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Jun 26, 2018 - Tailoring the Seebeck Coefficient of PEDOT:PSS by Controlling Ion. Stoichiometry in Ionic Liquid Additives. Amir Mazaheripour,. †,⊥...
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Cite This: Chem. Mater. 2018, 30, 4816−4822

Tailoring the Seebeck Coefficient of PEDOT:PSS by Controlling Ion Stoichiometry in Ionic Liquid Additives Amir Mazaheripour,†,⊥ Shubhaditya Majumdar,‡,⊥ Dakota Hanemann-Rawlings,‡ Elayne M. Thomas,† Christine McGuiness,§ Lauriane d’Alencon,∥ Michael L. Chabinyc,† and Rachel A. Segalman*,†,‡ †

Materials Department, University of California, Santa Barbara, California 93106, United States Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States § Corporate R&I, Solvay, Durand Building, Stanford, California 94305, United States ∥ R&I Center de Paris, Solvay, 52 Rue de la Haie Coq, Paris 93308, France

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‡

S Supporting Information *

ABSTRACT: Mixing simple additives into poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) dispersions can greatly enhance the thermoelectric properties of the cast films with little manufacturing cost, but design rules for many of these additives have yet to emerge. We show that controlling stoichiometry in ionic liquid (I.L.) additives can decouple morphological and electronic modifications to PEDOT:PSS and enhance its power factor by over 2 orders of magnitude. Blending I.L. additives with a 1:1 stoichiometry between cationic imidazolium (Im + ) derivatives and anionic bis(trifluoromethane)sulfonamide (TFSI−) groups into PEDOT:PSS dispersions raised the film conductivity to ∼1000 S/cm. The Seebeck coefficient, which gives insight into the electronic structure as well as thermoelectric performance, remained unchanged. This behavior mimics that of popular high-boiling solvent additives such as dimethyl sulfoxide and ethylene glycol, which restructure the film morphology to enhance carrier mobility. Blending I.L. additives with a 4:1 stoichiometry between Im+ and TFSI− groups raises the conductivity in a similar manner but also enhances the Seebeck coefficient. This selective Seebeck enhancement proceeds from the interaction of excess Im+ with anionic poly(styrenesulfonate) (PSS−) groups, similar to previous studies using inorganic salts, that results in a shift in charge carrier populations. Inorganic salts by themselves cannot raise the conductivity of PEDOT:PSS to appropriate values since they lack the solvent restructuring effect. These I.L. additives combine the effects of high-boiling solvents and diffuse ions, with the ability to tailor the Seebeck coefficient through ion stoichiometry.

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INTRODUCTION Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), a widely used hole transport layer in optoelectronic devices,1 has emerged as a material of choice for bioelectronics2−5 and thermoelectrics4−6 owing to its high conductivity, mechanical flexibility, ion permeability, and thermal/electrochemical stability.1−8 Additionally, it is industrially produced as an aqueous dispersion that can be easily cast onto a variety of surfaces. Notably, transport properties in PEDOT-based devices can be greatly enhanced by mixing the dispersion with small volumes of low-cost additives prior to casting.9 This approach can be essential to the fabrication of certain PEDOT-based devices and is compatible with large scale processing protocols. Efforts to understand the effects of processing additives on PEDOT:PSS often consist of measuring the changes in electrical conductivity (σ) and more recently the Seebeck coefficient (S).6−14 The conductivity depends on the mobility and concentration of the charge carriers present in the film. © 2018 American Chemical Society

PEDOT:PSS is often cast from a complex dispersion, and prior research has shown that slowing down the drying rate during film formation improves the conductivity by enhancing the charge carrier mobility.9−17 The Seebeck coefficient describes the amount of voltage obtained from a given temperature gradient (−ΔV/ΔT) and is an important parameter in the S 2σ

thermoelectric figure of merit, ZT = κ , where κ is the thermal conductivity. According to the Mott relationship, S is sensitive to the change in conductivity as a function of energy (σ(E)) at the Fermi level,

