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Identifying Key Properties of Electrolytes for Light-Emitting Electrochemical Cells Shi Tang, Jonas Mindemark, C. Moyses Araujo, Daniel Brandell, and Ludvig Edman Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5022905 • Publication Date (Web): 14 Aug 2014 Downloaded from http://pubs.acs.org on August 20, 2014

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Chemistry of Materials

Identifying Key Properties of Electrolytes for Light-Emitting Electrochemical Cells Shi Tang,† Jonas Mindemark, ‡ Carlos Moyses Graca Araujo,# Daniel Brandell, ‡ and Ludvig Edman*,† †

The Organic Photonics and Electronics Group, Department of Physics, Umeå University, SE-901 87 Umeå, Sweden Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden # Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden ‡

ABSTRACT: The electrolyte is a key component in light-emitting electrochemical cell (LECs), as it facilitates for in-situ electrochemical doping and associated attractive device features. LiCF3SO3 dissolved in hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) constitutes an electrolyte with which we have attained a high stability and efficiency for polymer LECs, but the turnon time of such devices is unfortunately slow. By replacing hydroxyl with methoxy as the TMPE end group, we herein realize LECs with a desired combination of high efficiency, good stability, and fast turn-on time. Specifically, we show that the turn-on time to high luminance (300 cd/m2) at a current density of 7.7 mA/cm2 is lowered from 1740 to 16 s, that the efficiency is improved by ~20 %, and that the other device properties either are maintained or improved. In a parallel modelling and experimental effort, we demonstrate that the faster kinetics following the shift in the TMPE end group is attributed to a marked decrease in both the inter- and intra-molecular interactions of the electrolyte, as manifested in a lowered electrolyte viscosity, faster ion transport and more facile ion release during doping.

INTRODUCTION The light-emitting electrochemical cell (LEC) is currently attracting significant interest as a cost-efficient alternative to the organic light-emitting diode (OLED),1-6 but key device properties – notably the turn-on time, stability, and efficiency – are in need of further improvement for most applications. A fundamental feature of the LEC, which distinguishes it from the OLED, is the existence of an electrolyte (i.e. mobile ions) that is blended with the light-emitting organic semiconductor (being either a conjugated polymer7 or a small molecule8-11) in the active layer.12, 13 Following the application of a voltage between the two electrodes, the mobile ions in a functional LEC redistribute to allow for efficient charge injection and electrochemical p-type and n-type doping of the organic semiconductor at the anodic and cathodic interfaces, respectively. After a turn-on time, a light-emitting p-n junction forms in the bulk of the active layer.14-20 It has recently been demonstrated that the electrolyte also plays a key role in the quest for improved LEC operation. First, the concentration of mobile ions in the active layer dictates the maximum attainable doping concentration of the organic semiconductor.21, 22 As a “too high” doping concentration is concomitant with a poor efficiency via, e.g., excitonpolaron quenching and a short lifetime via, e.g., excitonelectrolyte induced side reactions, a powerful method towards improved LEC operation includes an optimization of the electrolyte concentration.23-25 Second, the turn-on time is dependent on the electrolyte conductivity (and the active material thickness).26, 27 As the doping-optimization procedure typically results in a relatively low electrolyte concentration, it is of

interest to utilize electrolytes with high ion mobility for the realization of satisfying device turn-on kinetics. Third, an insufficient chemical and electrochemical stability of the electrolyte will result in side reactions, as manifested in a short operational lifetime and poor efficiency.28 The electrolyte in polymer LECs commonly comprises mobile ions dissolved in an ion-conducting medium (here termed the “ion transporter”), but some reports on ion-transporter-free ionic liquids also exist.29-32 Nevertheless, the best polymer LEC performance to date stems from systems comprising an ion transporter.33-35 The pioneering work in the LEC field was performed with LiCF3SO3 dissolved in high-molecular-weight poly(ethylene oxide) (PEO) as the electrolyte,15, 36 and more recent studies on concentration-optimized systems based on this electrolyte have disclosed relatively good stability and efficiency.37 One disadvantage with using high-molecularweight polymers, such as PEO, as the ion transporter are that they are immobile and as such remain in the p-n junction region during light-emission, which is anticipated to have negative consequences on the device stability and efficiency.23, 38 Cao and co-workers inadvertently addressed this issue in an early study by introducing a mobile small molecule, a crown ether, as the ion transporter, and reported LEC devices with a fast response.39, 40 In a more recent study, a cross-linkable trimethylolpropane trimethacrylate small molecule was used by Pei and co-workers for the attainment of a fast turn-on time following an in-situ cross-linking of the methacrylate functional groups.41 The same group also reported a good stability and high efficiency for a short-chain PEO ion transporter, which also comprised cross-linkable methacrylate end-

