Mechanisms Governing Solar-to-Hydrogen Conversion Efficiency in Multijunction Silicon Nanowire Photocatalysts

In the nearly 50 years since the first demonstration of photoelectrochemical water-splitting, solar fuels such as hydrogen have yet to become a practical source of alternative energy in large part due to the high cost and low efficiency of solar fuel systems. Si-based particle suspension reactors (PSRs), which consist of light-absorbing photocatalytic particles in solution that can individually split water, are a simple and potentially low-cost paradigm to generate hydrogen and enable solar fuel generation on a large scale. Multijunction silicon nanowires (MJ SiNWs) can absorb light across the visible spectrum, have p-n junctions that facilitate charge separation, and have recently demonstrated the ability to split water. Here, we present the bottom-up vapor-liquid-solid synthesis of single nanowires with multiple p-i-n junctions consisting of degenerately-doped segments and abrupt dopant transitions <10 nm. MJ SiNWs are functionalized for water-splitting with oxygen and hydrogen evolution co-catalysts via a spatioselective photoelectrodeposition process. Photovoltaic and electrochemical characterization of single-nanowire devices show tunable photovoltages exceeding 10 V under 1-sun illumination and water splitting activity at infrared wavelengths up to ~1050 nm. Experimental techniques and finite-element modeling have allowed us to systematically isolate and investigate aspects of STH including light absorption and charge carrier activity. Our studies indicate that photocatalyst performance and efficiency is dictated by nanowire dopant profile and surface composition, as well as the photonic characteristics of the sub-wavelength nanowire diameter. Unlike wider bandgap oxide and chalcogenide particles previously studied for PSRs, MJ SiNWs bring the photonic advantages of a tunable, mesoscale geometry and the cost and environmental advantages of Si to the PSR design, providing a new approach for water-splitting reactors.