Photoelectrochemical Applications of SrNbO₂n Thin Films Synthesized Via a Two-Step Process at Reduced Temperatures

Oxynitride materials often exhibit a perovskite structure in which the oxygen and nitrogen occupy the same lattice sites. Such oxynitride perovskites, for example SrNbO2N with a favorable bandgap of 1.8 eV for tandems, are studied as photoanodes for photoelectrochemical (PEC) water splitting applications. These photoanodes are an essential component to the OER side of the water splitting cell. In literature, SrNbO2N PEC electrodes are often prepared using ammonolysis of deposited Sr2Nb2O7 oxide at high temperatures. To transform Sr2Nb2O7 into SrNbO2N, the ammonolysis temperature has to reach over 900 °C for the time period of minutes to hours. This often leads to intermixing and reactions between the layers in the thin film device stack. In this presentation, PEC measurements of photocurrent are presented for SrNbO2N on n-Si in ferro/ferricyanide, at varying thicknesses and Sr:Nb ratios. For the deposition of these films, we show a two-step low-temperature synthesis approach to SrNbO2N on Si for tandem PEC applications. In the first step, combinatorial RF sputtering (sometimes with a nitrogen plasma cracker) is used to deposit Sr1+yNb1-yO2-xN1+x at ambient temperature with varying Sr:Nb and O:N ratios. In the second step, these films are annealed in a nitrogen atmosphere using Rapid Thermal Annealing (RTA). We surveyed a series of temperatures and times to determine the minimum temperatures needed to obtain highly crystalline SrNbO2N for PEC applications without reaction with the Si substrate. The resulting film morphology, crystalline structure, and other characterization across the combinatorial spread was characterized. Photocurrent was observed in these materials over a range of varying thicknesses and compositions using a solar simulator and a standard 3-electrode setup.