Fabrication of a Monolithic Heterostructured c-Si/BiVO4 Core-Shell Photoanode for Unassisted Solar Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting is a promising way of storing solar energy in hydrogen, which can be used as a carbon-free fuel or a chemical feed stock. A tandem-junction PEC device offers a high theoretical solar-to-hydrogen (STH) efficiency due to the additive voltages across two photoabsorbers and the better utilization of the solar spectrum. Modeling STH efficiency as a function of band gap for a tandem device shows that over 20% STH can be achieved with band gaps of 1.2 and 1.8 eV.1 One promising photoabsorber material combination is c-Si paired with a metal oxide, especially BiVO4. In addition to the appropriate band structure alignment of both semiconductors, c-Si and BiVO4 are low cost, earth abundant, and industrially scalable. However, the key challenges for these materials are the instability of c-Si in aqueous environment and the poor electronic properties of BiVO4, a common limitation for most metal oxides. Hence, only a very few c-Si/metal oxide systems have been demonstrated for unassisted water splitting.2 In this work, we present the fabrication of a monolithic heterostructured c-Si/BiVO4 core-shell tandem based monolithic device capable of performing spontaneous water splitting without any precious metal. The wafer scaled device architecture was prepared by (1) nanostructuring of the d c-Si substrate to serve as both a bottom absorber and a scaffold to improve the charge separation of BiVO4; (2) tuning the c-Si p-n junction depth to maximize the device photovoltage; (3) engineering the SnO2 interfacial layer to passivate the c-Si/BiVO4 interface and protect the c-Si p-n junction; (4) conformally coating a BiVO4 thin film on the nanostructured Si; and (5) decorating the c-Si and BiVO4surfaces with both oxygen and a hydrogen evolution catalysts. (1) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. ChemSusChem 2014, 7(5), 1372-1385. (2) (a) Shaner, M. R.; Fountaine, K. T.; Ardo, S.; Coridan, R. H.; Atwater, H. A.; Lewis, N. S. Energy Environ. Sci. 2014, 7 (2), 779-790; (b) Liu, C.; Tang, J.; Chen, H. M.; Liu, B.; Yang, P. Nano Lett. 2013, 13 (6), 2989-2992; (c) Xu, P.; Feng, J.; Fang, T.; Zhao, X.; Li, Z.; Zou, Z. RSC Adv. 2016, 6 (12), 9905-9910.