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groups.34 Our group introduced hydroxyl end-capped trimethylolpropane ethoxylate (TMPE-OH) as an oligomeric ion transporter.38 With TMPE-OH-based LECs, we have attained an operational stability of 5600 h at a luminance of >100 cd/m2 for an encapsulated device operating in the ambient35 and a power conversion efficacy of 15.6 lm/W for a device with an external light-outcoupling structure,33 but the turn-on time is slow unless a high pre-bias is applied. Here, we present experimental and modeling data that demonstrate that the slow turn-on of TMPE-OH-based LECs can be attributed to the formation of a hydrogen-bonding network in the electrolyte phase, and a strong coordination of the Li cation to the TMPE-OH transporter molecule; the former results in a high viscosity with a concomitant low ion mobility, whereas the latter is anticipated to result in a slow cation release during n-type doping. To address these issues, we have designed and synthesized a new TMPE homologue, TMPEOCH3, in which the highly polar hydroxyl end-groups are replaced with less polar methoxy groups. As desired, the new LiCF3SO3 - TMPE-OCH3 electrolyte displays a much improved ion mobility and easier cation release, as manifested in a significantly shorter turn-on time for corresponding LEC devices. We also find that TMPE-OCH3-based LECs display an improved efficiency, and a more stable drive voltage during long-term operation; and tentatively attribute this to an improved cathodic stability of the electrolyte. EXPERIMENTAL SECTION Materials and characterization. The ion transporter methoxy-capped trimethylolpropane ethoxylate (TMPE-OCH3) was prepared through methylation of hydroxyl-terminated trimethylolpropane ethoxylate (TMPE-OH, Aldrich, Mw = 450 g/mol) with methyl iodide, using a procedure analogous to that described by Cooper and Booth for the methylation of poly(ethylene oxide) and poly(propylene oxide).42 The ion transporters were dried in a vacuum oven at p < 102 Pa and T = 343 K before use, while the salt LiCF3SO3 (Aldrich) and the conjugated polymer Super Yellow (Livilux PDY-132, Merck, Germany) were used as received. The rheological properties of the electrolytes, i.e. the LiCF3SO3 salt dissolved in an ion transporter in a 3:10 mass ratio, were characterized at 398 K through rotational viscometry (TA Instruments Advanced Rheometer AR2000), using a cone-and-plate geometry (40 mm diameter, 2° steel cone). Cyclic voltammetry (CV) measurements were carried out with an Autolab PGSTAT302 potentiostat using the general purpose electrochemical software (GPES). A Au-coated glass substrate was used as the working electrode, a Pt rod was the counter electrode, and a Ag wire was the quasi-reference electrode. For the characterization of the ion transporters, the electrolyte comprised 0.1 M LiCF3SO3 and 0.2 M ion transporter dissolved in anhydrous CH3CN. For the CV study of Super Yellow, a thin film of Super Yellow was deposited onto the Au-surface of the working electrode and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Fluka) in anhydrous CH3CN was used as the electrolyte. Directly after each CV scan, a calibration scan was run with a small amount of ferrocene/ferrocenium ion (Fc/Fc+) added to the electrolyte. All CV potentials are reported vs. Fc/Fc+. The reduction/oxidation onset potentials were defined as the intersection of the baseline with the tangent of the current at the half-peak-height. CV sample preparations and

