By Prof. Thomas Hannappel, Ilmenau University of Technology, Germany.
The individual processes in a solar-driven photoelectrochemical (PEC) cell can be divided into three essential steps: (i) light absorption, (ii) transport of charges, and (iii) charge separation at the solid liquid interfaces. The consideration of all energetic and kinetic processes leads to the conclusion that only individual, single absorber materials would be suitable for water splitting or CO2 reduction with band gaps, which are too large for an efficient exploitation of the sun light and, therefore, efficient solar fuel production. Alternatively, tandem layer structures are capable to create a sufficient photo voltage, i.e. a sufficient splitting of the quasi-Fermi levels, and in addition to explore the solar spectrum most efficiently [1,2]. When using tandem cells, solar-to-fuels (STF) efficiencies can be achieved clearly exceeding 20% [3,4]. The difficulty arises to achieve a stable performance and to describe the microscopic processes at the challenging solid-liquid interfaces. Based on surface chemistry observed in model experiments , we firstly applied interfacial functionalization processing sequences for highly efficient III-V-semiconductor tandem absorbers , secondly studied in situ interfacial chemistry on the atomic scale, and thirdly prepared well-defined surfaces to explore different surface reconstructions on their initial interaction with water and oxygen. In order to realize cost-competitive tandem devices structures, low-defect III-V semiconductor integration into the mature silicon technology needs to be tackled, which is a complex challenge .
T. Hannappel1, H.-J. LewerenzϮ,2, H. A. Atwater2, O. Supplie1, W.-H. Cheng2, M. H. Richter2, M.M. May3, F. Dimroth4, D. Lackner4
(1) Ilmenau University of Technology, Institute of Physics, Dep. Photovoltaics, Germany
(2) California Institute of Technology, Joint Center for Artificial Photosynthesis, Pasadena, USA
(3) Helmholtz-Zentrum Berlin für Materialien und Energie, Solar Fuels Institute, Germany
(4) Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
Ϯ Deceased, April 24, 2019
 M.M. May, H.-J. Lewerenz, D. Lackner, F. Dimroth, T. Hannappel, Nat. Commun. 6 (2015) 8256;
 W.-H. Cheng, M.H. Richter, M. M. May, J. Ohlmann, D. Lackner, F. Dimroth, T. Hannappel, H.A. Atwater, H.-J. Lewerenz; ACS Energy Lett. 3 (2018) 1795
 M. M. May, D. Lackner, J. Ohlmann, F. Dimroth, R. van de Krol, T. Hannappel, K. Schwarzburg, Sustainable Energy & Fuels 1 (2017) 492
 T. Hannappel, M. M. May, H.-J. Lewerenz; Cambridge: Royal Society of Chemistry 2013, RSC Energy and Environment Series No. 9, ISBN 978-1-84973-647-3
 W.-H. Cheng, M.H. Richter, M.M. May, J. Ohlmann, D. Lackner, F. Dimroth, T. Hannappel, H.A. Atwater, H.-J. Lewerenz, in review, arXiv:1706.01493 (2017)
 B. Kim, K. Toprasertpong, A. Paszuk, O. Supplie, Y. Nakanoa, T. Hannappel, and M. Sugiyama, Solar Energy Materials & Solar Cells, in press, DOI:10.1016/j.solmat.2017.06.060 (2017).
 O. Spplie, O. Romanyuk, C. Koppka, M. Steidl, A. Nägelein, A. Paszuk, L. Winterfeld, A. Dobrich, P. Kleinschmidt, E. Runge, T. Hannappel, Progr. Cryst. Growth Charact Mat. 64 (2018) 103
Thomas Hannappel is W3 full professor at Ilmenau University of Technology, Institute of Physics. He was scientific director of the solar centre at CIS Research Institute Erfurt, director of the Institute “Materials for photovoltaics” at Helmholtz-Zentrum Berlin, department head at the Hahn-Meitner-Institute Berlin, and lecturer at the Free University Berlin. He started in situ studies on the Si/III-V interface at NREL and received his PhD at Berlin University of Technology with studies on photo-induced charge carrier dynamics performed at Fritz-Haber-Institute of the Max-Planck-Society. Current research is focused on the development of highly efficient solar energy conversion including solar fuels production, high-performance optoelectronic materials, crystal growth, and critical interfaces.