Disentangling Redox Properties and Capacitance ... - ACS Publications

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Disentangling Redox Properties and Capacitance in SolutionProcessed Conjugated Polymers Anna M. Ö sterholm,*,† James F. Ponder, Jr.,†,‡ Michel De Keersmaecker,† D. Eric Shen,† and John R. Reynolds*,† †

School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Department of Chemistry, Imperial College London, London W7 2AZ, U.K.

Chem. Mater. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/03/19. For personal use only.

S Supporting Information *

ABSTRACT: The unique ability of combined ionic and electronic transport in conjugated, semiconducting polymers has resulted in the emergence of a variety of redox-based technologies ranging from energy storage and conversion, to bioelectronics, to on-demand color control. Although conjugated polymers have been extensively studied for decades, the recent revival of organic bioelectronics, in particular, has demonstrated that there needs to be a better understanding of the interplay between mixed ion and electron transport and the underlying film morphology. Many of the conjugated polymers that are effectively doped electrochemically and that exhibit a combination of high capacitance, fast and reversible redox switching, and exceptional stability lack long-range order making it more challenging to evaluate how the morphology evolves as a function of oxidation state. Here, we demonstrate how readily accessible electrochemical and spectroscopic techniques can offer a great deal of insight into ion and electron transport in redox-active conjugated polymers regardless of their degree of order. Furthermore, we show how numerous redox properties, including onset of oxidation, capacitance, and conductance profile, of five dioxythiophene-based copolymers can be manipulated by the size and polarity of the functional groups that are incorporated to provide solution processability.

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INTRODUCTION Conjugated polymers have been widely explored for use in solid state organic electronics, such as organic photovoltaics and organic field effect transistors and, to this end, many of the structure−property relationships governing optoelectronic and transport properties, as well as solid-state morphology, have been defined, and advancements in both backbone and side chain engineering have had a large impact on device performance.1−10 Unlike in the solid-state, electrochemical devices rely on reversible transport of both ions and electrons through the active layer and, as a result, understanding of the structure−property relationships governing mixed conduction has become a resurgent research area.11−16 Electrochemical doping of conjugated polymers is a dynamic process where large variations in microstructure, energetic landscape, ion content, and charge carrier concentration are linked to the oxidation state of the polymer. Consequently, the choice of the backbone, solubilizing groups, and electrolyte influences ion and electron transport and, ultimately, the performance of any conjugated polymer-based redox-active device. Although these types of devices have been explored for decades, much of the early work on mixed transport focused on electropolymerized materials where the film morphology is templated by the electrolyte during synthesis, rather than by © XXXX American Chemical Society

solvent−polymer and polymer−polymer interactions that drive morphology evolution in solution-processed materials. With the recent revival of organic bioelectronics, in particular, there has been a substantial interest in trying to better understand mixed transport in solution-processed conjugated polymer systems and elucidate how the morphology evolves as a function of charge carrier concentration.11−13,17 To date, the bioelectronics research has focused heavily on materials with backbone motifs that have proven successful in the solid-state arena. Specifically, repeat units structures that induce rigidity and planarity to enhance intermolecular π-stacking and decrease conformational disorder to afford high solid-state mobility have been the focus, rather than studying repeat unit structures that have proven successful in other redox-active applications. This recent work on mixed transport has mainly focused on the fruit fly of solid-state organic semiconductors, poly(3-hexylthiophene) (P3HT) and its analogues,18−22 as well as polymers incorporating thienothiophene and benzodithiophene units.17,23 Separately, the composite conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrene sulReceived: February 4, 2019 Revised: March 25, 2019

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DOI: 10.1021/acs.chemmater.9b00528 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Scheme 1. General Polymerization Scheme for ProDOT(X)-DMP Copolymers with Various Side Chainsa

a

Where X = n-octyl (nOct, green), 2-ethylhexyl (EtHx, black), 2-butyloctylester, (BOE, magenta), 2-hexyldecyl (HxDc, red), and tetraester (tE, blue).

