Electrochromic devices fabrication

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

The LbL strategy to chemically modify electrodes with redox polyelectrolyte films has become an important tool for the fabrication of devices and electrodes with important future applications in biosensors, electrochromic devices, electrocatalysts, corrosion-resistant coatings, and so on. 

Compared with PEO electrolytes, PDVF, and PMMA electrolytes exhibited higher ionic conductivities. In particular, PMMA has attracted increasing attentions due to its low cost, high solvent retention ability, high transparency, and processibility. The first allpolymer electrochromic device was obtained based on a gel electrolyte and PEDOT-PSS [poly(styrene sulfonate)] electrochromic material (Argun et al., 2003). The fabricated device exhibited a maximum transmittance change of 51% at 540 nm. In addition, this device was fairly stable and only 5% contrast loss was observed after 32,000 cycles. 

Boehme, J.L., D.S.K. Mudigonda, and J.P. Ferraris. 2001. Electrochromic properties of laminate devices fabricated from polyaniUne, poly(ethylenedioxythiophene), and polyJM-methylpyrrole). Chem Mater 13 4469-4472. 

Lu, W., A.G. Fadeev, B. Qi, and B.R. Mattes. 2004. Fabricating conducting polymer electrochromic devices using ionic liquids. J Electrochem Soc 151 H33-H39. 

Polymer-based, variable light transmission electrochromic devices have been fabricated with counter-electrodes that are either optically passive or electrochromic in the complementary mode. Two devices with pMeT, electrochemically grown by monomer oxidation on ITO-coated glass as the electrochromic electrode, having colourless counter-electrodes in both oxidized and reduced states, and operating in liquid electrolyte, such as PC-LiC104, have been developed [29, 30]. 

Pol mier devices such as sensors, organic field effect transmitters, printed circuit boards, electrochromic devices, nonvolatile memory devices, or photovoltaic devices can be fabricated (22). 

These examples of electrochromic devices serve to display the breadth of their applicability to any number of systems. Polymeric electrochromics, particularly those based on thiophene and its derivatives, show promise for use in display technologies. The processability of many of these systems makes them specifically suited for large-area applications, such as smart windows, billboards or organic photovoltaic cells (solar cells), which currently suffer from large environmental and practical costs when fabricating large-area devices. 

The first example is the work of Lu et al. [124] who fabricated polypyrrole (PPy)ATi02 coaxial nanocables, where the conductivity of PPy was integrated with the photocatalytic activity of Ti02 for applications in electrochromic devices, nonlinear optical systems, and photoelectrochemical devices. The synthetic approach consisted in (1) preparation of Ti02 fibers by sol-gel electrospinning and calcination of the polymer (PVP in the specific case), (2) physical adsorption of Fe " oxidant on the surface of Ti02 nanofibers, and (3) polymerization of pyrrole (from vapor) on the surface of Ti02 nanofibers. 

Most recently Beaupre et al. developed a flexible electrochromic device using textile in 2006 [71]. The structure is made with a transparent electrode, covered with spray-coated electrochromic polymer, a gel electrolyte and finally with a conductive textile. The textile electrode is made with a textile fabric coated with copper and nickel. The other electrode is made of glass or polyester (PET) coated with ITO. Two electrochromic conductive polymers have been tested. Similar colours and colour changes are obtained for structures using two PET-ITO electrodes, or two glass-ITO electrodes, or one textile electrode with one PET-ITO electrode. The colour change is visible but slow. When a plastic electrode and a textile electrode are used, the structure is flexible. A similar structure, using a copper-coated textile cathode, was described by Zhan et al. in 2013 [72]. 

In 2014 Yan et al. developed a flexible, extensible electrochromic device [74]. The electrochromic material (tungsten trioxide) is electrochemically deposited on a flexible, extensible and transparent polydimethylsiloxane (PDMS) matrix coated with silver. The entire structure is extensible (up to 50%) and presents a visible colour change, from white to blue. The colouration occurs in 1 s and the discolouration in 4 s. After 100 cycles, the contrast between the oxidized and the reduced state only decreases by 19%. A small display composed of three independently controlled pixels was realized. The PDMS-based structure has been combined with a cotton fabric in order to form a textile-based electrochromic device. However, the textile itself can hardly be considered as indispensable for the working of this structure. It is more a plastic-based electrochromic structure that can be fixed on textile than a textile electrochromic structure. 

Other successful electrochromic devices have been realized by Kelly et al. using polyaniline-impregnated fibres [79]. In situ electrochemical polymerization of polyaniline is used to bind poly aniline to a PET or viscose spacer fabric. The fabric is then impregnated with an electrolyte and sandwiched between two electrodes. For the bottom electrode, carbon black or silver ink can be printed directly on the fabric. Polyaniline colour changes from green to blue through oxidation—reduction processes. However, the lifetime of this structure is also short and does not exceed dozens of oxidation—reduction cycles. 

Electrochromic devices, where a transparent window can be turned opaque at selected regions of the electromagnetic spectrum under a potential, are another area in which PA copolymers and substituted PAs may have potential applications. Promising research opportunities also exist in the fabrication of PA-based controlled-release devices, non-linear optical devices, light-emitting diodes, and radar shielding. The readers are encouraged to consult some older [191-196] and more recent [197-199] review articles on these subjects. 

---