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

The effect of absorption length, carrier diffusion length, and surface area of an oxynitride photoabsorber on the photoelectrochemical performance is investigated, and how to best fabricate optical-quality thin films of band gap-tunable oxynitrides is also discussed. We targeted the stoichiometric compound SrNbO2N as an optimal wide-band-gap photoabsorber (1.9 eV) for use with silicon (1.1 eV) in a tandem structure photoelectrochemical cell. The preparation of perovskite oxynitrides at high temperatures as isolated powders is often straightforward, but it is difficult to integrate them as thin films in tandem junction devices with low-temperature materials. Here, we develop the first method to prepare optical-quality SrNbO2N thin films of tunable thickness and roughness on a single-crystal silicon substrate. To achieve this, an interfacial layer of ultrathin tantalum nitride (TaN) was used as a barrier to reduce the interdiffusion of silicon and oxygen during oxynitride synthesis. We prepared SrNbO2N films 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 was evaluated using a low-barrier sacrificial electron donor. Photocurrent density and photovoltage revealed a significant (and nonlinear) dependence on film thickness and roughness. Absorption length, carrier diffusion length, and surface area were each found to play key roles, and it is imperative to balance these properties to achieve optimal device performance.