polymers experience during electrochemical doping, which should improve both reversibility and redox stability. Here, we evaluate a family of soluble 3,4-propylenedioxythiophene (ProDOT)-based copolymers that lack long-range order and tune the redox properties and capacitance by varying the side chains. We use a backbone motif that was recently shown to be equally effectively doped in both organic and aqueous electrolytes, and that gave promising performance both as the active material in accumulation mode OECTs, as well as a polymer electrochrome.31 Because they can be symmetrically and orthogonally substituted with two side chains per monomer unit, ProDOT-based systems are typically exceptionally soluble, they are adaptable to a wide variety of polymerization chemistries,32 and they allow for straightforward functionalization with a range of side chains.33−35 The position and nature of the solubilizing groups can substantially alter both the optical and electrochemical properties of conjugated polymers through a combination of electronic and steric effects.36,37 However, because the side chains are not directly attached to the conjugated backbone, but through a neopentyl linkage within the 3,4-dioxypropylene bridge, we not only enhance redox stability but also exclude any large electronic effects on the redox properties, allowing us to evaluate the effects of steric strain, intermolecular interactions, and lyophilicity of the solubilizing groups. To evaluate how the solubilizing groups influence the redox properties, we go beyond the optical moving front experiments25,38−40 that are currently used for attempting to elucidate mixed transport in these types of polymers, by using a combination of techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), in situ UV−vis− NIR spectroelectrochemistry, and in situ conductance. We show that, in spite of all of the polymers studied having a similar structure near the conjugated backbone, and having almost identical absorbance spectra, changing the number/ arrangement of carbons and oxygens in the side chain far from the backbone has a substantial influence on numerous redox properties, including onset of oxidation, capacitance, and conductance profile. Importantly, we show how a range of readily accessible electrochemical and spectroscopic techniques can offer insight into trends and acquire a better understanding of mixed transport, especially in materials that lack long-range order.

fonate) (PEDOT:PSS), has been thoroughly evaluated for these applications because of its aqueous electroactivity and high solid-state conductivity.24−28 High-resolution X-ray diffraction techniques have shown that ions enter and preferentially reside in amorphous or side chain rich domains of both P3HT and its oligoethersubstituted analogue.18,21,29 However, increasing the ion content or degree of hydration causes the order of the crystalline domains to be disturbed and the π−π and lamellar stacking to be irreversibly altered. Studying P3HT-based organic electrochemical transistors (OECTs) in situ and in operando with electrochemical strain microscopy showed that ordered film domains, not surprisingly, exhibited less ion uptake than disordered domains.20 Two recent studies on oligoether-functionalized polythiophene showed that highly crystalline films had a higher solid-state mobility but lower OECT mobility and ionic conductivity than the amorphous analogue.21,22 These studies demonstrated that the design principles that have proven successful for solid-state applications, where new materials are often tailored for increased crystallinity and order to obtain enhanced electron transport, can be counterproductive for redox applications that rely on efficient ion uptake. In addition to these observations, a recent study on the influence of disorder on charge transfer characteristics in electrochemical transistors highlighted the importance of tailoring new redox-active materials, not just to have high transconductance, but also a broad window of high transconductance.30 One of the reasons polythiophene derivatives have received little attention in the area of redox-active devices in the past, is due to the relatively high oxidation potential, limited aqueous redox activity and stability, and their tendency to crystallize. So, while polythiophene derivatives serve as valuable model systems and provide some insight into the interplay between crystalline and amorphous regions and how they influence mixed transport, new materials and approaches are needed to carry these principles beyond their limitations, especially since it has been shown that ions mainly traverse amorphous film domains.22 However, many highly redox-active conjugated polymers lack long-range order, which means that X-ray techniques are not necessarily able to provide insight into how the choice of backbone or solubilizing groups influence film morphology and how that morphology is altered during electrochemical doping. These polymers do, however, have the advantage of not undergoing the same kind of phase changes and irreversible crystallite deformation that highly ordered B


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Chemistry of Materials

Figure 1. (a) Cyclic voltammograms of an as-sprayed EtHx film in 0.5 M lithium bistrifluoromethanesulfonylimide (LiTFSI)/propylene carbonate (PC) between −0.8 and 0.8 V. (b) Absorption spectra comparing EtHx in CHCl3 (20 μL/mL, black line), a spray cast film on indium tin oxide (ITO) immersed in LiTFSI/PC (green line), and the same film held at −0.8 V (red line) after 10 CV cycles. (c) Onset of oxidation as a function of side chain measured by DPV after electrochemical annealing. (d) Normalized absorption spectra of electrochemically annealed films.

