FOR THE NEW WORLD RECORD IRECT SOLAR WATER-SPLITTING EFFICIENCY
An international team of researchers has now succeeded in raising the efficiency of producing hydrogen from direct solar water-splitting to a record 19%. They did so by combining a tandem solar cell of III-V semiconductors with a catalyst of rhodium nanoparticles and a crystalline titanium dioxide coating. For the monolithic photocathode investigated here, the research teams combined additional functional layers with a highly efficient tandem cell made of III-V semiconductors developed at Fraunhofer ISE. This enabled them to reduce the surface reflectivity of the cell, thereby avoiding considerable losses caused by parasitic light absorption and reflection.
“This is also where the innovation lies,” explains Prof. Hans-Joachim Lewerenz, Caltech, USA: “Because we had already achieved an efficiency of over 14% 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.”
Under simulated solar radiation, the scientists achieved an efficiency of 19.3% in dilute aqueous perchloric acid, while still reaching 18.5% in an electrolyte with neutral pH. These figures approach the 23% theoretical maximum efficiency that can be achieved with the inherent electronic properties for this combination of layers. “The crystalline titanium- dioxide layer not only protects the actual solar cell from corrosion, but also improves charge transport thanks to its advantageous electronic properties,” says Dr Matthias May, who carried out part of the efficiency determination experiments at the HZB Institute for Solar Fuels in the forerunner laboratory to the Solar-Fuel Testing Facility of the Helmholtz Energy Materials Foundry (HEMF).