Photovoltaics are common renewable-energy supply systems and sunlight is available but not all day. One solution is storing sunlight in the form of chemical energy, by using sunlight to make hydrogen. Hydrogen can be stored easily and safely. If solar cells are combined with catalysts and additional layers to form a monolithic photoelectrode as a block. The photocathode is immersed in an aqueous medium and when light falls on it, hydrogen is made on the front side and oxygen on the back.
For the monolithic photocathode investigated here, the research teams combined functional layers with a highly efficient tandem cell made of III-V semiconductors developed at Fraunhofer ISE. They reduced the surface reflectivity of the cell. "This is also where the innovation lies," explains Prof. Hans-Joachim Lewerenz, Caltech, USA: "Because we had already achieved an efficiency of over 14 per cent for an earlier cell in 2015, which was a world record at the time. Here we have replaced the anti-corrosion top layer with a crystalline titanium dioxide layer that not only has excellent anti-reflection properties, but to which the catalyst particles also adhere." And Prof. Harry Atwater, Caltech, adds: "In addition, we have also used a new electrochemical process to produce the rhodium nanoparticles that serve to catalyse the water-splitting reaction. These particles are only ten nanometres in diameter and are therefore optically nearly transparent, making them ideally suited for the job." Under simulated solar radiation, an efficiency of 19.3 per cent in dilute aqueous perchloric acid, while still reaching 18.5 per cent in an electrolyte with neutral pH was achieved. These figures approach the 23 per cent theoretical maximum efficiency with electronic properties for these layers. "This work shows that tailor-made tandem cells for direct solar water-splitting have the potential to achieve efficiencies beyond 20 per cent. One approach for this is to choose even better band-gap energies for the two absorber materials in the tandem cell. And one of the two could even be silicon," explains Prof. Thomas Hannappel, TU Ilmenau. Teams at Fraunhofer ISE and TU Ilmenau are working to design cells that combine III-V semiconductors with lower-priced silicon, which could reduce costs.
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