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RESULTS AND DISCUSSION Polymer Design and Synthesis. A family of alternating copolymers were designed and synthesized via direct (hetero)arylation polymerization consisting of a 3,4-propylenedioxythiophene (ProDOT, I) functionalized at the 2-position of the propylene bridge with the various side chains shown in Scheme 1, co-polymerized with 3,3-dimethyl-3,4-dihydro-2Hthieno[3,4-b][1,4]dioxepine (DMP, II). This repeat unit affords polymers with low oxidation potential, high solubility (>40 mg/mL in certain cases), efficient electrochemical doping, and high redox stability even in the presence of air and moisture. Synthetic details, 1H NMR, 13C NMR, elemental analysis indicating proper structures and purity as well as molecular weight and dispersity data can be found in the Supporting Information (Figures S1−S19). As we will show in the following section, all copolymers used in this study have the same optical gap indicating that they exceed the critical degree of polymerization. Because of their high solubility and lack of aggregation in many common organic solvents, we have shown for several dioxythiophene-based polymers that differences in chain conformation and interchain interactions on a length scale that can be manipulated by changing solvent or deposition method alter the charge capacity, but not the optical properties or the extent of electrochemical doping.41 Because we have observed differences in charging capacity, all polymer films in this study were cast under identical conditions, from the same low boiling point solvent (CHCl3), using the same weight percent of polymer, and without any thermal annealing or other post-treatments to minimize the influence of processing-induced discrepancies.42 Although disordered polymers have the advantage of not exhibiting the same ion sensitivity as semicrystalline polymers

that exhibit irreversible morphological changes during electrochemical doping, great care was taken to ensure that the electrolyte concentration was the same in all experiments and that the solutions were thoroughly purged with argon, as electrochemical measurements and electrochemical impedance spectroscopy, in particular, are extremely sensitive to salt and oxygen concentrations.43 Onset of Oxidation and i−V Characteristics. A unique property that is often observed for solution-processed polymer films is an electrochemical annealing or break-in effect.44 This describes how the initial current response, and sometimes even the absorption spectrum of a freshly coated film changes over the course of the first few redox cycles as the flux of ions and solvent through the film is equilibrated. During this process, the packing and microstructure are altered as the polymer chains are reorganized to stabilize the formed charge carriers and accommodate the necessary counter ions needed for charge balance. The electrochemical annealing effect on the redox response is highlighted in the cyclic voltammogram in Figure 1a for EtHx (others in Figure S20 in the Supporting Information), where the onset of oxidation (Eonset) of the first cycle is observed near 0.0 V, followed by two oxidation peaks at 0.22 and 0.47 V (vs Ag/Ag+). Upon subsequent scans, the redox behavior is markedly different indicating that the film undergoes large structural changes during electrochemical doping. The Eonset decreases to −0.54 V from 0.0 V, and a new oxidation peak appears at −0.25 V (Ep,1, Figure 1a) arising from film domains that were not present in the as cast film. The two oxidation peaks that were observed during the first doping cycle (Ep,2 and Ep,3) are still present during subsequent cycles, but with a lower peak current density. BOE and HxDc behave similarly to EtHx, where large changes in the redox C


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Figure 2. (a) Absorbance as a function of potential for EtHx from −0.8 to 0.8 V at 0.05 V increments (Eonset: red spectrum, Ep,1: green spectrum, Ep,2: blue spectrum, Ep,3: cyan spectrum, Ep,full: magenta spectrum). (b) Cyclic voltammograms of all copolymers in 0.50 M LiTFSI/PC at 50 mV/s (10th cycle shown). (c) Comparison of doping kinetics of nOct (green), EtHx (black), BOE (magenta), HxDc (red), and tE (blue) in 0.50 M LiTFSI switching between −0.8 V and 0.8 V with varying pulse lengths (10 sec to 0.25 sec).

