Literature Seminar
Tungsten trioxide has garnered interest for applications in energy efficient materials. Reversible
electrochromism resulting in optical modulation makes tungsten trioxide a prime candidate for smart
windows and digital displays. Optical modulation in transition metal oxides (TMOs) is often attributed to
cation insertion; however, the physical mechanism responsible for these optical changes has not been
described precisely in literature. Understanding the mechanism of optical modulation in electrochromic
TMOs would provide precise tuning of optical properties improving functionality. To advance the field of
electrochromic materials and to gain quantitative knowledge about optical absorption in TMOs, a
discussion of polaron hopping is necessary. The absorbance of energy attributed to polaron hopping is
computed by accounting for the electronic transition between adjacent atoms in a material with
localized electron density. To characterize the optical absorbance in amorphous tungsten oxide, Triana
et al.(1) analyzed experimental spectra of oxygen deficient and lithiated films, calculated refractive index
and extinction coefficients, computed the complex dielectric function and optical conductivity, and
accurately modeled the effect of small-polaron hopping on the magnitude of optical absorbance. They
concluded that the observed optical change from transparent to dark blue amorphous tungsten oxide
films due to the absorption of infrared and visible light was responsible for increasing optical
conductivity and is a result of polaron hopping. Triana et al.’s approach can be applied to other TMOs to
characterize optical modulation of small-polaron hopping.
(1) Triana, C. A.; Granqvist, C. G.; Niklasson, G. A. Electrochromism and small-polaron hopping in oxygen
deficient and lithium intercalated amorphous tungsten oxide films. J. Appl. Phys. 2015, 118, 024901.
https://doi.org/10.1063/1.49
