, University of Cambridge, Cambridge, , United Kingdom
3, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, , Germany
4, Technische Universität Ilmenau, Ilmenau, , Germany

We report latest results using a dual III-V semiconductor based tandem solar photoelectrochemical cell with record solar-to-fuel efficiency for hydrogen generation. Unassisted water splitting was carried out with a tandem photocathode device featuring a cathode surface with a highly transparent catalyst Rh layer supported on an anatase TiO2, which in turn coats a PO4-terminated AlInP photoelectrode window layer of the tandem photoelectrode in order to achieve optimal band alignment. To maximize the photocurrent density, we have also employed several light management strategies that enable increased transparency of the metal catalyst ensembles. Two pathways have been pursued: i) an optically transparent but dense Rh metal nanoparticle layer, and ii) a general approach to design of high-activity effectively transparent catalysts that employ opaque catalyst materials, in which a tailored mesophotonic dielectric cone structure is used as a light coupler to efficiently guide incident light into the light absorber. We have observed solar-to-hydrogen conversion efficiencies as high as 19.3% at low pH, but saw a time-dependent loss of device performance related to loss of catalyst activity. The origin of the activity losses was analyzed using surface analysis methods (XPS, UPS, AFM) and strategies for improvement are addressed. We also have demonstrated hydrogen generation with an 18.5% STH efficiency operating with a tandem photoelectrode with improved stability at neutral pH. This development also opens up a new route for other photoelectrochemical applications, as for example, aqueous CO2 reduction is best performed near pH 7. The catalyst integration and device design criteria for direct photoelectrochemical CO2 reduction for production of higher value solar fuels will also be discussed.