change in film morphology is observed after electrochemical annealing (Figure S23). For the studied polymers, the Eonset is side chain dependent where, for example, nOct with the smaller and linear side chains has a lower Eonset (−0.65 V) than EtHx with branched chains (−0.54 V).8,10 Further increasing the side chain length increases the Eonset to −0.38 V for BOE and −0.35 V for HxDc and tE (Figure 1c). These differences are in spite of the polymers all having the exact same backbone, and presumably the same ionization potential in the absence of any intermolecular interactions. By comparing the absorbance spectra, we can determine whether there are differences in effective conjugation length that could be caused by twisting along the backbone induced by differences in the steric bulk of the side chains, or if the electrochemical annealing process simply results in differences in how electrochemically accessible different domains in the film are. From the normalized absorbance spectra in Figure 1d of the electrochemically annealed films, it is evident that the spectra of the films are almost identical, independent of side chain identity with only slight differences in the relative intensity of the aggregation peak to λmax (0−0:0−1 ratio) that decreases in the following order: tE > EtHx > nOct > HxDc > BOE. First, this suggests that BOE is the least aggregated and tE is the most aggregated. Second, because there are no large spectral differences, the backbone conformation and the effective conjugation length are not side chain dependent, which demonstrates that the DMP unit indeed acts as a spacer that is able to minimize steric interactions between repeat units. Thus, the trend in Eonset can be attributed to differences in the electrochemical accessibility, which encompasses both ion accessibility, as well as how easily electrons can be extracted from parts of the film that are in close proximity to the underlying electrode. This difference in electrochemical accessibility is highlighted by comparing nOct and tE where the latter has an Eonset that is 0.3 V higher despite the fact that the effective conjugation length is identical and that tE has a higher population of aggregated domains. Because the absorption spectra are extremely sensitive to any change in the polymer oxidation state, the potential-dependent changes in the optical response during the early stages of oxidation provide further insight into the difference in electrochemical accessibility (Figures 2 and S25). Upon oxidation, and in conjunction with the depletion of the π−π* transition from the neutral polymer, new charge carrier

response are observed after the first doping cycle, whereas the first CV cycle of as-cast films of nOct and tE bear a much stronger resemblance to the subsequent cycles. This suggests that nOct and tE undergo a less dramatic structural change during electrochemical annealing. From the optical absorption spectra, we can elucidate the changes occurring during electrochemical annealing, and distinguish between possible changes in effective conjugation length and degree of aggregation, as aggregated and unaggregated domains have different spectral signatures. Using the spectra in Figure 1b of EtHx as an example (others in Figure S21), in dilute chloroform solutions where the polymer is well-dissolved, the absorbance is believed to arise solely from chains in an unaggregated state. Upon solidification, the absorption spectra broaden and the onset of absorption is red shifted, a common feature observed for conjugated polymers as they adopt a more planar conformation in the solid-state. Electrochemical annealing gives rise to a new absorption peak at ∼605 nm that we attribute to the formation of aggregated domains with stronger intermolecular interactions. This is verified from solution spectra where gradual addition of a poor solvent (methanol) to the polymer containing CHCl3 solution induces aggregation and results in the appearance of a peak at 605 nm (Figure S22a). Further evidence is shown in Figure S22b where the electrochemically induced aggregates are broken up upon heating the film to 80 °C.45 In contrast to EtHx, BOE, and HxDc, the solution and as-cast spectra of nOct and tE already show evidence of an aggregation induced absorbance just above 600 nm (Figure S21), which is in good agreement with the CV results. Based on these observations, we can conclude that the large oxidation peak in the first cyclic voltammogram of the as-cast films of EtHx (Figure 1a), BOE (Figure S20b), and HxDc (Figure S20c) arises from electrochemical oxidation of the mainly disordered and unaggregated bulk. Reorganization and aggregation that occurs during electrochemical annealing then gives rise to new domains in the film that oxidize more readily with a peak centered at ca. −0.25 V (Ep,1), while the remaining disordered domains continue to oxidize at higher potentials (≥Ep,2). For nOct (Figure S20a) and tE (Figure S20d) the oxidation peak corresponding to Ep,1 is already present in the first CV cycle as a result of the already present aggregated domains. The electrochemically induced aggregation occurs at a shorter length scale than we can detect using atomic force microscopy (AFM), as no noticeable D


