Electrochromic organic and polymeric materials for display applications

An electrochromic material is one where a reversible color change takes place upon reduction (gain of electrons) or oxidation (loss of electrons), on passage of electrical current after the application of an appropriate electrode potential. In this review the general field of electrochromism is introduced, with coverage of the types, applications, and chemical classes of electrochromic materials and the experimental methods that are used in their study. The main classes of electrochromic organic and polymeric materials are then surveyed, with descriptions of representative examples based on transition metal coordination complexes, viologen systems, and conducting polymers. Examples of the application of such organic and polymeric electrochromic materials in electrochromic displays are given

Establishing Dual Electrogenerated Chemiluminescence and Multicolor Electrochromism in Functional Ionic Transition-Metal Complexes

A combination of electrochromism and electroluminescence in functional materials could lead to single-layer dual electrochromic/electroluminescent (EC/EL) display devices, capable of simultaneous operation in emissive and reflective modes. Whereas such next generation displays could provide optimal visibility in any ambient lighting situation, materials available that exhibit such characteristics in the active layer are limited due to the required intrinsic multifunctionality (i.e., redox activity, electroluminescence, electrochromism, and ion conductivity) and to date can only be achieved via the rational design of ionic transition-metal complexes. Reported herein is the synthesis and characterization of a new family of acrylate-containing ruthenium (tris)­bipyridine-based coordination complexes with multifunctional characteristics. Potential use of the presented compounds in EC/EL devices is established, as they are applied as cross-linked electrochromic films and electrochemiluminescent layers in light-emitting electrochemical cell devices. Electrochromic switching of the polymeric networks between yellow, orange, green, brown and transmissive states is demonstrated, and electrochemiluminescent devices based on the complexes synthesized show red-orange to deep red emission with λmax ranging from 680 to 722 nm and luminance up to 135 cd/m2. Additionally, a dual EC/EL device prototype is presented where light emission and multicolor electrochromism occur from the same pixel comprised of a single active layer, demonstrating a true combination of these properties in ionic transition-metal complexes

Mapping the Broad CMY Subtractive Primary Color Gamut Using a Dual-Active Electrochromic Device

Although synthetic efforts have been fruitful in coarse color control, variations to an electrochromic polymer (ECP) backbone are less likely to allow for the fine control necessary to access the variations and shades of color needed in display applications. Through the use of thin films of cyan, magenta, and yellow ECPs, non-emissive subtractive color mixing allows the color of an electrochromic device (ECD) to be selected and tailored, increasing access to various subtle shades and allowing for a non-emissive display to exhibit a wide range of colors. Using a dual-active ECD, subtractive color mixing utilizing the cyan–magenta–yellow (CMY) primary system was examined. The bounds of the gamut, or the subset of accessible colors, using these three 3,4-propylenedioxythiophene (PProDOT)-derived materials in combination with the recently recognized 3,4-propylenedioxypyrrole-based minimally color changing polymer (MCCP) were mapped, highlighting the benefit of applying subtractive color mixing toward the development of full-color non-emissive displays. Here, we demonstrate that ECPs are suitable for the generation of a wide gamut of colors through secondary mixing when layered as two distinct films, exhibiting both vibrantly colored and highly transmissive states

Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-to-Transmissive Electrochromic Conjugated Polymers

