Electrochromic Devices Based on Porous Tungsten Oxide Thin Films

Tungsten oxide (WO3) has been one of the most widely studied materials due to its multifunctional properties that have wide spectrum of applications in electrochromic devices, catalysers, gas sensors, optical switching devices, and so forth. Over the past three decades, intensive research has been carried out towards improving WO3-based electrochromic (EC) devices due to their important energy saving properties. A typical asymmetric EC device consists of an ion conducting (IC) layer, which also acts as a counter electrode and a WO3 electrochromic layer as the working cathode, sandwiched between two transparent conductive electrodes (TEs). When a low voltage (1–3 V) is applied across the TEs, Li+ ions from the counter electrode and electrons (e−) from the TE move to the EC WO3 layer. This double injection results in the formation of colored LixWO3 bronzes, with a controllable degree of coloration as a function of (Li ion concentration). When the applied voltages are reversed, leading to a bleached state.

Recent developments in the synthesis of transition metal oxides in the form of porous thin films have opened up opportunities in the construction of electrochromic devices with enhanced properties. In this paper, synthesis, characterization and electrochromic applications of porous WO3 thin films with different nanocrystalline phases, such as hexagonal, monoclinic, and orthorhombic, are presented. Asymmetric electrochromic devices have been constructed based on these porous WO3 thin films. XRD measurements of the intercalation/deintercalation of Li+ into/from the WO3 layer of the device as a function of applied coloration/bleaching voltages show systematic changes in the lattice parameters associated with structural phase transitions in LixWO3. Micro-Raman studies show systematic crystalline phase changes in the spectra of WO3layers during Li+ ion intercalation and deintercalation, which agree with the XRD data. These devices exhibit interesting optical modulation (up to ~70%) due to intercalation/deintercalation of Li ions into/from the WO3layer of the devices as a function of applied coloration/bleaching voltages. The obtained optical modulation of the electrochromic devices indicates that, they are suitable for applications in electrochromic smart windows.

 

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