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Chemistry of Materials bands at ∼900 and >1800 nm originating from radical cations/ polarons appear at Eonset. At Ep,1 all polymers have reached the same oxidation state, as evidenced by the λmax absorbance that has decreased to an optical density of 0.52 ± 0.02 (green spectra). Because the magnitude of Ep,1 increases from nOct < EtHx ≈ BOE < HxDc < tE (Figure 2b), it means that nOct reaches the same oxidation state as the other polymers, but at a lower potential (−0.3 V), while tE requires the highest potential (−0.2 V) to reach a comparable oxidation state. Furthermore, the aggregation induced absorbance shoulder at 605 nm is mostly depleted in nOct at Ep,1 while it is still prominently present in the tE spectrum. These results further flesh out the differences in accessibility and how enhanced electrochemical accessibility allows the aggregated domains throughout the film to oxidize more readily at lower potentials. By Ep,2 there are no visible vibronic features remaining in any of the spectra, suggesting that the chains exist in unaggregated domains. This has also been demonstrated for chemically doped polymer films where dopant anions first enter more ordered film domains that are able to more effectively delocalize and stabilize of charge carriers.29,46 As the oxidation progresses and the electrolyte uptake increases, the side chains and the underlying microstructure they afford have a less drastic, albeit not negligible, influence on the i−V response (Figure 2b). While Ep,2 and Ep,3 are measured at the same potential for all the polymers, the relative intensity and shape of the peaks differ, providing additional information about redox kinetics. For example, the redox peaks in nOct and EtHx are fairly broad and have similar current densities suggesting the redox sites can interact due to short separation. The redox peaks in BOE and HxDc, on the other hand, are more well-defined suggesting that the redox sites in these two are well-separated and less interactive, but are of equal or fairly close energy, resulting in electron transfer occurring at similar potentials.47 One other way that CV is able to probe differences in the bulk is through comparing the scan rate dependence of the various redox peaks (as opposed to just plotting peak current as a function of scan rate), exemplified by EtHx and HxDc in Figure S26. The presence of three oxidation peaks of varying relative intensity, and the fact that the relative intensity changes as the scan rate is increased, demonstrates that the different film domains with different degrees of intermolecular interactions give rise to variations in how the ions are able to move through the various film domains. The decrease in the normalized current density as a function of scan rate for Ep,1, i.e., the redox peak primarily associated with the oxidation of aggregated domains, suggests that ion transport through these domains is more tortuous and therefore more sensitive to changes in the scan rate with the effect being more noticeable in materials that have a lower degree of electrochemical accessibility and where the redox sites are less interactive.48 Measuring the intensity of the π−π* transition as the film is switched between the neutral and the fully oxidized state as a function of switching time provides another way of evaluating the doping kinetics. For this experiment, the polymers were spray cast on ITO to the same optical density (1.05 ± 0.03) to ensure that the amount of chromophore was comparable allowing us to make quantitative comparisons of the side chain effects. The polymers with branched aliphatic side chains (EtHx, HxDc, and BOE) exhibit sub-second doping kinetics, whereas it takes 1.1 and 1.3 s, respectively, for nOct and tE to convert from the fully neutral to the fully oxidized state (Figure

2c). While we see subtle and meaningful differences in the doping kinetics, it is important to note that all polymers switch rapidly between the neutral and oxidized forms, demonstrating that the side chains are compatible with the electrolyte to allow for rapid ion transport through the films. From these experiments, we are able to identify how differences in the electrochemical accessibility are dictated by the structure of the side chains. This manifests itself both in the differences shown in the Eonset, as well as the scan rate dependence. In contrast to this, we observe that the overall shape of the absorbance spectra, as well as the optical band gap, is nearly identical across the studied polymers, showing only a difference in the population of aggregates. We can understand these observations by considering that the propylenedioxythiophene moiety is designed such that the side chains are located away from the thiophene backbone. As a result, differences in the side chain bulk minimally impact optical properties, which are dictated mainly by the conjugated backbone. However, redox properties depend on both ion and electron transport, where ions must migrate through side chains, and therefore are sensitive to small changes in structure. Insight into Ion and Electron Transport by Disentangling the Capacitance. The electrochemical capacitance of a polymer film, determined by both charge carrier concentration and ion content, and how it changes as a function of oxidation state is an important property for any redox-active application. It directly relates how a polymer will perform in a charge storage device or as a transducer material in a sensor, it determines the charge-to-switch/coloration efficiency of an electrochromic device, and, finally, it is one figure-of-merit when evaluating active materials for electrochemical transistors. However, as was recently demonstrated by Tybrandt and co-workers using PEDOT:PSS as a model compound, the ideal capacitive charging approximation used to describe the charging behavior of conjugated polymers is simplistic and does not accurately describe the charging behavior observed experimentally.49,50 A polymer film’s capacitance is composed of redox-based faradaic and double-layer capacitive components that both contain information about both ion and electron transport. Understanding the relative contribution of these capacitance sources has recently been evaluated using sophisticated mapping techniques to describe how the microstructure of a polymer film affects ion uptake, which ultimately affects the capacitance.20 Synthetically, the design of specialized sidechains for thiophene-based polymers has been explored in order to deliberately shift the contribution from a predominantly double-layer to a predominantly redox-based capacitance.23 From an electrochemical standpoint, the specific film capacitance (C, F/g) is determined by the amount of charge (Q) stored in a given voltage window. The capacitance can be determined under dynamic conditions either, for example, by integrating the cyclic voltammogram, or from the slope of a galvanostatic discharge curve, or measured under steady-state conditions using EIS. The value obtained via CV is a bulk capacitance (Cbulk) with both faradaic and capacitive components that eventually become scan/discharge rate dependent at high scan rates. Here, we determine Cbulk by integrating the forward scan of a cyclic voltammogram recorded at 50 mV/s while ensuring that the integrated charge from the forward scan/oxidation was the same as that during the backward scan/reduction. We demonstrate how studying E


