Controlling the multi-level assembly and morphological properties of conjugated polymers through structural manipulation has contributed significantly to the advancement of organic electronics. In this work, a redox active conjugated polymer, TPT-TT, composed of alternating 1,4-(2-thienyl)-2,5-dialkoxyphenylene (TPT) and thienothiophene (TT) units is reported with non-covalent intramolecular Sâ¯O and Sâ¯H-C interactions that induce controlled main-chain planarity and solid-state order. As confirmed by density functional theory (DFT) calculations, these intramolecular interactions influence the main chain conformation, promoting backbone planarization, while still allowing dihedral rotations at higher kinetic energies (higher temperature), and give rise to temperature-dependent aggregation properties. Thermotropic liquid crystalline (LC) behavior is confirmed by cross-polarized optical microscopy (CPOM) and closely correlated with multiple thermal transitions observed by differential scanning calorimetry (DSC). This LC behavior allows us to develop and utilize a thermal annealing treatment that results in thin films with notable long-range order, as shown by grazing-incidence X-ray diffraction (GIXD). Specifically, we identified a first LC phase, ranging from 218 °C to 107 °C, as a nematic phase featuring preferential face-on π-π stacking and edge-on lamellar stacking exhibiting a large extent of disorder and broad orientation distribution. A second LC phase is observed from 107 °C to 48 °C, as a smectic A phase featuring sharp, highly ordered out-of-plane lamellar stacking features and sharp tilted backbone stacking peaks, while the structure of a third LC phase with a transition at 48 °C remains unclear, but resembles that of the solid state at ambient temperature. Furthermore, the significance of thermal annealing is evident in the â¼3-fold enhancement of the electrical conductivity of ferric tosylate-doped annealed films reaching 55 S cm-1. More importantly, thermally annealed TPT-TT films exhibit both a narrow distribution of charge-carrier mobilities (1.4 ± 0.1) × 10-2 cm2 V-1 s-1 along with a remarkable device yield of 100% in an organic field-effect transistor (OFET) configuration. This molecular design approach to obtain highly ordered conjugated polymers in the solid state affords a deeper understanding of how intramolecular interactions and repeat-unit symmetry impact liquid crystallinity, solution aggregation, solution to solid-state transformation, solid-state morphology, and ultimately device applications.
Mixed ionic/electronic conducting polymers are versatile systems for, e.g., energy storage, heat management (exploiting electrochromism), and biosensing, all of which require electrochemical doping, i.e., the electrochemical oxidation or reduction of their macromolecular backbones. Electrochemical doping is achieved via electro-injection of charges (i.e., electronic carriers), stabilized via migration of counterions from a supporting electrolyte. Since the choice of the polymer side-chain functionalization influences electrolyte and/or ion sorption and desorption, it in turn affects redox properties, and, thus, electrochemically induced mixed conduction. However, our understanding of how side-chain versus backbone design can increase ion flow while retaining high electronic transport remains limited. Hence, heuristic design approaches have typically been followed. Herein, we consider the redox and swelling behavior of three poly(propylenedioxythiophene) derivatives, P(ProDOT)s, substituted with different side-chain motifs, and demonstrate that passive swelling is controlled by the surface polarity of P(ProDOT) films. In contrast, active swelling under operando conditions (i.e., under an applied bias) is dictated by the local side-chain free volume on the length scale of a monomer unit. Such insights deliver important design criteria toward durable soft electrochemical systems for diverse energy and biosensing platforms and new understanding into electrochemical conditioning ("break-in") in many conducting polymers.
Understanding the impact of side chains on the aqueous redox properties of conjugated polymers is crucial to unlocking their potential in bioelectrochemical devices, such as organic electrochemical transistors (OECTs). Here, we report a series of polar propylenedioxythiophene-based copolymers functionalized with glyme side chains of varying lengths as well as an analogue with short hydroxyl side chains. We show that long polar side chains are not required for achieving high volumetric capacitance (C*), as short hydroxy substituents can afford facile doping and high C* in saline-based electrolytes. Furthermore, we demonstrate that varying the length of the polar glyme chains leads to subtle changes in material properties. Increasing the length of glyme side chain is generally associated with an enhancement in OECT performance, doping kinetics, and stability, with the polymer bearing the longest side chains exhibiting the highest performance ([µC*]OECT = 200 ± 8 F cm-1 V-1 s-1). The origin of this performance enhancement is investigated in different device configurations using in situ techniques (e.g., time-resolved spectroelectrochemistry and chronoamperometry). These studies suggest that the performance improvement is not due to significant changes in C* but rather due to variations in the inferred mobility. Through a thorough comparison of two different architectures, we demonstrate that device geometry can obfuscate the benchmarking of OECT active channel materials, likely due to contact resistance effects. By complementing all electrochemical and spectroscopic experiments with in situ measurements performed within a planar OECT device configuration, this work seeks to unambiguously assign material design principles to fine-tune the properties of poly(dioxythiophene)s relevant for application in OECTs.