∂ ln σ(E) |Ef .6−8 ∂E

Therefore, it can

provide insight into electronic structure (distribution of density of states, position of the Fermi energy, etc.) in addition to thermoelectric performance. Thermoelectric Received: May 18, 2018 Revised: June 26, 2018 Published: July 11, 2018 4816

DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

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Chemistry of Materials

conjunction with these ion-exchange effects, making them ideal for thermoelectric enhancements. However, it still remains unclear which features of the cation and/or anion in I.L. additives can control thermoelectric enhancements in PEDOT:PSS. Because the I.L.s have multiple features as solvents, ions, and acids/bases, all of which can contribute to the observed enhancements, identifying the relevant factors can be challenging. As a result, there are currently no design rules for using I.L.s to tailor the thermoelectric properties of PEDOT:PSS. Herein, we reveal an important design parameter in tuning the transport properties of PEDOT:PSS films with I.L. additives: the stoichiometry between the cationic and anionic I.L. groups. By blending PEDOT:PSS with various imidazolium:bis(trifluoromethane)sulfonamide (Im:TFSI)based I.L. additives, we show that the solvent effects can be decoupled from the electronic effects based on this stoichiometry. Additives with a 1:1 stoichiometry between Im+ and TFSI− groups show a two-orders-of-magnitude increase in conductivity and an unchanged Seebeck coefficient, mimicking the behavior of high-boiling solvents. Additives with a 4:1 stoichiometry between Im+ and TFSI− groups show an improvement in conductivity as well as a Seebeck enhancement, improving the power factor by over 2 orders of magnitude. This tailorable Seebeck coefficient proceeds from the ability of large, diffuse cations (Im+) in excess of their preferred counterions (TFSI−) to interact with PSS− groups and modify the charge-carrier populations in the PEDOT domains. Importantly, this ability is activated only when the concentration of the Im+ cations exceeds that of the TFSI− anions, allowing one to decouple solvent effects from electronic effects via I.L. stoichiometry. We examine the effects on the electronic structure of PEDOT domains through UV−vis-NIR absorbance spectroscopy, as well as the role of acid−base chemistry in driving the observed enhancements.

materials usually show opposing relationships between conductivity and S upon doping, as an increase in carrier concentration tends to increase conductivity while decreasing S. However, PEDOT:PSS additives have been shown to significantly enhance conductivity without decreasing S, leading to large enhancements in the power factor (S2σ).10−17 As such, strong efforts have been put forth to understand the role of additives in driving these observed enhancements. Mixing common additives with PEDOT:PSS dispersions enhances the charge carrier mobility in PEDOT:PSS films and leaves the Seebeck coefficient unchanged due to changes in morphology. Various high-boiling organic solvents mixed with PEDOT:PSS raise the pristine electrical conductivity value (∼0.3 S/cm for Clevios PH1000) by 1−3 orders of magnitude; diethylene glycol increases it to 10 S/cm, ethylene glycol to 639 S/cm, and dimethyl sulfoxide to 700 S/cm.10−17 Mixing these additives has been suggested to reduce interactions between the sulfonate groups and PEDOT chains and contribute to a solvent annealing effect that facilitates polymer movement. These effects lead to phase segregation and a structural reorganization of the PEDOT domains into longer and more continuous networks, enhancing charge carrier mobility.9−17 In these instances, the Seebeck coefficient does not increase and tends to stay constant as the amount of additive is increased.10−17 This is likely because the morphological rearrangements primarily serve to enhance carrier mobility, which by itself does not necessarily influence the Seebeck coefficient. Inorganic salts have been shown to enhance the Seebeck coefficient of PEDOT:PSS films through an ion-exchange reaction (cation:anion + PEDOT:PSS → cation:PSS + PEDOT:anion) that is favorable with larger and more diffuse cations.18,19 Larger and more diffuse cations are more likely to interact with the charge-diffuse PSS− groups and modify their interactions with the doped PEDOT domains, which enhances the Seebeck coefficient. However, inorganic salts do not enhance electrical conductivity as dispersion additives, as their small and localized ions do not enable the morphological rearrangements seen with the high-boiling solvents. Ionic liquid (I.L.) additives combine extremely high-boiling solvent effects with ion-exchange effects to offer the highest reported conductivity enhancements while also increasing the Seebeck coefficient. Badre et al. demonstrated that a ∼2% by weight mixture of 1-ethyl-3-methylimidazolium tetracyanoborate (EMIm:TCB) with PEDOT:PSS raised the electrical conductivity to ∼2000 S/cm, higher than any reported solvent additive.20 Notably, these I.L.s do not evaporate off during film annealing, allowing much more time for favorable solvent effects that improve the continuity between PEDOT domains. Later studies postulated that ion-exchange between the PEDOT:PSS and EMIm:TCB was also involved in reorganizing polymer networks.21 Liu et al. showed that 1-butyl-3methylimidazolium tetrafluoroborate (BMIm:BF4) and its bromide variant (BMIm:Br) increased the conductivity by an order of magnitude while also increasing the Seebeck coefficient by a factor of 2, an effect previously unseen for dispersion additives.22 Here, the Seebeck increase likely proceeds from an ion-exchange reaction similar to the inorganic salt additives, where the charge-diffuse imidazolium cation is especially suitable for interacting with PSS−.23 Unlike the inorganic salts, the charge-diffuse nature of both I.L. constituents enables the high-boiling solvent effects in