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redox processes occurring in these polymers, EIS spectra (Figure S27) were recorded at five potentials corresponding to ∼Eonset, just before the three redox peaks (Ep,1−3), and the fully oxidized state (Ep,full). The circuit that gave the best fit for the EIS spectra is the modified Randles circuit (Figure S27a), commonly used for analyzing conjugated polymers,51 and consists of an electrolyte resistance (Rs) in series with a parallel combination of a double layer capacitance (Cdl), a charge transfer resistance (Rct), and a finite length Warburg diffusion impedance element (ZD) that is defined in terms of a diffusional time constant (τ) and a diffusional pseudocapacitance (Cϕ). The Cdl is a measure of the non-faradaic double layer charging at the polymer/electrolyte interface and provides insight into how the accessible surface area, especially in the proximity of this interface, changes as a function of oxidation state. As a general trend, and not surprisingly, increasing the doping level results in an increase in the Cdl indicating that the accessible interfacial surface area increases as the films expand to accommodate additional ions and solvent molecules (Figure 3a and Table S1a). BOE and HxDc, bearing the longest alkyl chains, both have a moderate Cdl already around Eonset, and exhibit the smallest overall change increasing only by a factor of 2 over the course of the doping process. On the other hand, the Cdl in nOct and tE increases by a factor of 8 and 10, respectively. These potential dependent differences in the Cdl suggest that side-chain induced differences in polymer/ electrolyte interactions influence the extent that the accessible surface has to change during the doping process to accommodate the counter ions necessary for switching the films into their fully oxidized states. It is important to note that the Cdl measured by EIS makes up less than 5% of the overall mass capacitance in the fully oxidized state, and as will be discussed in more detail below, does not take into account all double layer-type charging in these polymer films. The low-frequency part of the Bode and Nyquist plots (Ep,3), the polymers are in comparable oxidation states but show large differences in the potential dependent conductance trends. This corresponds to a potential range, as noted earlier, where a substantial portion of the charge carriers cannot be accounted for by Cϕ suggesting that the conductance at high doping levels is affected by the relative difference in the amount of delocalized and electrochemically “trapped” charge carriers (linear vs nonlinear charge accumulation). For example, at high doping levels, nOct and tE that have the lowest Cϕ/CCV ratios (Table S3) have an almost negligible conductance. The fact that tE has a conductance that is almost an order of magnitude lower than the others suggests that, in addition to having fewer delocalized carriers, their mobility must also be much lower than for the other polymers, which is further supported by the Rct that for tE begins increasing above Ep,2. In this instance, there is no correlation between the potential dependent conductance and the low diffusional time constant suggesting that electron transport is the rate limiting process in tE. This demonstrates how fine tuning of the side chains gives rise to subtle differences in a variety of redox properties that culminate in large differences in materials performance. The large bulk of the tE side chains lead to films where charge transport through the bulk becomes compromised. nOct with short, linear chains initially allows for greater electrochemical accessibility at the polymer/electrode interface, but must undergo a relatively large change in its accessible surface area during the doping process. That, in addition to a low Cϕ/CCV ratio, negatively impact its charge transport properties and the breadth of its conductive window. The moderate length, branched alkyl chains, exemplified especially by the HxDc polymer, afford films that have a combination of low Rct, high Cϕ, and low diffusion resistance which combined result in polymers that are able to maintain a high conductance in a broad potential window. I


