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

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.

Relating the 3D Geometry and Photoelectrochemical Activity of WO3-Loaded n-Si Nanowires: Design Rules for Photoelectrodes

“…Furthermore, due to the grown structure of the nanowire array, shadowing causes lower PEC activity for the WO 3 located at the lower part than the top part of the nanowires. 42 The sketch in Figure 5 illustrates the relation between the active surface area of the WO 3 /Si nanowire array and the Si etching time. Longer etching time results in a loss of Si volume and surface area in the top part of the nanowire array (Figure S1) and therefore less WO 3 at the top part (Figure S2).…”

“…These factors result in a slower increase of the photocurrent density as a function of the Si etching time. Furthermore, due to the grown structure of the nanowire array, shadowing causes lower PEC activity for the WO 3 located at the lower part than the top part of the nanowires . The sketch in Figure illustrates the relation between the active surface area of the WO 3 /Si nanowire array and the Si etching time.…”

In Situ Magnetic Alignment of a Slurry of Tandem Semiconductor Microwires Using a Ni Catalyst

“…A B-doped p + radial emitter layer was formed on the n-Si planar or microwire array (2 cm x 1 cm wafer) using diffusion from solid-source BN dopant wafers (Saint Gobain-BN-975). [41,42] The silicon wafers were placed between the dopant wafers in a ceramic boat at 950 °C for 4 min under N 2 flow (200 sccm in a 1-inch diameter tube). The boat was slowly removed from the furnace over > 1 min while the temperature ramped down to 750 °C.…”

Photoelectrochemical water splitting: a road from stable metal oxides to protected thin film solar cells

“…7a). [126][127][128] Liu et al demonstrated a Z-scheme photoelectrode depositing TiO 2 nanorods on the top of Si microwires, making each of the tree-like structures a bias-free photoelectrode. 129 TiO 2 nanorods on top of Si microwires combine large and small bandgap materials too (Fig.…”