Electrochromic

Chemical species that can switch between different colours electrochemically are said to be elecrtrochromic (EC) and have been widely studied for more than 40 years. The colour change is commonly between a transparent (also called bleached) state and a coloured state, or between two coloured states. Electrochromism are exhibited by a number of materials, both inorganic and organic liquids and solids. Electrochromism is a reversible color change in material caused by an applied electric field or current. This change can be due to the formation of color centers (or defect complexes) of an electrochemical reaction that produces a colored compound. For example, in an inorganic solid the ion-insertion reaction might be:

Cathodic MO_y + 𝑥A⁺ + 𝑥e⁻ ⇄ A_xMO_y, colored when reduced, Where A⁺ = H⁺ , Li⁺, Na⁺, Ag⁺.

Anodic MO_y + 𝑥A⁻ + 𝑥h⁺ ⇄ A_xMO_y, colored when oxidized, Where A⁻ = F⁻ , CN⁻, OH⁻/ H⁺ and x is generally 0 < x < 1.

An electrochromic device can consist of many configurations depending on property requirements. The basic elements of an electrochromic device are: two conductor layers

may in some cases be an insulator) and an ion storage media which may be the integral part of the conductive layer.

Figure 2.1 Basic elements of an electrochromic device.[41]

This sandwich configuration allows a reversible chemical reaction to cycle between the electrochromic material and ion storage media, which simultaneous injection of electrons or holes and protons or ions, depending on the material.

Whilst many type of chemical species exhibit electrochromism, only those with favourable EC performance parameters are potentially useful in commercial applications.

Thus, most applications require EC materials with a high contrast ratio, colouration efficiency (absorbance change/charge injected per unit area), cycle life, and write-erase efficiency (% of the originally formed colouration that may be subsequently

electro-bleached). Whereas displays need fast response times, by contrast ‘smart windows’ can tolerate response times of up to several minutes.

Traditionally, EC materials have been divided into three categories: (i) metal oxides, e.g., tungsten oxide (WO3) or nickel oxide (NiO); (ii) transition metal complexes, e.g., Prussian blue and its analogs; (iii) organic molecules or polymers, e.g., viologen and poly(3,4-ethylenedioxythiophene) (PEDOT). The following are schemes of viologen and PEDOT.

Figure 2.2 Various charge states of Viologen.[12]

Figure 2.3 Coloration and De-coloration of PEDOT.[42]

In the past decade, a new type of EC material—metallo-supramolecular—has attracted attention and shown benefits as both inorganic and organic materials. The synthesis of various functional organic ligands with a single metal ion (Fe, Ru or Co) for application to electronic papers and smart windows has been reported. Reports also show that different metal centres can directly affect polymer characteristics, including viscosity, magnetic moment, fluorescence, and even electrochromism.

In general, organic polymers are synthesized via the polymerization of monomers, which is accompanied by the formation of covalent bonds. In contrast, metallo-supramolecular polymers,[36] which are synthesized via the 1:1 complexation of metal ions and ditopic organic ligands, are a new type of polymers, in which the polymer backbone consists of coordination bonds. The most distinguished difference in the polymer structure between the conventional organic polymers and metallo-supramolecular polymers is that the chain length of metallo-metallo-supramolecular polymers is not fixed in solution unlike that of conventional polymers because the complexation is an

equilibrium reaction.

Figure 2.4 Metallo-supramolecular polymer formation.[37]

The color change of this material comes from the intramolecular MLCT (metal to ligand charge transfer). The MLCT in the visible region occurs from the HOMO (the highest occupied molecular orbital) of the metal ions to the LUMO (the lowest unoccupied molecular orbital) of the ligand in the complex moieties of the polymers. The colour of metallo-supramolecular polymers is controllable by selecting the metal ions species with different HOMO potentials or altered the ditopic organic ligands with different LOMO potentials. Various colours can be realized by the proper combinations of metal ions and ditopic ligands in the metallo-supramolecular polymer synthesis.

Figure 2.5 A possible mechanism for the electrochromic change in FeMEPE.

2.1.1 Transmittance Attenuation

Electrochromic contrast is probably the most important factor in evaluating an electrochromic material. It is often reported as a percent transmittance (T) at a specified

wavelength where electrochromic materials has the highest optical contrast. For some applications, it is more useful to report a contrast over a specific range rather than a single wavelength. To obtain an overall electrochromic contrast, measuring the relative luminance change provides more realistic contrast values since it offers a perspective on transmissivity of a material as it relates to the human eye perception of transmittance over the entire visible spectrum.

2.1.2 Coloration Efficiency

The coloration efficiency, CE (), (also referred to as electrochromic efficiency) is a

practical tool to measure the power requirements of an electrochromic material. In essence, it determines the amount of optical density change (OD) induced as a function

of the injected/ejected electronic charge (Qd), i.e., the amount of charge necessary to produce the optical change. It is given by the following equation:

𝜂 =^∆𝑂𝐷

𝑄𝑑 = log^𝑇𝑏

𝑇𝑐⁄𝑄_𝑑 (Eq. 2-1)

Where  (cm²/C) is the coloration efficiency at a given OD and Tb and Tc are the bleached and colored transmittance values, respectively. The coloration efficiency is measured by a spectro-electrochemistry method.

2.1.3 Switching Speed

Switching speed is often reported as the time required for the coloring/bleaching process of an EC material. It is important especially for applications such as dynamic displays and switchable mirrors. The switching time of electrochromic materials is dependent on several factors such as the ionic conductivity of the electrolyte, accessibility of the ions to the electroactive sites (ion diffusion in thin films), magnitude of the applied potential, film thickness, and the morphology of the thin film. The sub-second switching

electrochromes. Many electrochromic applications require high-contrast ratios allowing devices to be prepared that are opaque in one state while being highly transmissive in the other. The switching time is proportional to the square film thickness (k), t∝k²/D, where D is the diffusion coefficient of ions dependent on the solvent, ion concentration and applied voltage.