(353e) Heterostructured c-Si/BiVO4 Core-Shell Tandem Photoanode for Unassisted Photoelectrochemical Water Splitting

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. In principle, a PEC electrode consists of a single active material capable of handling the key processes: absorption of solar photon energy, charge separation and transport, and electrocatalysis to make and break chemical bonds at the electrode surface. In practice, there is no existing material that can handle all of these functions simultaneously; thus, the multicomponent system that integrate different materials with more specialized functions is a more viable approach.

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.

In this work, we present the fabrication of a heterostructured c-Si/BiVO₄ core-shell tandem based device capable of performing spontaneous water splitting without any precious metal. The wafer scaled device architecture was prepared by (1) nanostructuring of the 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 BiVO₄ surfaces 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.