Electrochromism in MoOx films characterized by X-ray electron spectroscopy

The ability of molybdenum oxide thin films to reversibly exchange lithium ions (with an electrolyte) and electrons (with an external circuit) has been explored in optical electrochromic devices. The as-grown films are either transparent or slightly blue and show a deep blue color upon Li+/e(-) insertion. The thin film electronic band and crystallographic structure govern the kinetics of insertion reaction and the charge capacity of the material, as well as the optical behavior of the system. In this work, molybdenum oxide thin films were produced by r.f, reactive sputtering of a metallic molybdenum target in an O-2-Ar atmosphere. Thin films with distinct compositions, were obtained by varying the oxygen flow (phi) during deposition and maintaining all other deposition parameters constant. Lithium intercalation was promoted by electrochemical means, from an aprotic, Li+-containing electrolyte. The films of distinct compositions were analyzed by X-ray photoelectron spectroscopy in order to investigate the relation between MoO3 reduction and the electrochromic effect reported in the literature [1]. Data obtained on high oxygen flow samples show narrow Mo 3d lines, with binding energies characteristic of the 6+ state. Lowering the oxygen flow results in very broad 3d lines, indicating the presence of Mo6+, 5+ and 4+. This broadening of the 3d lines is also observed upon Li intercalation in high flow samples both on as-grown films and on films intercalated with Li+/e(-). (C) 1999 Elsevier Science B.V. All rights reserved.