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RESULTS AND DISCUSSION Effect of Ion Stoichiometry on the Thermoelectric Properties of PEDOT:PSS. Additives with a 1:1 Im:TFSI stoichiometry behave like high-boiling solvents when mixed with PEDOT:PSS dispersions, rearranging the film morphology to enhance the effective carrier mobility. The chemical structures of the I.L. additives are shown in the inset in Figure 1a. Im+ and TFSI− groups were chosen because of their large size and charge diffusivity, which should allow solvent-driven morphological rearrangement and ion exchange.23 Furthermore, TFSI− groups have been shown to be effective for conductivity as counterions to doped PEDOT.24 By blending increasing amounts of these additives with PEDOT:PSS dispersions and annealing the cast films, we obtained films with increasing conductivities, plotted as a function of the volume percentage of the additive in the solid film (see Experimental Section for details). Figure 1a shows the monotonic increase in film conductivity from ∼6 S/cm up to ∼1000 S/cm, similar to the maximum attainable value from dimethyl sulfoxide (DMSO), a popular high-boiling solvent additive (dashed line). Notably, the mass fraction of additive required in the dispersion to achieve this enhancement (∼0.6%) is far less for the I.L. additive than for DMSO (5%). This is likely because the I.L. is able to remain in the film during the entirety of the annealing process,20,21 allowing much more time for favorable morphological rearrangements that improve the continuity between PEDOT domains, 4817

DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

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increased (Supporting Information Figure S2), indicating ionexchange with the PEDOT:PSS consistent with the literature reports on inorganic salts.18,19 Considering that these studies demonstrated that chloride salts with more diffuse cations were more likely to associate with PSS− groups and raise the Seebeck coefficient, it is surprising that we see no Seebeck enhancement for the charge-diffuse Im:TFSI additives.23 We therefore conclude that the Im+ cations prefer associating with TFSI− over PSS−, preventing significant ion exchange and leaving the I.L. to purely mimic the effects of high-boiling solvents. We postulated that increasing the concentration of Im+ relative to TFSI− groups in the additive would be more likely to perform ion exchange and raise the Seebeck coefficient without eliminating the beneficial solvent effects that enhance conductivity. Additives with a 4:1 Im:TFSI stoichiometry increased conductivity in a similar manner to the 1:1 stoichiometries, indicating that the solvent effect was still present. The inset in Figure 2a shows the ionic composition of the additives. To

