Multilayered Hematite Nanowires with Thin-Film Silicon Photovoltaics in an All-Earth-Abundant Hybrid Tandem Device for Solar Water Splitting.

The concept of hybrid tandem device structures that combine metal oxides with thin-film semiconducting photoabsorbers holds great promise for large-scale, robust, and cost-effective bias-free photoelectrochemical water splitting (PEC-WS). This work highlights important steps toward the efficient coupling of high-performance hematite photoanodes with multijunction thin-film silicon photocathodes providing high bias-free photocurrent density. The hybrid PEC-WS device is optimized by testing three types of multijunction silicon photocathodes with the hematite photoanode: amorphous silicon (a-Si:H) tandem: a-Si:H/a-Si:H and triple junction with microcrystalline silicon (µc-Si:H): a-Si:H/a-Si:H/µc-Si:H and a-Si:H/µc-Si:H/µc-Si:H. The results provide evidence that the multijunction structures offer high flexibility for hybrid tandem devices with regard to tunable photovoltages and spectral matching. Furthermore, both photoanode and photocathode are tested under various electrolyte and light concentration conditions, respectively, with respect to their photoelectrochemical performance and stability. A 27 % enhancement in the solar-to-hydrogen conversion efficiency is observed upon concentrating light from 100 to 300 mW cm-2 . Ultimately, bias-free water splitting is demonstrated, with a photocurrent density of 4.6 mA cm-2 (under concentrated illumination) paired with excellent operation stability for more than 24 h of the all-earth-abundant and low-cost hematite/silicon tandem PEC-WS device.