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measurements were performed under N2 atmosphere in a glovebox ([O2] < 1 ppm, [H2O] < 0.5 ppm). Device preparation and measurement. For the fabrication of LEC devices, master solutions were prepared by separately dissolving the constituent materials in anhydrous tetrahydrofuran in a concentration of 6.5 mg/ml (Super Yellow) or 10 mg/ml (ion transporters and LiCF3SO3). The active-material solutions were prepared by mixing the master solutions in a mass ratio of {Super Yellow : ion transporter : LiCF3SO3} = {1 : 0.1 : 0.03}. The active-material solution was spin-coated onto carefully cleaned indium-tin-oxide (ITO) coated glass substrates (20 Ω/square, Thin Film Devices, USA) at 2000 rpm for 60 s. The thickness of the dry active layer was 100 nm, as established with a stylus profilometer (Dektak). Al cathodes were deposited by thermal evaporation at p < 2 × 10-4 Pa through a shadow mask, which defined the active lightemitting area of each device as 0.85 × 0.15 cm2. The devices were driven by, and the current/voltage measured with, a Keithley 2400 source-meter. The luminance was measured with a calibrated photodiode equipped with an eye-response filter (Hamamatsu Photonics), which was connected to a HP 34401A voltmeter via a current-to-voltage amplifier. All device preparations and measurements were performed at T = 298 K under N2 atmosphere in two interconnected gloveboxes ([O2] < 1 ppm, [H2O] < 1 ppm). Modelling. The free energy of the TMPE molecules and coordination complexes was calculated with Density Functional Theory (DFT) using the equation: G = Eelect + ZPVE + ∑ ν

(1) hν n + kT − T ( Svib + Srot + Strans ) + Gsolv . e hν /kT −1 2

where Eelect is the total electronic energy, ZPVE is the zeropoint energy contribution, the sum accounts for the vibrational contribution to the internal energy, and the forth term (where n = 8) accounts for the internal energy of the translational and rotational modes plus the PV term. The first four terms thus define the enthalpy of the system in the gas phase. The fifth term comprises the entropic contributions (vibrational, rotational, and translational) in the gas phase, while the last term accounts for the free energy of solvation. Energy optimization calculations were performed for non-coordinated Li+ ions and TMPE molecules (comprising 2 ethylene oxide groups per side chain) in the gas phase and in solution, and for a Li+ ion coordinated with either of the two TMPE molecules, using a number of different starting geometries. All calculations were carried out within the DFT framework using the hybrid density functional B3LYP 43-45 as implemented in the Jaguar 7.7 software package [B] and the level of 6-311G* basis set.46, 47 RESULTS AND DISCUSSION The TMPE oligomer comprises a trimethylolpropane core onto which three ethylene oxide-based arms are attached; the arms on average comprise two ethylene-oxide units. The ethylene oxide arms are capped by end-groups, and we have here studied two different variants for the end-group: methoxy (OCH3) and hydroxyl (-OH). The chemical structures of the two TMPE homologues are shown in Figure 1(a) and (b). The geometric configuration and binding energy for a Li+ ion coordinated to one of the TMPE homologues (with two ethylene


