From Geometry to Activity: A Quantitative Analysis of WO3/Si Micropillar Arrays for Photoelectrochemical Water Splitting

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 WO3/n‐Si and WO3/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 WO3/n‐Si micropillars, it is found that the photocurrent increases for WO3/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 WO3/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 WO3/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.