Integrated wireless monolithic solar water splitting devices, i.e. monoliths submerged in the electrolyte, is a promising approach for low-cost photoelectrochemical solar water splitting [1]. However, such a device design poses a significant limitation: the ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This turns out to be a bottleneck for reaching high efficiencies and maintain optimum performance when it comes to up-scaling.

In this work, we present a novel approach to tackle the aforementioned limitation. Our device design is based on the concept of porosity as an ionic shortcut, implemented in silicon-based monoliths for low-cost and scalable solar water splitting [2, 3]. Simulation and experimental results towards enabling the proof of concept device consisting of porous multi-junction thin-film silicon solar cells on perforated substrates are presented. Based on simulations, we highlight how porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. As a result, the overpotentials to drive the water splitting reaction can be reduced by more than 400 mV. Experimentally, the impact of porosity (square array of holes with a period of 100 μm and a diameter of 20 μm) on single-junction and multi-junction amorphous and microcrystalline thin-film silicon solar cells is explained. A minimal decrease in V_OC is seen, with porous triple-junction thin-film silicon solar cells reaching a value of 1.98 V. Additionally, we discuss the implementation of surface coatings by atomic layer deposition to i) alleviate the material degradation that occurs during silicon etching (electrical passivation) and ii) enable a chemically stable operation (protection against material corrosion). Finally, to demonstrate the beneficial effect of porosity on the hydrogen production we focus on two systems: i) a well-known simplified system based on platinum nanoparticle decorated porous silicon photocathodes (V_on ≈ 475 mV) immersed in a sulfite scavenger (cell potential ΔΕ⁰ = 170 mV) in absence of electrical bias and ii) the envisage device of porous multi-junction silicon solar cell monoliths in unbiased solar water splitting conditions. Overall, keeping a device oriented point of view, we discuss the results and challenges in our approach as well as some design guidelines.

References:

[1] S. Y. Reece et al., Science 334, 645-648 (2011).

[2] T. Bosserez et al., J. Phys. Chem. C 120 (38), 21242 – 21247 (2016)

[3] C. Trompoukis et al., Sol. Energy Mater. Sol. Cells 182, 196–203 (2018).