Electronic properties and ion migration of “in vacuo” lithiated nanoporous WO3:Mo thin films

Electrochromic (EC) glazing helps manage daylight and solar heat gains in building, thereby allowing a reduction in energy consumption caused by heating, cooling, and artificial lighting. This study relates the optical and electronic properties of nanoporous amorphous molybdenum-doped tungsten trioxide thin films (WO3:Mo) in the pristine state and upon lithiation. When such a film is used as a cathode in EC devices, the color neutrality can be improved with respect to pure WO3, and electrochromic transmission control can be achieved in the full spectral range of solar radiation. In situ x-ray photoelectron spectroscopy reveals that the coloration mechanism is related to the reduction of W6+ to W5+ and Mo6+ to Mo5+. In the initial stages of lithiation, Mo is preferably reduced followed by the reduction of W. Ultraviolet photoelectron spectroscopy highlights systematic trends in the position of the valence band edge and in work function. The occurrence of peaks at 2.2 and 0.8 eV is observed and is related to the formation of partially delocalized Mo5+ and W5+ midgap states. Visible/near-infrared spectrophotometry shows initial absorption mainly in the visible spectral range, followed by absorption in the near infrared. Both absorption bands can be associated with the midgap states due to the occurrence of Mo5+ and W5+, respectively. Lithiation of bilayers composed of WO3:Mo and WO3 shows that the Mo5+ states, which are energetically lower, trap preferentially the transferred charges. Furthermore, our results suggest that lithium ions diffuse rather freely in the direction perpendicular to the substrate. These findings pave the way to next-generation EC devices with color neutral and broadband modulation of spectral transmission and in principle also with dual-band modulation of visible and near-infrared light.