Light absorption mechanisms in sodium vanadium bronze thin films in electrochromic cells

Abstract

Light absorption mechanisms in electrochromic sodium vanadium bronze thin films presenting a phase mixture of hydrated NaV6O15 (predominant component) and Na1.1V3O7.9 were studied in this work. Light absorption spectra were analyzed by using the combination of our method of successive extraction of contributions in the weak absorption range and the standard method for determination of the optical bandgap in the strong absorption region, which is related to direct allowed transitions of electrons. Bandgaps connected to different contributions of indirect allowed, direct forbidden and direct allowed transitions have been determined for three values of the voltage (−2 V, 0 V and +2 V) applied on an electrochromic cell containing the studied sodium vanadium bronze thin film. Optical bandgap (direct allowed transitions) at zero voltage has been determined to be 2.986 eV. Three indirect allowed transitions with bandgap values of 1.398 eV, 2.377 eV and 2.605 eV have been identified, involving absorption/emission of phonons with energy of about 50 meV. Another one direct forbidden transition between points separated for 2.279 eV has been found. Positive voltage applied on the cell decreases all the bandgap values, while the negative voltage increases them.