Bright ways to utilize the sun: towards solar-to-fuel devices

Silicon (Si) is an attractive semiconductor material for a wide range of applications. Particular advantages result from the larger surface area of silicon microwires, and from surface functionalization with different materials that can be employed to tune the functionality of the substrate towards a desired application. For example, silicon can be used in photovoltaic (PV) cells, or it can be employed as one of the materials in a so-called solar-to-fuel (S2F) device, solely by changing the surface functionalization. In a simplistic view, a S2F device is constructed by adding electrocatalysts to a PV cell. However, to produce an efficient S2F device, silicon has to be modified in several manners, both regarding its structure (e.g. shaping into microwires) and using additional materials (e.g. catalysts, protective layer, junction, anti-reflection coating, etc.). The thesis discusses the use of silicon as a base material for a S2F device, employing structuring and modification.

In conclusion of the thesis, silicon microwires with radial p/n junctions were fabricated and optimized with respect to various parameters, i.e., junction depth, microwire height and pitch, and passivation and anti-reflection layers. The investigated microwires proved to be a very suitable template for the development of a hydrogen half-cell, i.e., a photocathode, in a silicon-based S2F device. The increased surface area of Si microwires is beneficial for both the light absorption capabilities and the catalyst activity. A strong dependence on the overall efficiency has been found on catalyst loading, both catalyst coverage in combination with microwire pitch. By careful design of the parameters mentioned above, a highly efficient, earth-abundant, Si-based photocathode has been fabricated that can function in both acidic and alkaline electrolytes. Lastly, a full simplistic wireless S2F was constructed, and experimentally validated. Micropores within a Si triple PV cell provided ionic short cuts, which decreased the overall ionic potential loss, and prevented a pH gradient within the device.