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.

Wireless photoelectrochemical (PEC) devices promise easy device fabrication as well as reduced losses. Here, the design and fabrication of a stand-alone ion exchange material-embedded, Si membrane-based, photoelectrochemical cell architecture with micron-sized pores is shown, to overcome the i) pH gradient formation due to long-distance ion transport, ii) product crossover, and iii) parasitic light absorption by application of a patterned catalyst. The membrane-embedded PEC cell with micropores utilizes a triple Si junction cell as the light absorber, and Pt and IrO _x as electrocatalysts for the hydrogen evolution reactions and oxygen evolution reactions, respectively. The solar-to-hydrogen efficiency of 7% at steady-state operation, as compared to an unpatterned η _PV of 10.8%, is mainly attributed to absorption losses by the incorporation of the micropores and catalyst microdots. The introduction of the Nafion ion exchange material ensures an intrinsically safe PEC cell, by reducing the total gas crossover to <0.1%, while without a cation exchange membrane, a crossover of >6% is observed. Only in a pure electrolyte of 1 m H ₂ SO ₄ , a pH gradient-free system is observed thus completely avoiding the build-up of a counteracting potential.