Energy levels analysis of magnetron sputtered MoOx thin films

Ultrathin molybdenum oxide films are promising as transparent carrier-selective layers in photovoltaic cells, yet the electronic structure of sputtered molybdenum oxide (MoOx) and, therefore, the nature of electronic transport remains insufficiently understood. In this study, we show that magnetron-sputtered MoOx thin films exhibit a largely invariant energy-level structure across variations in thickness, target material, and underlying layer, while remaining electrically nonmetallic. Films with thicknesses of ≈5 nm and ≈20 nm were prepared from either a metallic molybdenum target in a reactive process or a ceramic MoOx target on glass and, for some samples, on germanium or aluminum doped zinc oxide coated glass. The results from Kelvin probe force microscopy, ultraviolet photoelectron spectroscopy, UV–Vis–NIR spectroscopy, optical modeling, and four-point probe measurements were combined to determine the work function, valence-band edge energy, optical bandgap, and derived conduction-band edge energy. Neither film thickness, target type, nor underlying layer seems to create a clear systematic shift of the energy levels. Additional occupied states were observed at binding energies of about 1.0–1.6 eV and are consistent with defect-related intra-gap states. The Fermi level is found to be ∼0.1–0.5 eV above the conduction-band edge derived from the optical bandgap. The results are discussed with respect to the mechanisms underlying the carrier-selective transport in functional MoOx thin films.