From Geometry to Activity: A Quantitative Analysis of WO<sub>3</sub>/Si Micropillar Arrays for Photoelectrochemical Water Splitting
Yihui Zhao, Pieter Westerik, Rudi Santbergen, E. Zoethout, Han Gardeniers, Anja Bieberle‐Hütter
Abstract
Abstract The photoelectrochemical (PEC) activity of microstructured electrodes remains low despite the highly enlarged surface area and enhanced light harvesting. To obtain a deeper understanding of the effect of 3D geometry on the PEC performance, well‐defined WO 3 /n‐Si and WO 3 /pn‐Si micropillar arrays are fabricated and subjected to a quantitative analysis of the relationship between the geometry of the micropillars (length, pitch) and their PEC activity. For WO 3 /n‐Si micropillars, it is found that the photocurrent increases for WO 3 /n‐Si pillars, but not in proportion to the increase in surface area that results from increased pillar length or reduced pillar pitch. Optical simulations show that a reduced pillar pitch results in areas of low light intensity due to a shadowing effect. For WO 3 /pn‐Si micropillar photoelectrodes, the p–n junction enhances the photocurrent density up to a factor of 4 at low applied bias potential (0.8 V vs RHE) compared to the WO 3 /n‐Si. However, the enhancement in photocurrent density increases first and then decreases with reduced pillar pitch, which scales with the photovoltage generated by the p–n junction. This is related to an increased dead layer of the p–n junction Si surface, which results in a decreased photovoltage even though the total surface area increases.