Anna M. Österholm

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.

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.

Out of sight but not out of mind: the role of counter electrodes in polymer-based solid-state electrochromic devices

Throughout the literature, a variety of counter electrode materials have been used in conjugated polymer-based electrochromic devices (ECDs) without a comparative understanding of their effects on the electrochromic properties of the device. In this study, we show that poor ECD performance, often attributed to electrochromic polymer (ECP) stability, is in fact largely due to an inappropriate choice of counter electrode. We have compared a set of counter electrode materials used in the ECD literature in magenta-to-clear and black-to-clear devices and evaluated how they affect the device parameters including contrast, switching time, stability, and voltage requirements. We demonstrate that through the appropriate choice of counter electrode material (i) the operating voltage can be lowered, (ii) no additional equilibration/break-in time is required, and (iii) the contrast and switching times of the ECP is maintained when incorporated into a device. Furthermore, we show that even unencapsulated ECDs with ECP-Magenta as the vibrantly colored material assembled and operated under ambient conditions can withstand over 10 000 switches without compromising contrast or switching time.

Solution Processed PEDOT Analogues in Electrochemical Supercapacitors

We have designed fully soluble ProDOTx-EDOTy copolymers that are electrochemically equivalent to electropolymerized PEDOT without using any surfactants or dispersants. We show that these copolymers can be incorporated as active layers in solution processed thin film supercapacitors to demonstrate capacitance, stability, and voltage similar to the values of those that use electrodeposited PEDOT as the active material with the added advantage of the possibility for large scale, high-throughput processing. These Type I supercapacitors provide exceptional cell voltages (up to 1.6 V), highly symmetrical charge/discharge behavior, promising long-term stability exceeding 50 000 charge/discharge cycles, as well as energy (4-18 Wh/kg) and power densities (0.8-3.3 kW/kg) that are comparable to those of electrochemically synthesized analogues.

Understanding the effects of electrochemical parameters on the areal capacitance of electroactive polymers

A number of variables contribute to the electropolymerization, and the electrochemical properties, of electroactive polymers. However, few studies have attempted to acquire a unified understanding of the effects of all these variables, specifically as it relates to the capacitance of the material, as the number of experiments and resources required is large. Here, the effects of seven variables on the areal capacitance of the electropolymerized dimethyl derivative of poly(3,4-propylenedioxythiophene) (PProDOT-Me2) films are analyzed utilizing a fractional factorial design of experiments to reduce the number of experiments an order of magnitude. From this analysis, PProDOT-Me2 films were electropolymerized from an optimal set of variables to reproducibly afford films displaying the highest capacitances observed within this study. Devices were assembled from the optimized conditions, and the capacitance, energy, and power densities are reported in a framework that allows for meaningful comparison and understanding relative to commercially available supercapacitors. The supercapacitors fabricated in this study show promise towards being integrated as power sources for low-power, lightweight and flexible organic electronic devices.

Transparent Wood Smart Windows: Polymer Electrochromic Devices Based on Poly(3,4-Ethylenedioxythiophene):Poly(Styrene Sulfonate) Electrodes

Abstract Transparent wood composites, with their high strength and toughness, thermal insulation, and excellent transmissivity, offer a route to replace glass for diffusely transmitting windows. Here, conjugated‐polymer‐based electrochromic devices (ECDs) that switch on‐demand are demonstrated using transparent wood coated with poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a transparent conducting electrode. These ECDs exhibit a vibrant magenta‐to‐clear color change that results from a remarkably colorless bleached state. Furthermore, they require low energy and power inputs of 3 mWh m−2 at 2 W m−2 to switch due to a high coloration efficiency (590 cm2 C−1) and low driving voltage (0.8 V). Each device component is processed with high‐throughput methods, which highlights the opportunity to apply this approach to fabricate mechanically robust, energy‐efficient smart windows on a large scale.

Chemical Oxidation of Polymer Electrodes for Redox Active Devices: Stabilization through Interfacial Interactions

To achieve optimal performance in a conjugated polymer-based electrochemical device, i.e. for a supercapacitor to reach full depth of discharge or for an electrochromic device (ECD) to achieve maximum contrast, the two electrodes must be in different oxidation states when the device is assembled. Here, we evaluate the use of chemical oxidation as a scalable postprocessing method to adjust the redox state of polymer-coated electrodes. We evaluate how the extent of oxidation depends on both the redox properties of the conjugated polymer and on the choice of chemical oxidant, and how these parameters affect the functionality of the film. Comparing Ag(I) and Fe(III) oxidants, we find that it is not the oxidizing power that determines the extent of doping but rather the redox potentials of the polymers, with the more easily oxidized polymers doping to a higher extent. Because the polarity and surface energy of the polymer changes upon oxidation, we also show how a phosphonic acid surface pre-treatment improves interfacial adhesion between the polymer and a transparent oxide electrode (ITO). Finally, as a proof of principle, we demonstrate how chemical oxidation of the organic counter electrode a minimally color changing dioxypyrrole polymer enhances the device contrast of an ECD, confirming that this approach is a promising route toward high-throughput manufacturing of ECDs and other polymer-based electrochemical devices.

