Electrochromic Devices with Ion Conductor Layers Prepared by Sol-gel Method and Their Performances

Electrochromic device (ECD) is comprised of five superimposed layers, i.e., transparent conducting layer, electrochromic layer, ion conductor layer, ion storage layer and another transparent conducting layer, respectively. It can modulate the amount of solar radiation transmitting through it under applied voltage, often used as smart windows or facades to adjust the light and heat entering buildings proactively, which can sufficiently reduce the energy consumption. The existing ECDs are mainly fabricated by a magnetron sputtering method, which has a low productivity and high cost, especially for the ion conductor layer. Sol-gel method is a simple and relatively cheap method for preparing thin films, and massive researches are conducted on sol-gel electrochromic thin films such as tungsten oxide, nickel oxide etc.. In this work, an ion conductor layer was prepared by a sol-gel method, and its application in electrochromic device and their performance were also investigated.

Lithium tantalite sol was synthesized with lithium acetate (LiAc) and tantalum pentaethoxide (TaEt) as precursors and 1,2-Propanediol as a solvent. The molar ratio of LiAc and TaEt was 1:1. Lithium tantalite thin films were prepared by a spin coating method and heat-treated in air at 400 ℃ for 2 h. Some of the sol was dried and the subsequently formed powder was heated at 600–700 ℃. The phase compositions of the films and powders were characterized by X-ray diffraction (XRD), and the electrochemical properties of thin films were analyzed.

The electrochromic devices with a structure of ITO/NiO/LiTaO₃/WO₃/ITO were prepared by PVD method and subsequent sol-gel method. The tungsten oxide or nickel oxide thin film layer was firstly sputtered on the ITO glass and then lithium tantalite thin films were spin-coated onto the as prepared films. After heating process at 400 ℃ for 2 h, another layer of the tungsten oxide or nickel oxide films were sputtered, and another layer of ITO thin film was finally sputtered as a conducting electrode. The electrochromic devices were noted as EC-N when lithium tantalite was spin-coated onto nickel oxide films and as EC-W when lithium tantalite was spin-coated onto tungsten oxide films. The transmittance changes were measured by UV-vis-IR spectrometer.

The XRD patterns show that powders dried and heated from the sol are a polycrystalline lithium tantalite, and thin films are amorphous. The analysis of electrochemical impedance spectroscopy indicates that the sol-gel lithium tantalite thin film has an ionic conductivity of 7.44×10^-7 S/cm at room temperature, which is comparable to that of sputtered film. As for the devices, the XRD patterns show that nickel oxide thin film layers are both poly crystalline in EC-N and EC-W, while tungsten oxides is polycrystalline in EC-W and amorphous in EC-N. The XRD patterns and XPS spectra also show the existence of WO_3-x phase in poly cry stalline tungsten oxide films.

For electrochromic properties of the two groups of devices, transmittance regulations are measured at a fixed wavelength of 550 nm. For both groups, the differences of transmittance between colored and bleached state increase with increasing voltage. The maximum of transmittance change for EC-N is 26% at 5 V, with transmittance of around 72% at bleached state and 46% at colored state. Its coloring time is 50 s and bleaching time is 56 s. The results of cyclic test show that the differences of colored and bleached states keep stable, no obvious degeneration occurs after 60 cycles. The transmittance changes for EC-W have a lower value when applied the same voltages as in EC-N, and the switching speed is much slower. Its transmittance change is 14% at 3 V, while the coloring and bleaching time are both larger than 2 min, exceeding the setting range. It can be concluded that EC-N has a better property for regulation of visible light, and crystallinity can slow the switching process.

The transmittance spectra of colored states in vis-NIR range show that EC-W has a significant modulating performance for NIR, which can be attributed to the existence of WO_3-x components. As reported, oxygen deficient components can generate a local surface plasmon re sonance effect that can re sult in absorption in the spectral range of NIR.

Electrochromic devices fabricated by a process that lithium tantalite was spin-coated on nickel oxide had better electrochromic properties in a visible spectral range, with larger optical modulations and a higher switching speed less than 1 min. The devices with lithium tantalite spin-coated on the tungsten oxide films could have a slower coloring and bleaching rate, while showing a better near-infrared modulation effect. These results indicated a routine for the realization of electrochromic devices that could modulate light and heat simultaneously and independently.