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 in our group developed a unique method to split water using photoanodes made of quarter-wave (~25 nm thick) hematite (α-Fe₂O₃) films on metallic specular reflectors¹. The next goal is to couple ultrathin film hematite 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 ultrathin film hematite photoanodes with conventional PV cells using wavelength-selective dielectric mirrors instead of metallic specular reflectors. Toward this end we develop distributed Bragg reflectors (DBRs) that reflect short wavelength photons (400<λ2 and Nb₂O₅ designed to achieve the desired spectral response. Photoelectrochemical measurements showed that using DBRs enhances the photocurrent of 10 nm thick hematite photoanodes by 37% in the potential plateau (@1.6V_RHE) compared to similar photoanodes on transparent substrates. Adding FeNiO_x co-catalyst and an identical cell in a 90° V-shape configuration increased the photocurrent by a factor of 4 at the reversible potential (1.23 V_RHE). Calculations show that a 30° V-shape configuration would absorb ~57% of the maximal hematite absorption within 10 nm hematite layer. Stable self-assisted photoelectrochemical water splitting tests using hematite photoanodes, silicon PV cells and DBRs were demonstrated.

1. Dotan et al., Resonant light trapping in ultrathin films for water splitting, Nat. Mater. 12, 158–164 (2013).