Organic Electronics for Electrochromic Materials and Devices. Hong Meng

Electrochromism is the phenomenon that describes the optical (absorbance/transmittance/reflectance) change in material via a redox process induced by an external voltage or current [1]. Usually the electrochromic (EC) materials exhibit color change between a colored state and colorless state or between two colors, even multicolor. In nature, its origin is from the change of occupation number of material's internal electronic states. As the core of EC technology, the EC materials have built up many categories during decades of development, for example, according to the coloration type, it could be classified as anodically coloring materials (coloration upon oxidation) or cathodically coloring materials (coloration upon reduction) [2]. Based on the light absorption region in the solar radiation, which consists of these three parts: ultraviolet (UV), visible (Vis), and near‐infrared radiation (NIR) lights (Figure 1.1), it could be divided into visible EC materials (wavelength: 380–780 nm), which can be seen by the human eye and therefore are suitable for smart window and indicator applications, and NIR EC materials (wavelength: 780–2500 nm), which have great potential for thermal regulation technologies and even in national defense‐related applications [3]. And on the basis of materials species, there are mainly inorganic, organic, and hybrid EC materials [4, 5] (https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg). Inorganic EC materials are transition metal oxides (TMOs) (WO₃, NiO, TiO₂, and Prussian Blue [PB]), organic EC materials including small molecules (e.g. viologen), conjugated polymers (e.g. poly(pyrrole), poly(thiophene), and poly(carbazole)) and aromatic polymers (e.g. polyimides [PIs] and polyamides [PAs]), organic–inorganic hybrid materials referring to metallo‐supermolecular polymers, and metal–organic framework (MOF). Among them, inorganic materials exhibit excellent long‐term stability compared with organic ones; however, considering the structure variety, flexibility, and low‐cost solution processability, organic EC materials are superior to inorganic materials. The organic–inorganic hybrid materials are designed to combine advantages of both organic and inorganic materials.

Figure 1.1 Solar irradiance spectrum above atmosphere and at the surface of the Earth.

      Source: Nick84: https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg, Licensed under CC BY‐SA 3.0.

EC materials exhibit color changes during the redox process; therefore the electrochromic devices (ECDs) generally consist of three elements: electrodes, EC materials, and electrolyte solution. The electrodes offer a constant supply of electric current, and ions are conducted by the electrolyte solution. Then the EC materials undergo electrochemical oxidation and/or reduction, which results in changes in the optical bandgap and colors. As shown in Figure 1.2, a typical ECD has five layers: two transparent conducting oxide (TCO) layers, EC layer, ion‐conducting layer (electrolyte solution), ion storage layer. Particularly, the ion storage layer acts as the “counter electrode” to store ions and keep electric charge balance. And according to the exact state of EC materials, there are three types of ECD: film type (I), solution type (II), and hybrid type (III). The Type I ECD is the most common; many kinds of EC materials are suitable for this type including TMOs, conjugated/non‐conjugated polymers, metallo‐supermolecular polymers, and MOF/covalent organic framework (COF) materials, which using spin‐coating, spray‐coating, and dip‐coating processes to form uniform films; these films won't dissolute in electrolyte solutions. Type II ECD requires that the EC materials have good solubility in electrolyte solutions. Therefore many organic small molecules such as viologen, terephthalate derivatives, and isophthalate derivatives are appropriate for this type of device. Meanwhile the fabrication method for this type of device is the most simple one. It just needs to dissolve the electrolyte and EC material in a specific solvent and inject into the prepared conducting cell. Type III ECD uses film‐type EC materials as working electrode and solution‐type EC materials as ion storage layer.

Figure 1.2 The scheme of three types of electrochromic devices.

1.2 The History of Electrochromic Materials

The word “electrochromism” was invented by John R. Platt in 1960 [6], in analogy to “thermochromism” and “photochromism.” However, the EC phenomenon could be traced to the nineteenth century, as early as 1815. Berzelius observed the color change of pure tungsten trioxide (WO₃) during the reduction when warmed under a flow of dry hydrogen gas. Then from 1913 to 1957, some patents described the earliest form of ECD based on WO₃ [7, 8]. Therefore the origins of electrochromism are the nineteenth and twentieth centuries. After then, electrochromism technology began to undergo rapid development, especially the exploration of many classes of EC materials. As showed in the technology roadmap (Figure 1.3), we summarized several generations of EC materials during long‐term development.

Figure 1.3 The roadmap of EC materials development.

The first‐generation EC material is TMOs (e.g. WO₃, NiO, and PB). Among them, WO₃ plays an important role in the electrochromism field; as the first founded EC material, it has already realized commercialization in smart windows application. PB was discovered as a dye by Diesbach in 1704, and then the electrochemical behavior and EC performance of PB was firstly reported by Neff at 1978 [9]. Benefitted from the structure stability and reversible redox process of those inorganic TMOs, the electrochromism based on the thin‐film TMOs are widely investigated, including the development of new TMOs materials, introduction of new nanostructures, and different element doping.

      Following the first‐generation TMO EC materials, organic small molecule EC materials have emerged since 1970. Among them, viologen as the most representative small molecule was first discovered by Michaelis and Hill in 1932 [10], and because of the violet on the reduction, these 1,1′‐disubstituted‐4,4′‐bipyridine compounds were named “viologen.” Then in 1973, Shoot made a new flat alphanumeric display using heptyl viologen; this can be regarded as the beginning of the use of viologen for electrochromism [11]. After a century's development, viologen already has been successfully commercialized. Besides the viologen, other small molecules EC materials such as terephthalate derivatives, isophthalate derivatives, methyl ketone derivatives, and some dye molecules have also attracted much attentions from scientists due to their simple structure and low cost.

      The third‐generation EC materials are conjugated polymers. In 1983, Francis Garnier and coworkers firstly characterized the EC properties of a series of five‐membered heterocyclic polymers including poly(pyrrole), poly(thiophene), poly(3‐methylthiophene), poly(3,4‐dimethylthiophene), and poly(2,2′‐dithiophene). Since then, conjugated polymers were given rise to the rapid emerge as a new class of electrochromism [12]. Five years later, Berthold Schreck observed the electrochromism phenomenon of poly(carbazole), which showed a color change from pale yellowish to green together with the conductivity enhancement [13]. To date, the conjugated polymer EC system has been well developed, from better understandings on mechanisms to completed color pallette with soluble or electro‐deposited polymers, and even full‐color display samples or roll‐to‐roll fabricated flexible