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Chemistry of Materials derivatives (tE) led to increased intermolecular interactions of the polymers already in solution. The consequence of this manifests itself in different ways for the two polymers. In the case of tE, the bulky side chains lead to poor electrochemical accessibility, low conductance, increased Rct at high doping levels, and a low Cϕ/Cbulk ratio suggesting that the polymer chains are isolated from one another preventing facile bulk transport. In contrast, the nOct polymer with the least bulky side chains initially has the greatest electrochemically accessibility, but at intermediate doping levels we see from the optical spectra that oxidation of the neutral polymer proceeds more slowly and there is a significant drop in Cϕ, resulting in its mass capacitance being 40% lower than for its branched analogue, EtHx. While we cannot claim that functionalizing a conjugated polymer with branched aliphatic chains is the be-all-end-all solution for designing highly redox active polymers, for this family of polymers as exemplified especially by HxDc, it provided a combination of low Rct, high Cϕ, and low diffusion resistance which combined result in polymers that are able to maintain a high conductance, high bulk capacitance, and fast doping kinetics.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.M.Ö .). *E-mail: [email protected] (J.R.R.). ORCID

Anna M. Ö sterholm: 0000-0001-6621-8238 Michel De Keersmaecker: 0000-0002-0956-1539 John R. Reynolds: 0000-0002-7417-4869 Notes

The authors declare the following competing financial interest: Electrochromic polymer technology developed at the Georgia Institute of Technology has been licensed to NXN Licensing. AMÖ , DES, and JRR serve as consultants to NXN Licensing.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the funding from the Office of Naval Research (grants N00014-16-1-2165 and N00014-18-1-2222), along with the Air Force Office of Scientific Research (FA9550-18-1-0184). The authors gratefully acknowledge Dylan Christiansen and Dr. Bing Xu for their assistance in the molecular weight determinations.

EXPERIMENTAL SECTION

The electrochemical experiments were carried out in a three-electrode cell under an inert argon atmosphere using glassy carbon (0.07 cm2) as the working electrode, a glassy carbon rod as the counter electrode, and Ag/Ag+ pseudoreference electrode (inner solution: 10 mM AgNO3 in 0.5 M LiTFSI-ACN, E1/2 vs ferrocene: 75 ± 5 mV). Lithium bistrifluoromethanesulfonylimide (LiTFSI, 98%, Acros Organics) was dissolved at a concentration of 0.50 M in propylene carbonate (PC, 99.5% purity, Acros Organics) that had been purified using a solvent purification system from Vacuum Atmospheres. All CV and DPV data were measured on at least five separate films to ensure reproducibility. The bulk capacitance was calculated by integrating the forward scan of a cyclic voltammogram recorded at 50 mV/s. In order to extract the differential capacitance from the same cyclic voltammograms at the various peak potentials, we measured the charge in a 10 mV window around the peak potentials (±5 mV). For the impedance measurements, the sine-wave perturbation signal contained excited frequencies between 10 mHz and 100 kHz and the amplitude of the excitation signal was set to 10 mV. Data validation was based on the Lissajous plots. Fitting the experimental data to the modified Randles circuit gave goodness-of-fit values ranging from 0.001 to 0.0001 for all polymers at all the measured potentials demonstrating that this model accurately describes the redox processes occurring in these materials. The potential dependent conductance measurements were performed using a Pt interdigitated microelectrode with 50 digits, 5 μm in width and 5 μm separation as the working electrode in a four-electrode cell. One of the working electrodes was held at a given potential while cycling the second working electrode ±5 mV at 0.5 mV/s. The slope of the resulting i−V curve was used to determine the film conductance. For the spectroscopic measurements, the films were deposited via spray casting onto ITO coated glass slides to an optical density of 1.05 ± 0.05. The doping kinetics were determined using chronoabsortometry, where the absorbance at λmax related to the π−π* was continuously monitored as the films switched between the two extreme oxidation states (−0.8 and 0.8 V) for various pulse lengths (30−0.25 s). Additional experimental details can be found in the Supporting Information.

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Details on the synthesis and characterization of the copolymers, cyclic voltammetry, differential pulse voltammetry, AFM, UV−vis−NIR spectroscopy, and electrochemical impedance spectroscopy (PDF)

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REFERENCES

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