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

• Published in: Journal of applied physics Vol. 118; no. 2; p. 1
• Main Authors: Triana, C. A., Granqvist, C. G., Niklasson, G. A.
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 14.07.2015
• Subjects: Absorption spectra; Applied physics; Electrochromism; Electron phonon interactions; Electrons; Fermi level; Insertion; Lithium; Mathematical models; Oxide coatings; Oxygen; Polarons; Reflectance; Superposition (mathematics); Thin films; Tungsten; Vacancies
• ISSN: 0021-8979; 1089-7550; 1089-7550
• DOI: 10.1063/1.4926488

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.