Figure 1. (a) Electrical conductivity of I.L.-blended PEDOT:PSS films (after annealing) vs % volume of I.L. in the solid film for 1:1 Im:TFSI-based I.L.s. Both I.L.s increase the conductivity to ∼1000 S/ cm, acting like high-boiling solvents. A schematic representation of cation and anion species in the I.L. additives is shown in the inset. (b) Seebeck coefficient of I.L.-blended PEDOT:PSS film vs % volume of I.L. in the solid film for the 1:1 I.L., where the value remains unchanged from pristine PEDOT:PSS.

whereas the DMSO evaporates off. In addition, the diffuse nature of charge in the Im+ and TFSI− groups contributes greatly to this plasticization ability, as an equivalent NaCl additive (see Experimental Section for details) was not able to enhance the conductivity from the pristine value (see Supporting Information Figure S2). The conductivity remained at a maximum until roughly 2 wt% dispersion additive, after which point the solution became too viscous to cast. At such a low weight percentage, the noticeable increase in viscosity is unusual relative to other additives and suggests a strong level of interaction between the I.L. and the PEDOT:PSS. This interaction may also be at play in the superior solvent effects of these I.L. additives. Impressively, the insulating additive occupies ∼44% of the film volume at this point and still maintains the high conductivity, consistent with the previous literature reports on EMIm:TCB and BMIm:BF4.20,21 The Seebeck coefficient remains constant for the 1:1 Im:TFSI dispersion additives, similar to high-boiling solvents, indicating that there is no significant ion-exchange with the PEDOT:PSS. Figure 1b shows the Seebeck coefficient as a function of volume percentage of the additive in the solid film. Like the conductivity behavior, this Seebeck behavior is also similar to that of DMSO (dashed line) and other high boiling solvent additives that rearrange morphology but do not ionexchange with the PEDOT:PSS.10−17 When NaCl was added to PEDOT:PSS dispersions in molar quantities comparable to those of the I.L. groups, the Seebeck coefficient slightly

Figure 2. (a) Electrical conductivity of I.L.-blended PEDOT:PSS films (after annealing) vs % volume of I.L. in the solid film for 4:1 Im:TFSI-based I.L.s. A schematic representation of cation and anion species in the I.L. additives is shown in the inset. (b) Seebeck coefficient of I.L.-blended PEDOT:PSS film vs % volume of I.L. in the solid film for the 4:1 I.L.s, where the conductivity monotonically increases from pristine PEDOT:PSS.

achieve a 4:1 ratio between 1-methylimidazolium (MIm+) and TFSI−, 4 equiv of 1-methylimidazole base was mixed with 1 equiv of HTFSI acid until a liquid was formed, which was then added to the dispersion (see the Experimental Section). The excess imidazole base is protonated at the acidic pH (∼2) of the dispersion to form the MIm+ cation. A 4:1 ratio between 1ethyl-3-methylimidazolium (EMIm+) and TFSI− was achieved by adding 3 equiv of EMIm:Cl to 1 equiv of EMIm:TFSI and repeating a similar procedure (see Experimental Section for 4818

DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

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the ability of ion stoichiometry to selectively tailor thermoelectric properties in I.L. additives. Charge-Carrier Populations. Absorbance spectroscopy confirms the suspected shift in the relative populations of the charge carriers in PEDOT domains for the 4:1 Im:TFSI additives. In the case of conjugated polymers such as PEDOT, there are two primary charge carriers: polarons, which dominate at lower doping levels, and bipolarons, which dominate at higher doping levels.24−31 The optical transitions of polarons and bipolarons in PEDOT:PSS have been assigned with the polaron absorbance at ∼1.5 eV followed by a continuous (almost linear) increase in absorbance from bipolarons at energies less than ∼1 eV.24−31 For the 4:1 Im:TFSI additive, the ratio between the 1.5 eV peak and sub 1 eV slope is much higher relative to the 1:1 analogue, which suggests a relatively higher concentration of polarons (Figure 3) according to the analysis described in refs 24−31. A shift

