Earth-abundant water-splitting catalysts coupled to silicon solar cells for solar-to-fuels conversion

Abstract

Direct solar-to-fuels conversion can be achieved by coupling semiconductors with water-splitting catalysts. A 10% or higher solar to fuels conversion is minimally necessary for the realization of a robust future technology. Many water-splitting devices have been proposed but due to expensive designs and/or materials, none have demonstrated the necessary efficiency at low-cost that is a requisite for large-scale implementation. In this thesis, a modular approach is used to couple water-splitting catalysts with crystalline silicon (c-Si) photovoltaics, with ultimate goal of demonstrating a stand-alone and direct solar-to-fuels water-splitting device comprising all non-precious, technology ready, materials. Since the oxygen evolution reaction is the key efficiency-limiting step for water-splitting, we first focus on directly interfacing oxygen evolution catalysts with c-Si photovoltaics. Due to the instability of silicon under oxidizing conditions, a protective interface between the PV and OER catalyst is required. This coupling of catalyst to Si semiconductor thus requires optimization of two interfaces: the silicon|protective layer interface; and, the protective layer|catalyst interface. A modular approach allows for the independent optimization and analysis of these two interfaces. A stand-alone water-splitting device based on c-Si is created by connecting multiple single junction c-Si solar cells in series. Steady-state equivalent circuit analysis allows for a targeted solar-to-fuels efficiency to be designed within a predictive framework for a series-connected c-Si solar cells and earth-abundant water-splitting catalysts operating at neutral pH. Guided by simulation and modeling, a completely modular, stand-alone water-splitting device possessing a 10% SFE is demonstrated. Importantly, the modular approach enables facile characterization and trouble-shooting for each component of the solar water-splitting device. Finally, as direct solar water-splitting is far from a mature technology, alternative concepts are presented for the future design and integration of solar water-splitting devices based on all earth-abundant materials.