Creating Functional Oxynitride-Silicon Interfaces and SrNbO2N Thin Films for Photoelectrochemical Applications br

The effect of absorption length, carrier diffusionlength, and surface area of an oxynitride photoabsorber on thephotoelectrochemical performance is investigated, and how to bestfabricate optical-quality thinfilms of band gap-tunable oxynitridesis also discussed. We targeted the stoichiometric compoundSrNbO2N as an optimal wide-band-gap photoabsorber (1.9 eV) foruse with silicon (1.1 eV) in a tandem structure photo-electrochemical cell. The preparation of perovskite oxynitrides athigh temperatures as isolated powders is often straightforward, butit is difficult to integrate them as thinfilms in tandem junctiondevices with low-temperature materials. Here, we develop thefirstmethod to prepare optical-quality SrNbO2N thinfilms of tunable thickness and roughness on a single-crystal silicon substrate. Toachieve this, an interfacial layer of ultrathin tantalum nitride (TaN) was used as a barrier to reduce the interdiffusion of silicon andoxygen during oxynitride synthesis. We prepared SrNbO2Nfilms of varying thicknesses (20-440 nm) on doped n+-Si(100) surfaces.The roughness factor (0.14-21) was scaled with thickness. The intrinsic photoelectrochemical activity of these devices wasevaluated using a low-barrier sacrificial electron donor. Photocurrent density and photovoltage revealed a significant (and nonlinear)dependence onfilm thickness and roughness. Absorption length, carrier diffusion length, and surface area were each found to playkey roles, and it is imperative to balance these properties to achieve optimal device performance.