Molybdenum doping effect on sol-gel Nb2O5:Li+ thin films: Investigation of structural, optical and electrochromic properties

Abstract

Niobium pentoxide (Nb2O5) has attracted interest in the field of electrochromic devices because of its thermodynamic and chemical stability. However, its performance can be improved by doping with other transition metals or lithium salts, which leads to a change in its crystalline structure and properties. This work describes a study of the influence of molybdenum (Mo) doping on thin films of Nb2O5:Li+ properties. It was observed that the Mo dopant enhanced the cycling stability and reversibility of Nb2O5:Li+ films. The X-ray diffraction (XRD) pointed that the incorporation of the dopant leads to the mixture of the crystalline phases, monoclinic LiNb3O8 and orthorhombic T-Nb2O5. The addition of 7.5 mol% of Mo increased the charge density to 24.5 mC cm-2, ionic conductivity to 1.95 × 10-7 S cm-1, and the reversibility to 95% after 50 color/bleaching cycles. The color change at 500 nm was from 84 to 34%, and the scanning electronic microscopy (SEM) revealed homogeneous and mostly flat surface. The atomic force microscopy (AFM) showed the samples’ surface roughness rising from 0.49 for undoped to 1.15 nm for 7.5 mol% Mo doped film. All collected results suggest that doping improved the performance of Nb2O5:Li+ thin films over cycles, an important property for application in electrochromic devices.