Doped tungsten oxide nanocrystals for next generation electrochromic windows
Doped tungsten oxide (WO₃ [subscript -x]) nanocrystals (NCs) have lots of potential for next generation electrochromic windows compared to bulk thin films. The difference lies in intrinsic WO₃ [subscript -x] NC properties of shape and crystalline anisotropy with localized surface plasmon resonance absorption in the shorter wavelength near-infrared range, which can directly affect the major electrochromic performance of switching speed, optical modulation, and cycling stability. This work illustrates how doped WO₃ [subscript -x] NC properties are effectively utilized for enhancing the electrochromic performance. First, how shape anisotropic properties can generate highly porous film is studied using different aspect ratio of WO₂.₇₂ nanorods. By changing the nanorod interaction to electrostatic repulsion from solution ligand-stripping chemistry, highly porous mesoporous thin film from randomly packed nanorods is fabricated. Incorporating guest inorganic materials of niobium polyoxometalate clusters followed by chemical condensation, dual-band modulation of electrochromic films on flexible substrates are demonstrated, tackling cycling stability, and optical modulation issues. Second, how doped semiconductor NCs are effectively used for dynamic Bragg stacks with targeted performance of ‘on and off’ reflectance is studied. Dynamic reflectance tuning can affect the color tuning as well as efficient heat blocking. Judicious NC selection of indium tin oxide and WO₃ [subscript -x] NCs from mechanistic understanding of electrochemical modulation of optical properties, optimization of film processing, and reliable refractive index data from in situ ellipsometry enable accurate Bragg stack optimization from simulation and experimental realization. Third, using monoclinic WO₂.₇₂ nanorods as a model system having different size of three intracrystalline tunnel sites, we study spectroelectrochemical properties with different cation system (lithium, sodium, and tetrabutylammonium ion). In doing so, Al₂O₃ atomic layer deposition is employed to prevent electrolytes degradation and allows to study spectroelectrochemical properties. Na⁺ electrolytes system gives higher coloration efficiency than Li⁺ electrolytes and mainly capacitive charging behavior owing to its occupancy in the hexagonal tunnel sites. The results of these studies suggest general approach to improve electrochromic performance where shape and crystalline anisotropic properties of doped metal oxide nanocrystals can be effectively utilized for impacting spectroelectrochemical properties.