Optical and electrochromic properties of annealed lithium-molybdenum-bronze thin films

Optical band gaps, Urbach inverse slopes, and coloration bands of various samples of annealed, microcrystalline Li_xMoO₃-bronze thin films in the concentration range 0<x<0.6 were determined over the photon energy range from 0.4 eV to 4.2 eV. On investigation, it is learned that the measured, optical band gaps do not shift rigidly over the annealing temperature range 293≤T≤423 K and, therefore, do not reveal the Burstein-Moss effect or reflect any stable, crystallographic phase transformation during any investigated annealing cycle. A model relating the temperature-dependent optical gap to the real part of the refractive index has also been developed, and this model fits very well to the annealed data within a maximum error of about 20%. Next, using an oscillator model, a phonon energy of ∼0.08 eV was obtained, which is very close to the characteristic phonon energy of the material, MoO₃. Using this model, it becomes more certain that the contributions to the Urbach absorption edge for the annealed-molybdenum bronzes are coming from the structural and compositional disorder. In another finding, it was found that the absorption-peak energy for the annealed data was about 1.5–1.6 eV, which is still broad and asymmetrical, and therefore, it is almost of the Mo⁶⁺ (or Mo⁴⁺)-Mo⁵⁺ intervalence or polaronic type. Using the polaron model, the half-bandwidth of the coloration bands of investigated, annealed Li_xMoO₃-thin films was found to be almost constant, which is consistent with the nonrigid band behavior.