Conjugated electrochromic (EC) polymers for flexible EC devices (ECDs) generally lack a fully colorless bleached state. A strategy to overcome this drawback is the implementation of a new sidechain-modified poly(3,4-ethylene dioxythiophene) derivative that can be deposited in thin-film form in a customized high-throughput and large-area roll-to-roll polymerization process. The sidechain modification provides enhanced EC properties interms of visible light transmittance change, Δτ v = 59% (ΔL* = 54.1), contrast ratio (CR = 15.8), coloration efficiency (η = 530 cm² C −1 ), and color neutrality (L* = 83.8, a* = −4.3, b* = −4.1) in the bleached state. The intense bluecolored polymer thin films exhibit high cycle stability (10 000 cycles) and fast response times. The design, synthesis, and polymerization of the modified 3,4-ethylene dioxythiophene derivative are discussed along with a detailed optical, electrochemical, and spectroelectrochemical characterization of the resulting EC thin films. Finally, a flexible see-through ECD with a visible light transmittance change of Δτ v = 47% (ΔL* = 51.9) and a neutral-colored bleached state is developed.
The cycling stability of flexible electrochromic devices (ECDs) under humid atmospheres is limited by irreversible indium tin oxide (ITO) reduction. A strategy to limit this degradation was developed and tested for model ECDs based on a sidechain-modified poly(3,4-ethylene dioxythiophene) (PEDOT) derivative and Prussian blue (PB). This work reveals that the cycling stability is reduced by dissolution of the ITO thin films and formation of metallic indium particles on the surface of the ITO layers. The ITO degradation strongly depends on the applied electrode potentials in combination with moisture ingress into the ECDs. To avoid ITO reduction in ECDs, efforts were made to adjust the electrode potentials. ECDs equipped with an auxiliary reference electrode were set up to gather knowledge on the actual electrode potentials. By adjusting the electrode charge density ratio, it was possible to narrow the overall cell voltage window to an extent in which irreversible ITO reduction no longer occurs. Detailed investigation of ECDs with the optimized cell configuration (charge density ratio) showed that the overall device performance with regard to visible light transmittance change and response time is not impaired and that the cycling stability under humid atmosphere (90% rH) is dramatically improved. Thus, the proposed strategy offers an excellent perspective for the commercialization of flexible ECDs upon their enhanced durability.
Large‐area electrochromic devices (ECDs) based on a cathodically‐coloring, side chain‐modified poly(3,4‐ethylene dioxythiophene) (PEDOT) derivative and anodically‐coloring Prussian blue (PB) are assembled by a customized sheet‐to‐sheet (S2S) lamination process. The ECDs with two complementary switching “half‐cells”, based on flexible PET‐ITO substrates, offer enhanced optical properties in terms of visible light transmission change (4–53%), contrast ratio (CR = 93.4) and color neutrality (L* = 77.9, a* = −5.9, b* = −0.6) in the bleached state. The cycling stability is monitored for up to 10 000 switching cycles (97% charge retention). The reported optical, electrochemical, and in operando (in situ) spectroelectrochemical data are obtained from small laboratory‐scale ECDs (active area: 5 × 5 cm2). These results, the homogeneity of the large‐area devices and the scalability of the S2S lamination process are confirmed by measurements on the large ECDs with an active area of 45 × 65 cm2. This large‐area electrochromic film technology with high optical contrast and enhanced cycling stability offers an excellent perspective for further development and scale‐up towards pilot production and the commercialization of flexible ECDs upon their unique materials and processing characteristics.
Printable organic electrochromic materials are the key component of flexible low power and low weight displays and dynamic shading systems. A vast number of more or less well‐performing materials is reported in the literature, but only a very limited number of them have been tested in an industrially‐relevant environment so far. Upscaling requires simplicity of synthesis, overall sustainability, low cost and compatibility with simple and high throughput wet‐chemical deposition techniques, such as slot‐die coating or inkjet printing. In the present paper, an original process is described that enables the controlled oxidative polymerization of a water insoluble, functionalized 3,4‐ethylene dioxythiophene (EDOT) derivative. This process leads to the formation of an ink that consists solely of active polymeric material (no dispersing agents) and has suitable rheological properties for use in roll‐to‐roll slot‐die coating or ink‐jet printing. The straightforward deposition, followed by a simple thermal treatment, directly yields stable and homogeneous thin films with state‐of‐the‐art electrochromic performance.
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