(418f) Flexible Platform for Studying Charge Transport in Tantalum Nitride and Oxynitride Photoanodes for Solar Water Splitting

Tantalum nitride (Ta₃N₅) and oxynitride (TaON) are promising semiconductors for use in photoanodes to split water using sunlight because their band edges straddle the water redox potentials and they absorb a large portion of the solar spectrum. Previous work suggests several charge transport processes, such as bulk hole and electron transport, may limit their photoconversion efficiency. We have developed a flexible platform for the synthesis of very uniform films which we employ to study charge transport within these materials. Electron beam evaporation was used to deposit a Pt back contact on quartz followed by a Ta layer of well-controlled thickness. Subsequent oxidation and nitridation, which required significant optimization to obtain phase-pure materials, yields crack-free, uniform films of either Ta₃N₅ or TaON. We measured the photoactivity of these materials as a function of film thickness under both broadband and monochromatic illumination. A major benefit to our approach is that, by omitting the Pt back contact, the exact same synthetic route yields films on a transparent substrate. This process enables us to quantify the optical absorption of the materials which is required to determine an absorbed photon-to-current efficiency (APCE), a key metric for decoupling absorption from photoactivity to focus on charge transport. The trends in the APCE as a function of film thickness allow us to distinguish between the effects of hole and electron transport and determine the limiting mechanism for each material and device architecture. These insights reveal strategies for designing an improved tantalum nitride or oxynitride photoanode structure.