A series of conjugated polymers were designed and synthesized to extract structure–property relationships with the goal of yielding yellow-to-transmissive switching electrochromes. The polymers are based on repeat units of propylenedioxythiophene (ProDOT) in alternation with a variety of arylenes including 1,4-phenylene (ProDOT-Ph), 2,7-fluorene (ProDOT-Fl), 2,7-carbazole (ProDOT-Cbz), 2,5-dimethoxy-1,4-phenylene (ProDOT-Ph­(MeO)₂), and 2,7-pyrene (ProDOT-Py). Additionally, a random copolymer containing ProDOT and two different arylene units was produced: ProDOT-phenylene-ProDOT-dimethoxyphenylene (R-ProDOT-Ph/Ph­(MeO)₂) and two polymers with a ProDOT dimer in alternation with pyrene and phenylene composed ProDOT₂-pyrene (ProDOT₂-Py) and ProDOT₂-phenylene (ProDOT₂-Ph), respectively. The polymers were synthesized using Suzuki polycondensation. Examinations of the optoelectronic properties via UV–vis–NIR spectroscopy, differential pulse voltammetry, and spectroelectrochemistry show that varying the electron richness of the polymer by utilizing more electron rich arylenes, dimers of ProDOT, or less electron rich arylenes, the oxidation potential could be decreased or increased, respectively, ranging from 270 to 650 mV. Through subtle C–H <i>ortho</i> interactions from the arylene unit, yellow neutral state colors were maintained with transmissive or near-transmissive oxidized states. Colorimetry utilizing <i>L</i>*<i>a</i>*<i>b</i>*, where <i>a</i>*<i>b</i>* values correlate to the chroma or saturation of a color (note: −<i>a</i>* and +<i>a</i>* correspond to green and red and −<i>b</i>* and +<i>b</i>* correspond to blue and yellow, respectively) and <i>L</i>* represents the lightness, was used to show the maintenance of yellow colors in the neutral states. Herein, the yellow polymers had <i>L</i>* values above 84.0, <i>a</i>* values ranging from −11.6 to 24.8, and <i>b</i>* values greater than 47.6. In the oxidized states, the most transmissive forms had <i>L</i>* values above 70.0, <i>a</i>* values ranging from −2.1 to 2.0, and <i>b</i>* values ranging from −6.8 to −0.1. These structure–property relationships grant access to conjugated polymers with high energy absorbance in the visible, while allowing variability in redox potentials, providing a deeper understanding in yielding yellow-to-transmissive electrochromic polymers

Optimization of PEDOT Films in Ionic Liquid Supercapacitors: Demonstration As a Power Source for Polymer Electrochromic Devices

We report on the optimization of the capacitive behavior of poly­(3,4-ethylenedioxythiophene) (PEDOT) films as polymeric electrodes in flexible, Type I electrochemical supercapacitors (ESCs) utilizing ionic liquid (IL) and organic gel electrolytes. The device performance was assessed based on figures of merit that are critical to evaluating the practical utility of electroactive polymer ESCs. PEDOT/IL devices were found to be highly stable over hundreds of thousands of cycles and could be reversibly charged/discharged at scan rates between 500 mV/s and 2 V/s depending on the polymer loading. Furthermore, these devices exhibit leakage currents and self-discharge rates that are comparable to state of the art electrochemical double-layer ESCs. Using an IL as device electrolyte allowed an extension of the voltage window of Type I ESCs by 60%, resulting in a 2.5-fold increase in the energy density obtained. The efficacies of tjese PEDOT ESCs were assessed by using them as a power source for a high-contrast and fast-switching electrochromic device, demonstrating their applicability in small organic electronic-based devices

An Electrochromic Painter’s Palette: Color Mixing via Solution Co-Processing

Electrochromic polymers (ECPs) have been shown to be synthetically tunable, producing a full palette of vibrantly colored to highly transmissive polymers. The development of these colored-to-transmissive ECPs employed synthetic design strategies for broad color targeting; however, due to the subtleties of color perception and the intricacies of polymer structure and color relationships, fine color control is difficult. In contrast, color mixing is a well-established practice in the printing industry. We have identified three colored-to-transmissive switching electrochromic polymers, referred to as ECP-Cyan (ECP-C), ECP-Magenta (ECP-M), and ECP-Yellow (ECP-Y), which, via the co-processing of multicomponent ECP mixtures, follow the CMY color mixing model. The presented work qualitatively assesses the thin film characteristics of solution co-processed ECP mixtures. To quantitatively determine the predictability of the color properties of ECP mixtures, we estimated mass extinction coefficients (ε_mass) from solution spectra of the CMY ECPs and compared the estimated and experimentally observed color values of blends via a calculated color difference (Δ<i>E</i>_ab). The values of Δ<i>E</i>_ab range from 8 to 26 across all mixture compositions, with an average value of 15, representing a reasonable degree of agreement between predicted and observed color values. We demonstrate here the ability to co-process ECP mixtures into vibrantly colored, visually continuous films and the ability to estimate the color properties produced in these mixed ECP films

