Photophysics of amorphous WO3 thin films as transparent conductive oxides in the Near-IR

We present thin films of tungsten oxide WO3, fabricated by non-reactive RadioFrequency sputtering deposition and post-growth thermally annealed in air at 300°C for 8 hrs, that are simultaneously electrically conductive and optically transparent in the visible and near-infrared (NIR) spectrum. Their figure-of-merit as transparent-conductive-oxide (TCO) outperforms commonly used competitors (ITO, FTO, AZO) in the NIR up to 2500nm. Fabricated WO3 films are amorphous, uniform, well adherent to substrate and with subnano roughness. A thorough analysis, including SEM, EDX, XRD, Raman spectroscopy, electrical resistivity measurements, optical cw spectroscopy and spectroscopic ellipsometry, provides morphological, structural, optical and electrical information, also including some indication on chemical composition. Despite being characterized by a fully stoichiometric W/O ratio, the electrical conductivity of films is over 5000 times higher than that of crystalline WO3. By Raman spectroscopy applied to the amorphous phase and ultrafast Transient Absorption optical Spectroscopy, we analyse and discuss the nature of the defects related to W-O coordination that can be at the origin of the electrical conductivity, and their dynamical photophysics, compatible with optical transparency.