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oxide units in the branching arms) were calculated with firstprinciple DFT using a geometry-optimization routine, with a number of different starting geometries tested to avoid trapping in local energetic minima. The resulting lowest-energy configuration for coordination complexes of Li+ - TMPEOCH3 and Li+ - TMPE-OH are depicted in Figure 1(c) and 1(d), respectively. For both complexes, the (central yellow) Li+ ion is coordinated to five (red) oxygens, with three being positioned close to the TMPE molecular center, and two being located on the peripheral positions of one of the three branching arms. The remaining four oxygens, located on the peripheral positions of the two remaining branching arms, are effectively left noncoordinated. As a consequence, the coordination-rich TMPE arm is observed to wrap closely around the central cation. A total Li+ coordination number of 5 is a common feature for dissolved Li salts.48-50 The Li+-O coordination bond distance ranges between 1.99 and 2.30 Å for both complexes. However, the two systems are distinguished in that the closest, and most tightly bonding, oxygen is located in the hydroxyl endgroup for TMPE-OH and in the ether group closest to the molecular center for TMPE-OCH3. Moreover, the four noncoordinated oxygens have an average distance to the central Li+ ion of 3.87 Å in TMPE-OH, while the corresponding value is 4.24 Å in TMPE-OCH3, thus indicating a stronger cation complexation also from these parts of the TMPE-OH molecule. This implies a closer wrapping of the TMPE molecule around the Li+ when it is hydroxyl-capped. A comparison of the total free energy of the investigated systems reveals that both coordination complexes are thermodynamically distinctly favored over the corresponding noncoordinated structures. Interestingly, we detect a marked difference in the binding between the two coordination complexes, as the total free energy of the hydroxyl-capped complex is 30.3 kJ/mol lower than that of its OCH3-capped counterpart. This energy difference yet again implies a distinctly stronger coordination of the Li+ ion to the TMPE-OH molecule. With the above observations taken into account, we anticipate that a LiCF3SO3 - TMPE-OCH3 electrolyte should allow for a more facile release of “free” Li+ ions from its coordination shell during, e.g., electrochemical doping.

Figure 1. Chemical structures of the ion transporters: (a) TMPEOCH3 and (b) TMPE-OH. Geometric configurations, as calculated with DFT, for coordination complexes of (c) Li+ - TMPE-OCH3 and (d) Li+ - TMPE-OH. The central yellow atom is the dissolved Li+-ion and the nine red atoms are oxygens, with five being coordinated to the central Li+ ion in both complexes, as indicated by the thin (orange-red) lines.

The viscosity of the electrolytes, i.e. LiCF3SO3 dissolved in either TMPE-OCH3 (solid red squares) or TMPE-OH (open black circles), was determined by measuring the shear stress as a function of shear rate. As seen in Figure 2, the rheological behavior of both electrolytes have the characteristics of a Newtonian fluid in that the shear stress is directly proportional to the rate of deformation at all measured shear rates. A reliable value for the viscosity (η) for the entire shear rate interval is thus obtained as the slope of the shear stress versus the shear rate. Importantly, the shift of the TMPE end-group from -OH to -OCH3 is found to have a dramatic influence on the rheological properties, as manifested in a decrease in the viscosity from 7.38 to 0.156 Pa⋅s. We attribute the much higher viscosity of the LiCF3SO3 - TMPE-OH electrolyte to the existence of a strong hydrogen-bonding intermolecular network, which should be considerably weaker or effectively absent, in the LiCF3SO3 - TMPE-OCH3 electrolyte.51 More specifically, the polar hydroxyl end-groups will act as intermolecular physical “cross-linkers” between different TMPE-based coordination complexes, with a concomitant significant increase in the viscosity. It is conceptually straightforward to connect an increased viscosity with a decreased ion mobility, and it is also wellestablished for liquid electrolytes that the ion mobility scales with the inverse of the viscosity, as implied by the classical Stokes-Einstein equation. When the electrolyte is blended with a solid conjugated polymer to form a multi-phase active material, as utilized in the subsequently studied LEC devices, an analytical relation between the viscosity and the ion mobility is lacking. Nevertheless, it is highly plausible that a decrease in the electrolyte viscosity by a factor of ~50 will result in significantly increased ion mobility in the active material and that LiCF3SO3 - TMPE-OCH3-based active materials accordingly will display much improved ion transport over their currently employed LiCF3SO3 - TMPE-OH counterparts. LiCF3SO3 - TMPE-OCH3

2000 Shear stress (Pa)

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Figure 2. Viscosity measurements of a LiCF3SO3 - TMPE-OCH3 electrolyte (solid red squares) and a LiCF3SO3 -TMPE-OH electrolyte (open black circles).



