Thin metal oxide films have attracted a lot of attention in the past years due to their unique ability to act as electrode contact layers in novel electronic and optoelectronic devices. Prominent examples are molybdenum oxide (MoO_x) and titanium oxide (TiO_x) thin films used as hole and electron contact layers, respectively, in organic and hybrid photovoltaics. Amongst many different methods used for fabrication of these films, reactive sputtering remains as a promising technique, due to the unique composition tuning and industrial scale processing possibilities[1]. In the work presented here, crystalline MoO_x and TiO_x layers are developed from reactive sputtering and vacuum annealing, and implemented as contact layers in organic and hybrid solar cells.

The film composition is characterized using X-Ray Photoelectron Spectroscopy (XPS), work function using Low Energy Electron Microscopy (LEEM) and Ultraviolet Photoelectron Spectroscopy (UPS), structure using Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD), morphology using Atomic Force Microscopy (AFM) and optical properties using UV-VIS spectroscopy. Importantly, we find that both the structure and work function of the developed thin films can be tuned by the annealing process, spanning an almost 2eV tuning range in the case of MoO_x.[2] Furthermore, due to the formation of the crystalline films with a low defect density, made possible via the reactive sputtering method, more efficient and stable contact layers for photovoltaic devices are developed. Non-encapsulated DBP/C₇₀ solar cell devices based on the sputtered MoO_x are demonstrated to remain with impressive 80% of the initial performance after 240 hours of light soaking under 1 sun (1000W/m²) at ~60°C, which is superior to similar devices based on conventional thermally evaporated MoO_x layers. Fabricated PTB7/PCBM solar cells devices based on the sputtered TiO_x are demonstrated to lead to s-shape-free high performing devices, otherwise typically appearing when employing TiO_x as contact layers. The underlying film properties leading to these appealing device properties are evaluated based on the extensive surface and film characterization performed.

 The work thus demonstrates a viable method for tuning the electronic and optoelectronic properties of metal oxide thin films, which can be applied in combination with a wide range of materials in e.g. organic and hybrid photovoltaics.