Orange and Red to Transmissive Electrochromic Polymers Based on Electron-Rich Dioxythiophenes

As the color palette of available solution processable electrochromic polymers expands, there has remained the need for red, orange, and yellow to transmissive switching materials. Here we report on the synthesis and characterization of two such polymers, the orange to transmissive switching (poly{3,4-di(2-ethylhexyloxy)thiophene}) electrochromic polymer-orange (ECP-orange) and the red to transmissive switching processable polymer (poly{3,4-di(2-ethylhexyloxy)thiophene-co-3,4-di(methoxy)thiophene}) electrochromic polymer-red (ECP-red). The ECP-orange has a bandgap of 2.04 eV, an absorption λmax centered at 483 nm, and an E1/2 of 0.37 V versus Ag/Ag+. The electrochromic contrast is 48% T at 483 nm with a time to reach 95% of the full optical contrast of 5.3 s for a film that has an absorbance of 0.98 au at λmax. Because of steric relaxations from the random copolymerization of a branched dialkoxy-substituted thiophene with a dimethoxy-substituted thiophene, the red to transmissive switching ECP-red has a bandgap of 2.00 eV, a λmax red-shifted by 42 to 525 nm, and an E1/2 decreased to 0.21 V versus Ag/Ag+. Additionally, the red polymer has a higher contrast of 60% T and a shorter time to reach 95% of the full optical contrast of 2.3 s. These two reported polymers allow the field of electrochromics to come closer to a full set of fully solution processable materials that yield films whose optical absorption covers the full visible spectrum while switching to a highly transmissive oxidized state as needed for full color displays

Poly[Bis-EDOT-Isoindigo]: An Electroactive Polymer Applied to Electrochemical Supercapacitors

Poly­[6,6′-bis­(ethylene-3,4-dioxythien-2-yl)]-<i>N</i>,<i>N</i>′-dialkylisoindigo (PBEDOT-iI) was synthesized and incorporated as an electroactive material into electrochemical supercapacitors (ESCs) in type I and type III configurations. In type I ESCs, PBEDOT-iI provides a specific power of ∼360 W/kg and specific energy of ∼0.5 Wh/kg, while retaining about 80% of its electroactivity over 10 000 cycles. In addition, we report on the use of PBEDOT-iI in type III supercapacitors where operating voltages as high as 2.5 V were achieved with specific energies of ca. 15 Wh/kg, albeit with limited stability

Blue-Violet Electroluminescence from a Highly Fluorescent Purine

Blue-Violet Electroluminescence from a Highly Fluorescent Purin

Four Shades of Brown: Tuning of Electrochromic Polymer Blends Toward High-Contrast Eyewear

We report a straightforward strategy of accessing a wide variety of colors through simple predictive color mixing of electrochromic polymers (ECPs). We have created a set of brown ECP blends that can be incorporated as the active material in user-controlled electrochromic eyewear. Color mixing of ECPs proceeds in a subtractive fashion, and we acquire various hues of brown through the mixing of cyan and yellow primaries in combination with orange and periwinkle-blue secondary colors. Upon oxidation, all of the created blends exhibit a change in transmittance from ca. 10 to 70% in a few seconds. We demonstrate the attractiveness of these ECP blends as active materials in electrochromic eyewear by assembling user-controlled, high-contrast, fast-switching, and fully solution-processable electrochromic lenses with colorless transmissive states and colored states that correspond to commercially available sunglasses. The lenses were fabricated using a combination of inkjet printing and blade-coating to illustrate the feasibility of using soluble ECPs for high-throughput and large-scale processing