An Investigation into the Resistance and Electron Selectivity of Ultra-Thin Titanium-Oxide Films

We investigated the properties and electron transfer rates between small solution probe molecules and TiO_x ultra-thin films over indium tin oxide (ITO). TiO_x and ITO are both increasingly common materials found in “inverted” organic photovoltaic (OPV) architectures. In order to selectively harvest electrons only from the acceptor phases in OPVs, we require an interlayer (5-50 nm) which is robust, easily processed at low temperatures, and will only allow for electrons to pass to the underlying contact (ITO).  We utilized a light induced chemical vapor deposition (LICVD) technique where a titanium alkoxide precursor was activated by heat and ultra-violet light to deposit ultra-thin and conformal TiOx films. Despite the inherent transparency and electron-selective properties, it is also intrinsically non-conductive. Therefore, we addressed the resistance of the ultra-thin films by decreasing the thickness. We have shown that as the film thickness was reduced to less than 10 nm, the net resistance of the film was shown to decrease according to our conductive-tip atomic force microscopy results. Conductivity maps indicate that our film becomes increasingly and uniformly conductive compared to the film thicknesses greater than 15 nm. In conjunction with conductivity studies, cyclic voltammetry was utilized to monitor the electron selective properties of TiOx, where probe molecules N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD), ferrocene (Fc) and tris(bipyridine)ruthenium(II) chloride (Ru(bpy)3) were utilized to indicate electron selectivity was preserved at a 10 nm film thickness. These findings will help foster our understanding into solar electric materials and provide insight into the successful building of an inverted OPV.