more details). Similar to the 1:1 additives, the conductivity enhanced monotonically with increasing volume percent (Figure 2a), indicating that solvent effects were still taking place. The conductivity enhancements were not as significant as those of the 1:1 additives, which may be related to the much higher viscosity of the 4:1 additives (solid at room temperature, whereas 1:1 additives were liquid), leading to a weaker solvent annealing effect. The maximum conductivity also plateaus at a film volume fraction of 32%, greater than that of the 1:1 analogues at 24%. This suggests that the volume from the excess Im+ is not contributing to the solvent annealing effect, requiring a higher volume percent of total additive to obtain a similar amount of 1:1 Im:TFSI in the film (see Supporting Information Figure S1 for further analysis). Another reason for the reduced conductivity enhancements involves a shift in charge-carrier populations10 from the ionexchange occurring in conjunction with the solvent annealing effect, which can be probed through the Seebeck coefficient. Unlike the 1:1 Im:TFSI additives, the 4:1 analogues enhance the Seebeck coefficient through an ion-exchange effect, demonstrating an ability to tailor the Seebeck coefficient in I.L. additives using ion stoichiometry. In excess of the TFSI− anions, the Im+ cations are able to interact with the PSS− much more effectively than the 1:1 additives, raising the Seebeck by up to a factor of ∼2 (Figure 2b). This is consistent with the literature reports on inorganic salts, where it is also shown that larger and more diffuse cations show greater Seebeck enhancements. The enhancement seen with the 4:1 analogues is much larger than the NaCl control at a similar cation concentration (Supporting Information Figure S3), verifying that additives with more diffuse cations are able to ionexchange more effectively with the PEDOT:PSS. Here, the excess MIm+ cations immediately interact with the PSS− groups upon protonation, and the excess EMIm+ cations are paired with Cl− anions and prefer to interact with the larger and more diffuse PSS− groups. The Seebeck enhancement ∂ ln σ signals an increase in ∂E |Ef in the PEDOT domains, which

Figure 3. UV−vis absorption spectra of PEDOT:PSS thin films with either a 4:1 or a 1:1 Im:TFSI ionic liquid additive. The pristine film is also shown.

can proceed from a shift in the relative populations of charge carriers10 as a result of the ion exchange, leading to a corresponding change in conductivity. This shift is more pronounced with the EMIm variant, which shows a greater Seebeck enhancement in conjunction with a lower conductivity. However, the Seebeck enhancement in this case does not compensate the conductivity loss, as the MIm variant has a much higher power factor at 15 μW/m-K2, making it the highest-performance thermoelectric material used in this study (see Table 1 for the full comparison). In summary, the 4:1 Im:TFSI additives are able to enhance the Seebeck coefficient as well as the conductivity by activating ion-exchange reactions in addition to solvent effects. These results overall demonstrate

toward an increasing population of polarons has been found with an increase in the Seebeck coefficient previously.10 This result implies that the increase of the Seebeck coefficient for the 4:1 Im:TFSI additives is driven by a shift in the relative charge-carrier populations in PEDOT, likely from the exchange of ions in the system. Another potential explanation involves a base-assisted dedoping of the PEDOT chains, which itself could increase the Seebeck coefficient, and would also manifest as an increase in polarons relative to bipolarons. This would be more likely to occur with the 4:1 MIm:TFSI additive as it has 3 excess equivalents of an imidazole-type base, whereas the 4:1 EMIm:TFSI has no excess base (the excess EMIm+ cations are paired with Cl−). The Seebeck enhancements likely do not proceed from a Brønsted base-assisted dedoping of PEDOT. Previous studies show that Seebeck enhancements to PEDOT:PSS can be achieved through Brønsted base-assisted dedoping of the PEDOT chains.29,30 To identify whether Brønsted base interactions from the I.L. additives are driving the dedoping, we compare the results from the MIm:TFSI additives to the EMIm:TFSI additives used in this study. If the excess imidazole base in the 4:1 MIm:TFSI additive were driving

