Electrochromism and small-polaron hopping in oxygen deficient and lithium intercalated amorphous tungsten oxide films

Thin films of LixWO3-z with 0 <= x <= 0.27 and 0 <= z <= 0.27 were prepared by sputter deposition followed by electrochemical lithiation. Kramers-Kronig-consistent complex dielectric functions were obtained for these films by numerical inversion of experimental spectra of optical transmittance and reflectance by using a superposition of Tauc-Lorentz and Lorentz oscillator models. Low-energy optical absorption bands were induced by oxygen vacancies and/or by electrochemical intercalation of Li+ species together with charge compensating electrons. The experimental optical conductivity was fitted to a small-polaron model for disordered systems with strong electron-phonon interaction, taking into account transitions near the Fermi level. The optical absorption is due to small-polaron hopping and associated with the formation of W5+ states due to transfer of electrons from oxygen vacancies and/or insertion of Li+ species. The results also show increases in the Fermi level, caused by oxygen deficiency or Li+ insertion, which occur along with a band gap shift towards higher energies for the Li+ intercalated films. (c) 2015 AIP Publishing LLC.