TANDEM PHOTOVOLTAIC-PHOTOELECTROCHEMICAL CELLS FOR SOLAR WATER SPLITTING

Conversion of solar power to fuel is a promising path for solar energy storage. One way to do this is by solar-powered photoelectrolysis: splitting water to hydrogen and oxygen using solar power. Previous work at the Rothschild group developed a unique method to split water using photoanodes made of quarter-wave (~25 nm thick) hematite (α-Fe₂O₃) films on metallic specular reflectors. Our next goal is to couple these photoanodes to photovoltaic (PV) cells in order to construct a stand-alone tandem system for solar energy conversion and storage.

This work explores an innovative tandem cell design that couples hematite photoanodes with conventional PV cells using wavelength-selective dielectric mirrors instead of metallic specular reflectors. Toward this end we develop distributed Bragg reflectors (DBR) that reflect short wavelength photons (400 < λ< 550 nm) back into the hematite photoanode while transmitting long wavelength photons (550 <λ< 1100 nm) to the PV cell. This design enables simple monolithic integration and the use of more compatible and cheaper materials. The DBR comprises a multilayer stack of SiO₂ and Nb₂O₅ that is designed, using optical modeling, to achieve the desired spectral response. Preliminary tests of the very first specimens revealed that hematite photoanodes on DBR structures reached photocurrent enhancement of ~40% compared to similar photoanodes on transparent substrates. Theoretical predictions show that additional enhancement of ~30% can be achieved by adding an identical cell in a 45° V-shape configuration. These results will be discussed at the meeting.

The research leading to these results has received funding from the European Research Council under the European Union`s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617516].