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

Abstract

Photoelectrochemical water splitting is one of many approaches being studied to harvest sunlight and produce renewable H2. 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 (Eg = 2.1 eV) on Ta foils were synthesized by oxidation of Ta metal in air at 550 °C and subsequent nitridation in NH3 at 900 °C. Scanning electron microscopy revealed that thermal stresses and difference...