Exploring unbalanced electrode configurations for electrochromic devices

Electrochromic cells consisting of a black-to-transmissive propylenedioxythiophene/benzothiadiazole copolymer active layer and a minimally colored dioxypyrrole-based charge storage layer were constructed, and the effect of the redox capacity ratio between the two layers was evaluated. This ratio was tuned between large excesses of either the active layer or the charge storage layer (an unbalanced configuration), to cells where both films had the same redox capacity (a balanced configuration), and the effects on cell voltage, optical contrast, switching time and switching stability were assessed. The inclusion of an auxiliary reference electrode allowed us to monitor the redox behavior of both the active layer and the charge storage layer during cell operation. With this setup, we show that the redox behavior of each layer is highly dependent on the redox capacity ratio between the two films. We also demonstrate that, in principle, any electrochromic material that has a colorless state can serve as an optically inactive charge storage material if it is present in sufficient excess in an electrochromic cell. Additionally, by monitoring the redox behavior of each individual layer during cell operation, we show that performance loss in electrochromic cells during long-term stability testing is not necessarily due to an irreversible electrochemical or structural degradation of the active material or the charge storage layer. Rather we observe a drift in the oxidation state of both layers, suggesting that the losses are reversible, and could be mitigated through proper electrochemical control of the cells. The main benefit of an unbalanced configuration is the substantial lowering of the operating voltage without compromising switching time or the contrast as long as transparency of the charge storage material is maintained.

Balancing Charge Storage and Mobility in an Oligo(Ether) Functionalized Dioxythiophene Copolymer for Organic- and Aqueous- Based Electrochemical Devices and Transistors

This work presents a soluble oligo(ether)-functionalized propylenedioxythiophene (ProDOT)-based copolymer as a versatile platform for a range of high-performance electrochemical devices, including organic electrochemical transistors (OECTs), electrochromic displays, and energy-storage devices. This polymer exhibits dual electroactivity in both aqueous and organic electrolyte systems, redox stability for thousands of redox cycles, and charge-storage capacity exceeding 80 F g-1 . As an electrochrome, this material undergoes full colored-to-colorless optical transitions on rapid time scales (<2 s) and impressive electrochromic contrast (Δ%T > 70%). Incorporation of the polymer into OECTs yields accumulation-mode devices with an ION/OFF current ratio of 105 , high transconductance without post-treatments, as well as competitive hole mobility and volumetric capacitance, making it an attractive candidate for biosensing applications. In addition to being the first ProDOT-based OECT active material reported to date, this is also the first reported OECT material synthesized via direct(hetero)arylation polymerization, which is a highly favorable polymerization method when compared to commonly used Stille cross-coupling. This work provides a demonstration of how a single ProDOT-based polymer, prepared using benign polymerization chemistry and functionalized with highly polar side chains, can be used to access a range of highly desirable properties and performance metrics relevant to electrochemical, optical, and bioelectronic applications.

All Polymer Solution Processed Electrochromic Devices: A Future without Indium Tin Oxide?

The growing range of applications for optoelectronic and electrochromic devices (ECDs) encourages the search for materials combining high electrical conductivity with optical transparency. Next generation transparent conducting electrodes (TCEs) are required to be inexpensive, lightweight, scalable, and compatible with flexible substrates to trigger innovations towards supporting sustainable living and reducing energy consumption. Here we show that PEDOT:PSS can be solution processed using blade coating and subsequently post-treated with nitric and acetic acid to raise its conductivity above 2000 S cm-1 with a film transparency of ∼95%. A combined grazing-incidence wide angle X-ray scattering, atomic force microscopy, and thickness analysis of the film indicates that the removal of excess insulating PSS- inducing reordering is the critical parameter for the claimed conductivity increase. We then investigate the impact of replacing indium tin oxide electrodes with PEDOT:PSS in ECDs. While electrochromic contrast and optical memory are comparable for devices constructed with both electrode materials, differences in switching kinetics are explored by comparing internal resistances, ion diffusion, and charging effects in the polymer films extracted by electrochemical impedance spectroscopy. While all these ideas are described based on a battery-type ECD configuration, these concepts are easily transferable to other types of redox-active devices.