Table 1. Conductivity, Seebeck Coefficient, and Power Factor for Films with Additives, at the Volume Percentage Corresponding to the Maximum Power Factor additive

% vol

σ (S/cm)

none 1:1 MIm 4:1 MIm 1:1 EMIm 4:1 EMIm

N/A 38 32 32 44

6 (1) 1120 (110) 520 (60) 810 (70) 140 (20)

S (μV/K) 12 10 17 12 20

(1) (1) (1) (1) (1)

S2σ (W/mK2) 0.8 1.2 1.5 1.1 5.7

× × × × ×

10−7 10−5 10−5 10−5 10−6

(0) (0.1) (0.2) (0.1) (1) 4819

DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

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Synthesis of PEDOT:PSS-I.L. Blends. The I.L.s were mixed with the PEDOT:PSS (as received) in different ratios after which they were vortexed for 1 min. The solutions were drop cast onto glass substrates having four evaporated gold electrodes. They were annealed for 60 min at 60 °C in ambient conditions. To ensure the absence of water in the samples, they were then moved to a glovebox and heated for an additional 120 min at 120 °C in a vacuum chamber. For the DMSO control, 5 wt % DMSO mixed with PEDOT:PSS, and the same procedure was repeated. For the NaCl control, solid NaCl was mixed with PEDOT:PSS at a molarity within the range of those used for the I.L. solutions (∼0.035 M), and the same procedure was repeated. Electrical Conductivity Measurements. The electrical conductivity (σ) was measured using a two-point probe system by measuring/sourcing current/voltage through the electrodes at varying separations (transmission line model) to remove contact resistance effects (details are in Supporting Information). The volume fraction of I.L. in the film is assumed to be equal to the volume ratio of I.L. to PEDOT:PSS in the dispersion, where PH1000 has 1.3% PEDOT:PSS by weight at a dry density of 1.014 g/mL. The resistance of the sample between any two electrodes was measured using a Keithley 2400. This was converted to electrical conductivity (σ), using the sample width (w), sample thickness (t), measured using a Bruker DektakXT Stylus profilometer, and electrode separation. A line was fit to the resistance vs electrode separation data, where σ = 1/(slope × w × t). Seebeck Measurements. The Seebeck coefficient (S) was calculated using the relationship S = −ΔVoc/ΔT by measuring an open-circuit thermovoltage (ΔVoc) between two electrodes on the sample, while a controlled temperature gradient (ΔT) is maintained between them using Peltier elements (details are in Supporting Information). Sufficient wait time was determined for each sample to ensure that any short time-scale transient effects in the thermovoltage do not affect the final steady-state values (details are in Supporting Information). The thermovoltage was measured using a Keithley 2400, and the temperature difference was measured using K-type thermocouples. The Peltier elements were controlled using Wavelength Electronics LFI3751 temperature controllers. The entire process of setting a temperature difference between the Peltier elements and simultaneously recording voltage and temperature as a function of time was automated through MATLAB. Three measurements were performed for each sample and then used to obtain the mean value and standard deviation, representing experimental uncertainty. UV−Vis Measurements. Ultraviolet/visible/near-infrared (UV/ vis/NIR) absorption spectra were obtained by using a Shimadzu UV3600 spectrometer.

the dedoping via Brønsted base interactions with the PEDOT chains, then the 4:1 EMIm:TFSI additive should not dedope the PEDOT chains, as it has no excess base, and therefore should not enhance the Seebeck coefficient. However, both analogues increase the Seebeck coefficient in a similar manner, with the EMIm actually showing a slightly larger increase (Figure 2). This demonstrates that Brønsted base-assisted dedoping of the PEDOT chains is not responsible for the observed Seebeck enhancements. The Seebeck enhancements are also not related to protic interactions between the Im+ cations and PSS− groups. We can compare the EMIm+ and MIm+ variants to understand the role of such protic interactions in the ion-exchange event. If protic interactions between the Im+ cations and the PSS− groups were driving the ion exchange, then the protic MIm+ cation (pKa ∼ 7) should show a far more significant Seebeck enhancement than the aprotic EMIm+ cation (pKa ∼ 23). However, again, the opposite is true. The differences between both the conductivity and Seebeck enhancements from the various additives seem to be driven purely by cation stoichiometry rather than cation pKa. Therefore, as hypothesized, this Seebeck enhancement is driven by ionic interactions between excess imidazolium cations and PSS− groups as opposed to Brønsted acid/base chemistry.