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Another key property of a functional electrolyte is its electrochemical stability window (ESW), within which it is electrochemically inert. In other words, the ESW defines the set of potentials for which it is not possible to neither reduce nor oxidize the electrolyte. Cyclic voltammetry (CV) is an often employed method of establishing the ESW, and the measured onset potentials for electrochemical reduction and oxidation of the two TMPE-based ion transporters are presented in tabular form in Figure 3(a). We have opted to also include the corresponding redox potentials for poly(ethylene oxide) (PEO), which by tradition has been the most common choice for the ion transporter in polymer LECs, and for the conjugated polymer Super Yellow, which is employed as the light-emitter in the subsequent device study.37 For Super Yellow, the reduction and oxidation potentials define the onset potentials for ntype and p-type doping, respectively. The CV traces for all four compounds are presented in Figure S1. By using the equation, Evacuum level = − e⋅(4.8 V+ VFc/Fc+)

(2)

we can translate the measured redox potentials into energy levels vs. the vacuum level.52 From the resulting energy-level diagram in Figure 3(b), we find that none of the ion transporters feature an ESW that completely encompasses the onset potentials for both n- and p-type doping of Super Yellow (as indicated by the horizontal dashed lines). It is particularly on the cathodic side that this represents a problem, as the onset potential for n-type doping of Super Yellow exceeds the LUMO level for all the investigated ion transporters by 0.2-0.5 V. It is thus the reduction of the ionic transporter that is the preferred thermodynamic reaction at the cathode. It is, however, important to point out that it is not always the thermodynamically preferred reaction that will prevail when the applied potential allows for a multitude of reactions, as kinetic factors can make another higher-energy reaction to be the preferred one, as outlined within the realm of the well-established Marcus theory.53 Nevertheless, the fact that the TMPE-OCH3 ion transporter features the highest LUMO level could be anticipated to be positive for the operational stability of Super Yellow-based LECs, particularly in the context of that the significant electron-polaron and exciton-binding energies, which are characteristic features of conjugated polymers, imply that

Figure 3. (a) CV redox potentials for three ion transporters and the light-emitting conjugated polymer Super Yellow. (b) The LUMO and HOMO energy levels for the ion transporters and Super Yellow.

the effective electron-transport and exciton-emission levels should be positioned at a lower energy than the n-type doping (or reduction) level, as measured with CV. On the p-type doping side, TMPE-OH is the preferred choice from an electrochemical stability perspective, as it features the highest oxidation potential. (Note that the slight 0.1 V difference for the oxidation of TMPE-OH in comparison to a previous study most probably originates from a low current, which makes an exact reading challenging for this particular reaction; see Figure S1(b).)38


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Figure 4. Schematic illustrating the various processes in effect during LEC turn-on. (a) The initial drift of ions within the electrolyte phase, with the “free” CF3SO3− anions drifting toward the positive anode, and the Li+ - TMPE coordination complexes drifting toward the negative cathode. (b) The subsequent electrochemical doping of the conjugated polymer, highlighting the ion ingress into the conjugated polymer phase. Note that the Li+ ion is proposed to be released from the solvating TMPE molecule before the doping event.

Before turning to the device data, we have elected to discuss our above findings in the context of anticipated ion kinetics during the device turn-on, with the aid of the schematic presented in Figure 4. It is well-established that the polar electrolyte and the non-polar conjugated polymer in the active material of an LEC phase separate,54, 55 and that the long-range motion of polar ions towards the electrode interfaces accordingly takes place within the electrolyte phase.56 This initial ion redistribution is depicted in Figure 4(a). As the ion mobility of the two investigated electrolytes differs markedly (as concluded from the viscosity data presented in Figure 2), the ion redistribution will be much more facile for the methoxy-capped electrolyte than for the hydroxyl-capped one. We further bring forward the hypothesis that the doping event will be preceded by an ion release from the solvation shell/environment for a strongly solvated ion. We note that the ion-release-from-the-solvent scenario at the interface between an electrolyte and a conjugated polymer faces been detected in so-called artificial-muscle devices.57,58 In our particular system, this process should constitute a release or separation of the Li+ cation from the solvating TMPE molecule, as depicted in Figure 4(b). As the Li+ cation exhibits a significantly stronger solvation with TMPE-OH than TMPE-OCH3, faster ion-release kinetics should be in place for the latter system. Thus, the LEC turn-on process, including the initial ion redistribution in the electrolyte phase and the subsequent ion ingress into the conjugated polymer phase, is anticipated to be much faster following the introduction of the new LiCF3SO3 TMPE-OCH3 electrolyte. A comparison of the optoelectronic performance of the two different electrolytes in champion LEC devices is provided in

