Polaron hopping induced dual-band absorption in all amorphous cathodic electrochromic oxides

Electrochromic oxides have tremendous potential applications in smart windows, displays, and camouflage due to their capability for selective modulation of visible and near-infrared optical spectra. Although these applications are dependent on the optical performance, the origin of the optical absorption in electrochromic oxides is not clear. Here, we demonstrate that the electrochromism of all amorphous cathodic electrochromic oxides can be described by a combination of polaron and bipolaron hopping. Based on the valences of the metallic constituents, we model experimental optical absorption spectra by polaron theory and assign two prominent absorption peaks to polaronic and bipolaronic charge transfer excitations. However, in the special case of V2O5, three peaks were necessary to fit the optical spectra. The activation energies of polaronic and bipolaronic hopping were remarkably similar for all the cathodic oxides studied. Within the framework of polaron absorption, V2O5 would be categorized as a cathodic oxide, rather than as a mixed anodic/cathodic material as in the conventional picture. We emphasize that our findings here not only offer a profound understanding of all amorphous cathodic electrochromic oxides but also pave the way for exploring electrochromic oxides with dual-band modulations.