Flexing the Spectrum: Advancements and Prospects of Flexible Electrochromic Materials

Abstract

The application potential of flexible electrochromic materials for wearable devices, smart textiles, flexible displays, electronic paper, and implantable biomedical devices is enormous. These materials offer the advantages of conformability and mechanical robustness, making them highly desirable for these applications. In this review, we comprehensively examine the field of flexible electrochromic materials, covering topics such as synthesis methods, structure design, electrochromic mechanisms, and current applications. We also address the challenges associated with achieving flexibility in electrochromic materials and discuss strategies to overcome them. By shedding light on these challenges and proposing solutions, we aim to advance the development of flexible electrochromic materials. We also highlight recent advances in the field and present promising directions for future research. We intend to stimulate further innovation and development in this rapidly evolving field and encourage researchers to explore new opportunities and applications for flexible electrochromic materials. Through this review, readers can gain a comprehensive understanding of the synthesis, design, mechanisms, and applications of flexible electrochromic materials. It serves as a valuable resource for researchers and industry professionals looking to harness the potential of these materials for various technological applications.

Keywords: flexible electrochromic materials; hybrid-based electrochromic materials; inorganic-based electrochromic materials; organic-based electrochromic materials.

Figure 1

A general schematic diagram of an electrochromic device showing the (a) bleached and (b) colored states. Adapted with permission from reference [11]. Copyright: the authors, some rights reserved, exclusive licensee MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY).

Figure 2

Types of electrochromic devices based on the solubility of the EC components.

Figure 3

General structure of flexible electrochromic materials.

Figure 4

(Top) Layered monolithic structure of flexible electrochromic device. (Bottom) Experimental setup for conducting bending tests on ECDs in both bleached and colored states. Adapted with permission from reference [80]. Copyright (2016), Elsevier.

Figure 5

Redox states of various organic-based EC materials: (a) viologen derivatives (R₁ and R₂ are alkyl or aromatic moieties); (b) tetracyanoquinodimethane; (c) tetrathiofulvalene; (d) quinone; (e) polythiophene; (f) polyaniline; and (g) polypyrrole.

Figure 6

Illustration presenting the proposed mechanism governing the color transition in the electrochromic device. The diagram also features the flexible device itself, showcasing its response to various bias conditions. Adapted with permission from reference [107]. Copyright (2019), American Chemical Society.

Figure 7

Coordination polymer films of metal(II) salts and P1 (poly(4-(2,2′:6,2″-terpyridyl)phenyliminofluorene)) were formed through sequential assembly. Spectroelectrochemistry was performed on these films using 12 dipping cycles on ITO-coated glass supports. Color transitions after 24 dipping cycles are shown in the inset pictures. The experiments were conducted using a 0.1 M TBAPF6/acetonitrile electrolyte/solvent couple at applied potentials (V). Adapted with permission from reference [119]. Copyright (2017), Elsevier.

Figure 8

(a) Demonstration of conductive polymer pattern fabrication on various substrates using solution-based methods. Steps include (i) monomer casting and oxidative polymerization, (ii) photolithography, (iii, vi) peeling off layers, (iv) PEDOT in ITO glass, and (v) gelation processes. (vii) The resulting patterns are transferred from glass to a flexible hydrogel substrate. (b) Photographs depict the process, and scale is represented by a 1 cm bar. Adapted with permission from reference [99]. Copyright (2016), American Chemical Society.

Figure 9

(a) A schematic diagram of RF rotating plasma, and (b) an illustration of the plasma polymerization process. Adapted with permission from reference [134]. Copyright (2017), John Wiley and Sons.

Figure 10

A modular roll-to-roll (R2R) coating machine enables the efficient deposition of PEDOT-EthC6 thin films onto PET-ITO substrates for large-scale production. (a) Modular R2R coating machine with unwinding unit, slot die, ISP, and oven section. (b) Slot die deposition. (c) ISP section (6 m), showing a yellowish colored wet film. Adapted with permission from reference [136]. Copyright: the authors, some rights reserved, exclusive licensee John Wiley and Sons. Distributed under a Creative Commons Attribution License 4.0 (CC BY).