Impact of Nanostructuring on the Photoelectrochemical Performance of Si-Ta3N5 Nanowire Photoanodes

The nanostructuring of light absorbing materials in photoelectrochemical applications can potentially improve the performance of charge transport limited semiconductors by increasing incident light absorption as well as the electrochemically active surface area. However, a drawback associated with an increase in electrode surface area is the increased effect of surface recombination on device performance. To understand the interplay of the positive and negative impacts of nanostructuring, we studied these effects by varying the nanowire length and thereby surface area, on the photoelectrochemical performance of tandem core-shell Si-Ta₃N₅photoanodes. Si-Ta₃N₅ nanowires of different lengths, 1.2 to 3.3 μm, were fabricated by changing the reactive ion etch duration by which the Si nanowires are formed and subsequently characterized by optical UV-Vis reflectance measurements, effective charge carrier lifetime measurements, and photoelectrochemical ferrocyanide oxidation. Overall, we show that as the nanowire length is increased, the photovoltage decreases due to decreasing effective carrier lifetimes that arise from higher surface recombination. On the other hand, the device photocurrent increases as the nanowires become longer due to increasing electrochemically active surface area and decreased light reflection, which in turn increases absorption due to light trapping within the nanowires. Balancing these effects is crucial toward developing high performance devices.