Transparent to Black Electrochromism-The "Holy Grail" of Organic Optoelectronics

Abstract

In the rapidly developing field of conjugated polymer science, the attribute of electrochromism these materials exhibit provides for a multitude of innovative application opportunities. Featuring low electric potential driven colour change, complemented by favourable mechanical and processing properties, an array of non-emissive electrochromic device (ECD) applications lays open ahead of them. Building up from the simplest two-colour cell, multielectrochromic arrangements are being devised, taking advantage of new electrochromic materials emerging at a fast pace. The ultimate device goal encompasses full control over the intensity and spectrum of passing light, including the two extremes of complete and null transmittance. With numerous electrochromic device architectures being explored and their operating parameters constantly ameliorated to pursue this target, a summary and overview of developments in the field is presented. Discussing the attributes of reported electrochromic systems, key research points and challenges are identified, providing an outlook for this exciting topic of polymer material science.

Keywords: displays; e-paper; electrochemistry; electrochromic device; electrochromism; pi-conjugated molecule; polymer; redox doping; smart window.

Figure 1

Theoretically attainable chromic states for a single working electrode modified with two polymer electrochromes.

Figure 2

Theoretically attainable chromic states for a two-electrode system, where each electrode has been modified with a different polymer electrochrome.

Figure 3

Schemes of three different designs of electrochromic devices: (A) First type with polymer matrix; (B) second type with thin polymer films applied on ITO; (C) third type with thin film of metal oxide modified by a redox chromophore.

Figure 4

Flexible and stretchable electrochromic devices (ECDs) presented by Yan at al. Reprinted with permission from [27]. Copyright 2014, American Chemical Society.

Figure 5

Spectroelectrochemistry of a copolymer of alkoxy derivatised poly(3,4-propylenedioxythiophene)-ProDOT and 2,1,3-benzothiadiazole, demonstrating black to transmissive switch upon doping of the polymer layer deposited at ITO electrode. Reprinted with permission and adapted from [30]. Copyright 2008, Springer Nature.

Figure 6

Polyamide composed of methoxy functionalised oligotriarylamine and diphenyl ether carboxylic acid units, demonstrating spectacular basic colour multielectrochromism, involving neat transmissive and black states. Reprinted with permission and adapted from [5]. Copyright 2012, The Royal Society of Chemistry.

Scheme 1

Viologen derivative electrochromes investigated by Alesanco et al. in [11].

Scheme 2

PrODOT/benzothiadiazole copolymer (ECP-Black) used as an active layer and 3,4-propylenedioxypyrrole (MCCP) used as a charge storage layer by Hassab et al. in [41].

Scheme 3

Triphenylamine derivative-based polyamides and a gel electrolyte containing an electrochromic viologen derivative, proposed by Liu et al. in [43].

Scheme 4

Alternant conjugated/non-conjugated polyamide copolymers, containing a novel fluorenyldiphenylamine system reported in [44].

Scheme 5

Diimide and triphenylamine moieties investigated by Sun et al. in [45].

Scheme 6

Phenylenediamine-based rhodamine systems used by Wang [46] to prospect bond-coupled electron transfer reaction in an electrochromic device.