Benchmarking , Hydroxyl Radical , Electric Capacitance , Kinetics , Polymers
In this short review, we provide an overview of our efforts in developing a family of anodically coloring electrochromic (EC) molecules that are fully transparent and colorless in the charge neutral state, and that can rapidly switch to a vibrantly colored state upon oxidation. We employ molecules with reduced conjugation lengths to center the neutral state absorption of the electrochrome in the ultraviolet, as desired for highly transparent and colorless materials. Oxidation creates radical cations that absorb light in the visible and near infrared regions of the electromagnetic spectrum, thus providing a host of accessible colors. Combining a density functional theory (DFT) computational approach fed back to the synthetic effort, target molecules are proposed, synthesized and studied, directing us to develop a complete color palette based on these high contrast ACE molecules. Utilizing pendant phosphonic acid binding substituents in concert with high surface area mesoporous indium tin oxide (ITO) electrodes, the electrochromes can be distributed throughout the oxide film, bringing high extent of light absorption and color density.
Electrochromic (EC) materials and devices provide a user-controlled, dynamic way of displaying information using low power, making them interesting for a range of applications in numerous markets, including logistics, retail, consumer goods, and health care. To optimize the cost while simplifying the production, expanding the color space, and enhancing the contrast and vibrancy of EC displays aimed for cost-sensitive products, we sought to reduce the number of layers as well as remove the underlying conducting layer that accounts for a substantial fraction of the cost of a printed label. Here, we show how conjugated electrochromic polymers, which are inherently semiconducting, can be used to accomplish this goal and afford printable EC displays with a flexible form factor. Using a combination of electrochemical probes, in situ spectroscopy, solid-state conductivity, and in situ conductance measurements, we have studied and compared five different EC polymers with conductivities spanning multiple orders of magnitude and colors that span most of the visible range, identifying polymers and properties that allow for switching from the colored to the clear state without an underlying conducting layer. Finally, we incorporate these EC polymers into optimized flexible devices without an underlying conductor and demonstrate that they are able to provide on-demand, reversible colored-to-clear switching on the order of seconds to minutes, with operating voltages below ±1 V, optical memories exceeding 60 min, and a shelf-life exceeding 12 months.
We address the nature of electrochemically induced charged states in conjugated polymers, their evolution as a function of electrochemical potential, and their coupling to their local environment by means of transient absorption and Raman spectroscopies synergistically performed in situ throughout the electrochemical doping process. In particular, we investigate the fundamental mechanism of electrochemical doping in an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymer. The changes embedded in both linear and transient absorption features allow us to identify a precursor electronic state with charge-transfer (CT) character that precedes polaron formation and bulk electronic conductivity. This state is shown to contribute to the ultrafast quenching of both neutral molecular excitations and polarons. Raman spectra relate the electronic transition of this precursor state predominantly to the Cß-Cß stretching mode of the thiophene heterocycle. We characterize the coupling of the CT-like state with primary excitons and electrochemically induced charge-separated states, providing insight into the energetic landscape of a heterogeneous polymer-electrolyte system and demonstrating how such coupling depends on environmental parameters, such as polymer structure, electrolyte composition, and environmental polarity.
We present evidence of pupil response modification, as well as differential theoretical melatonin suppression through selective and dynamic electrochromic filtering of visible light in the 400-500 nm range to minimize chronodisruptive nocturnal blue light exposure. A lower activation of intrinsically photosensitive retinal ganglion cells (ipRGCs), the first step for light to reach a human's internal clock, is related to melatonin secretion therefore avoiding detrimental effects of excessive blue light exposure. Pupillary Light Reflex and Color Naming were experimentally assessed under light filtered by two different coloration states (transmissive and absorptive) of these novel dynamic filters, plus an uncoated test device, in 16 volunteers. Also, different commercial light sources at illuminances ranging from 1 to 1000 lux were differentially filtered and compared in terms of theoretical melatonin suppression. Representative parameters of the pupil responses reflected lower pupil constriction when the electrochromic filters (ECFs) were switched on (absorptive state, blue light is absorbed by the filter) compared to uncoated filters (control sample), but failed to do so under transmissive state (blue light passes through the filter) indicating less activation of ipRGCs under absorptive state (although no significant differences between states was found). Out of eight colors tested, just one showed significant differences in naming between both filter states. Thus, the ECF would have some protecting effect on ipRGC activation with very limited changes in color perception. While there are some limitations of the theoretical model used, the absorptive state yielded significantly lower theoretical melatonin suppression in all those light sources containing blue wavelengths across the illuminance range tested. This would open the way for further research on biological applications of electrochromic devices.
Pupil/physiology , Reflex, Pupillary/physiology , Retinal Ganglion Cells/physiology , Adult , Color , Female , Humans , Light , Male , Melatonin/metabolism , Middle Aged , Photic Stimulation/methods
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.
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 â¼95%. 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.
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 3â mWh m-2 at 2â W m-2 to switch due to a high coloration efficiency (590â cm2 C-1 ) and low driving voltage (0.8â 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.
Bridged Bicyclo Compounds, Heterocyclic/chemistry , Construction Materials , Electrochemistry/methods , Polymers/chemistry , Polystyrenes/chemistry , Wood , Color , Electrodes
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.
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 50â¯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.
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.
Lenses , Polymers/chemistry , Color , Electrochemistry , Oxidation-Reduction/radiation effects , Polymers/radiation effects
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.