Figure 5. For simplicity we refer to LECs comprising TMPEOCH3 as the ion transporter for OCH3-LECs and those comprising TMPE-OH for OH-LECs. The device-to-device variation for the ≥4 devices tested in each category was ~10 % from the average value, and the presented and discussed data below are average values if nothing else is specified. As anticipated and desired, the turn-on kinetics are consistently markedly improved with the employment of our newly synthesized TMPE-OCH3 ion transporter; see Figure 5(a). Specifically, at a constant current density of j = 7.7 mA/cm2, the turn-on time to a high luminance of 300 cd/m2 is 16 s for an OCH3-LEC and 1740 s for an OH-LEC, whereas the average turn-on time to a lower luminance of 100 cd/m2 is 4 s for the former and 8 s for the latter. The same trend is observed at constant-voltage driving, with the turn-on time to 300 cd/m2 being decreased from 31 (120) to 22 (68) min at a low drive voltage of V = 4 (3.5) V. The potentiostatic data are presented in Figure S2. We further find that the maximum luminance, the electrical stability and the efficiency all are improved with the new electrolyte. The maximum luminance increases from 545 to 645 cd/m2 at j = 7.7 mA/cm2, while the drive voltage during the 130 h test period depicted in Figure 5(b) only increases with 11 % (from a lowest value of 3.5 V to 3.9 V) for the new OCH3-LEC but by 57 % (from 3.7 V to 5.8 V) for the OHLEC. The improvements in peak luminance and electrical stability also carry over to the device efficiency: The champion (and average) peak values for the current efficacy and power conversion efficacy increase from 7.1 (6.4) cd/A and 5.9 (5.2) lm/W to 8.4 (7.5) cd/A and 7.1 (6.3) lm/W at the corresponding peak luminance. This corresponds to an efficiency improvement of the order of 20 %, as presented in Figure 5(c). The only device parameter that was found to be invariant to the new electrolyte is the operational stability of the luminance, as exemplified by a comparison of the two luminance traces in Figure 5(b). The observed deviation between the markedly improved electrical stability (here, the drive voltage) and the unaffected luminance stability implies different origins for the lifetime-limiting reactions. In consideration of that it has been demonstrated that cathodic side reactions can be prominent in similar polymer LECs,28 and that the new electrolyte features an improved cathodic stability (see Figure 3), we propose that the increased electrical stability can be attributed to suppression of undesired electrochemical side reactions involving the electrolyte at the cathode. Specifically, with a lowered pile-up of side-reaction residues at the cathode, the “wasted” voltage drop over this residue region will be suppressed, as manifested in a lowered increase of the drive voltage, during the operation of the OCH3-LEC. The luminance decay is, in contrast, assigned to side reactions in the emission zone,59 and the independence of the electrolyte selection implies that the prime culprit is electrolyte-independent, and we propose that exciton-exciton or exciton-polaron interactions could play a key role in the deterioration of the luminance.



(a)

trolyte. By implementing the modified electrolyte into LEC devices, we are able to attain a desired combination of fast turn-on, good stability, and improved efficiency.

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ASSOCIATED CONTENT Supporting Information. Cyclic voltammetry data and potentiostatic data for devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Optoelectronic properties for ITO/{Super Yellow+ion transporter+LiCF3SO3}/Al LECs with the ion transporter being TMPE-OCH3 (solid red squares) or TMPE-OH (open black circles). The turn-on kinetics are presented in (a), the long-term stability of the luminance and the voltage in (b), and the power conversion efficacy in (c). The drive current density was j = 7.7 mA/cm2.

