Effect of Film Morphology and Thickness on Charge Transport in Ta3N5/Ta Photoanodes for Solar Water Splitting

Photoelectrochemical water splitting is one of many approaches being studied to harvest sunlight and produce renewable H-2. Tantalum nitride (Ta3N5) is a promising photoanode candidate as its band edges straddle the water redox potentials and it absorbs a large portion of the solar spectrum. However, reported photocurrents for this material remain far from the theoretical maximum. Previous results indicate Ta3N5 may be hindered by charge transport limitations attributed to poor bulk charge transport, charge transport across grain boundaries, and/or charge transfer across the interface at the back contact. The primary goal of this work was to study these mechanisms, especially bulk hole and electron transport, to determine which processes limit device efficiency. Crystalline thin films (60-780 nm) of Ta3N5 (E-g = 2.1 eV) on Ta foils were synthesized by oxidation of Ta metal in air at 550 degrees C and subsequent nitridation in NH3 at 900 degrees C. Scanning electron microscopy revealed that thermal stresses and differences in the density of the phases resulted in the formation of porous, textured films with high surface area. Films were characterized by their photon absorption, crystal grain size, and electrochemically active surface area. Trends in photoactivity as a function of film thickness under broadband illumination as well as in the incident photon-to-current efficiency revealed that minority charge carrier (hole) and majority carrier (electron) transport both play important roles in dictating photoconversion efficiency in Ta3N5 alms.