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CONCLUSION In summary, we have shown that I.L. additives can tailor the thermoelectric performance of PEDOT:PSS films by decoupling morphological and electronic effects. Additives with a 1:1 Im:TFSI ratio exhibit the behavior of high-boiling solvents, enhancing the conductivity while leaving the Seebeck coefficient unchanged. This likely proceeds from the preference of Im+ cations to associate with TFSI− over PSS−, preventing the ion exchange seen with inorganic salts. Additives with a 4:1 Im:TFSI ratio are able to enhance the Seebeck coefficient while still maintaining the solvent effects that enhance conductivity. Here, the excess Im+ cations are free to interact with the PSS− groups, which enhances the Seebeck coefficient by shifting the relative populations of polarons and bipolarons. Further exploration is needed to determine the mechanism by which ion exchange shifts the relative populations of the carriers, as well as whether there are consequences to carrier concentration. These results overall demonstrate the ability to tailor the thermoelectric properties of PEDOT:PSS by controlling ion stoichiometry in I.L. additives. We believe these novel design rules will be useful for the development of future high-performance PEDOT:PSS films for thermoelectrics and other applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02114. Experimental details, modified conductivity plot, and data from control experiments (PDF)

EXPERIMENTAL SECTION

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Materials. PEDOT:PSS (Clevios PH 1000) was purchased from Heraeus Precious Metals. 1-Ethyl-3-methylimidzole/chloride (EMIm:Cl) was purchased from Sigma-Aldrich, and 1-ethyl-3methylimidzole:bis(trifluorosulfonimide) (EMIm:TFSI) was purchased from EMD Performance Materials. 1-Methylimidazole (MIm) and HTFSI were also purchased from Sigma-Aldrich. Synthesis of Ionic Liquids. Solutions of MIm with HTFSI were carefully synthesized following the procedure described in refs 32 and 33. The two components were mixed together in a glovebox at the appropriate stoichiometry (either 1:1 or 4:1 MIm:TFSI) in a vial at 105 °C, which is above the melting point of HTFSI (∼60 °C) and MIm (∼90 °C). On melting, they were mixed by stirring overnight. For the 4:1 EMIm solution, 1 equiv of EMIm:TFSI was mixed with 3 equiv of EMIm:Cl, and the same procedure was repeated.

AUTHOR INFORMATION

Corresponding Author

* Email: [email protected]. ORCID

Amir Mazaheripour: 0000-0001-7393-1795 Elayne M. Thomas: 0000-0003-0072-4204 Michael L. Chabinyc: 0000-0003-4641-3508 Rachel A. Segalman: 0000-0002-4292-5103 Author Contributions ⊥

4820

A.M. and S.M. contributed equally to this work. DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

Article

Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge a grant from Department of Energy Office of Basic Energy Sciences (Grant No. DE-SC0016390) for materials characterization and analysis and support from Solvay Chemical for device fabrication. We also gratefully acknowledge the use of equipment in the UCSB Materials Research Laboratory Shared Experimental Facilities, supported by the MRSEC Program of the NSF under Award No. DMR 1720256, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org). We thank Bhooshan C. Popere for insightful discussions.

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DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822

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Chemistry of Materials Liquid [Im][TFSI], Probed by Pulsed-Field Gradient NMR and Quasi-Elastic Neutron Scattering. J. Phys. Chem. B 2012, 116, 8201− 8209.

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DOI: 10.1021/acs.chemmater.8b02114 Chem. Mater. 2018, 30, 4816−4822