By Prof. Thomas-Hannappel, Head of Department of Photovoltaics, Institute of Physics, Ilmenau University of Technology, Germany.
Monolithically integrated layer stacks from III-V semiconductors have been demonstrated to display the highest performance in basically all optoelectronic device structures such as in the field of solar energy conversion.
Record tandem cells in photovoltaics [1,2] as well as in direct solar-driven water splitting systems  have been developed lately from epitaxially prepared layer structures. Research and development of multi junction solar cells is meanwhile addressing four to five junction solar cells and recently a four-junction configuration with optimized band gaps including GaInP/GaAs and InGaAsP/InGaAs tandem cells has been realized yielding highest efficiencies up to 46% [1,2]. Regarding the design of a device for direct water-splitting, its architecture, and the given thermodynamic and kinetic energetic condition to split H2O, only absorbers with large band gaps or tandem structures with a sufficient voltage, i.e. with sufficient splitting of the quasiFermi levels, can produce H2. In this idea of direct water splitting, the entire device is exposed
to the electrolyte and needs to be developed with regard to several interconnected challenges such as sun light exploitation, passivation, catalysis, solid—liquid interfaces, stability, etc. Here, the latest achievement was a new record device with a solar-to-hydrogen (STH) efficiency of 14%  and with increased stability.
 F. Dimroth, M. Grave, T. Hannappel, K. Schwarzburg et al., “Wafer bonded four-junction GaInP/GaAs/GaInAsP/GaInAs concentrator solar cells with 44.7% efficiency“, Prog. Photovolt: Res. Appl. 22 (2014) 277
 T.N.D. Tibbits, P. Beutel, M. Grave, T. Hannappel et al., New efficiency frontiers with wafer-bonded multi-junction solar cells, Proc. Eur. Photovolt. Sol. Energy Conf. Exhib. 29 (2014) 1975 – 1978. doi:10.4229/EUPVSEC20142014-4CP.2.1.
 M.M. May, H.-J. Lewerenz, D. Lackner, F. Dimroth, T. Hannappel, ”Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure”, Nature Commun. 6 (2015) 8256.
Authors: T. Hannappel, O. Supplie, M.M. May, P. Kleinschmidt, H.-J. Lewerenz, F. Dimroth, D. Lackner, A. Paszuk, C. Koppka.
Thomas Hannappel is W3 full professor (physics) at Ilmenau University of Technology, Germany, department ‘Photovoltaics’, since 2011. Before, he was provisional head of the Institute “Materials for Photovoltaics” at the Helmholtz-Zentrum Berlin and lecturer at the Free University Berlin, where he received his state doctorate in 2005. At Technical University Berlin he obtained his PhD in Physics with studies on ultrafast dynamics of photo-induced charge carrier separation in dye solar cells, he performed at Fritz-Haber-Institute Berlin of the Max-Planck-Society. In 2003/04 he conducted research on silicon/III-V-interfaces at National Renewable Energy Laboratory, Colorado. His current investigations are focused on high-performance solar cells and critical interfaces and he is a key player in the fields solar energy conversion and reactions of critical semiconductor interfaces including silicon/ and germanium/III-V-interfaces, and nano- and quantum-structures.