CONCLUSION To conclude, we report on the designed synthesis of a modified oligomeric ion transporter for application in improved LEC devices. We show that the hydroxyl end-group in previously utilized TMPE-based ion transporters causes significant intermolecular hydrogen bonding and strong intramolecular cation coordination in the corresponding electrolyte, and correlate the strength of these interactions with sluggish ion kinetics and a slow LEC turn-on. By replacing the hydroxyl endgroup with a less polar methoxy end-group, we are able to significantly suppress these inter- and intramolecular interactions, and also improve upon the cathodic stability of the elec-

ACKNOWLEDGMENT This work was financially supported by the Swedish Foundation for Strategic Research, Vetenskapsrådet, Energimyndigheten, Kempestiftelserna, STandUP for Energy and the Knut and Alice Wallenberg foundation under contract KAW 2012.0083. L.E. is a “Royal Swedish Academy of Sciences Research Fellow” supported by a grant from the Knut and Alice Wallenberg Foundation.

REFERENCES 1. Sandstrom, A.; Dam, H. F.; Krebs, F. C.; Edman, L. Nat. Commun. 2012, 3, 1002. 2. Hernandez-Sosa, G.; Tekoglu, S.; Stolz, S.; Eckstein, R.; Teusch, C.; Trapp, J.; Lemmer, U.; Hamburger, M.; Mechau, N. Adv. Mater. 2014, DOI: 10,1002/adma.201305541. 3. Matyba, P.; Yamaguchi, H.; Eda, G.; Chhowalla, M.; Edman, L.; Robinson, N. D. Acs Nano 2010, 4, 637-642. 4. Liang, J. J.; Li, L.; Niu, X. F.; Yu, Z. B.; Pei, Q. B. J. Phys. Chem. C 2013, 117, 16632-16639. 5. Yu, Z. B.; Hu, L. B.; Liu, Z. T.; Sun, M. L.; Wang, M. L.; Gruner, G.; Pei, Q. B. Appl. Phys. Lett. 2009, 95, 203304. 6. Tordera, D.; Meier, S.; Lenes, M.; Costa, R. D.; Orti, E.; Sarfert, W.; Bolink, H. J. Adv. Mater. 2012, 24, 897-900. 7. Leger, J. M.; Rodovsky, D. B.; Bartholomew, G. R. Adv. Mater. 2006, 18, 3130-3134. 8. Abdellah, A.; Fabel, B.; Lugli, P.; Scarpa, G. Org. Electron. 2010, 11, 1031-1038. 9. Pertegas, A.; Shavaleev, N. M.; Tordera, D.; Orti, E.; Nazeeruddin, M. K.; Bolink, H. J. J. Mater. Chem. C 2014, 2, 16051611. 10. Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abruna, H. D.; Malliaras, G. G. J. Mater. Chem. 2007, 17, 2976-2988. 11. Buda, M.; Kalyuzhny, G.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 6090-6098. 12. Leger, J. M. Adv. Mater. 2008, 20, 837-841. 13. Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Angew. Chem. Int. Ed. 2012, 51, 8178-8211. 14. Dick, D. J.; Heeger, A. J.; Yang, Y.; Pei, Q. B. Adv. Mater. 1996, 8, 985-987. 15. Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086-1088. 16. Matyba, P.; Maturova, K.; Kemerink, M.; Robinson, N. D.; Edman, L. Nat. Mater. 2009, 8, 672-676. 17. Rodovsky, D. B.; Reid, O. G.; Pingree, L. S. C.; Ginger, D. S. Acs Nano 2010, 4, 2673-2680. 18. Meier, S. B.; van Reenen, S.; Lefevre, B.; Hartmann, D.; Bolink, H. J.; Winnacker, A.; Sarfert, W.; Kemerink, M. Adv. Funct. Mater. 2013, 23, 3531-3538. 19. Hu, Y.; Gao, J. J. Am. Chem. Soc. 2011, 133, 2227-2231.


Page 7 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20. Shoji, T. D.; Zhu, Z. H.; Leger, J. M. Acs Appl. Mater. Interfaces 2013, 5, 11509-11514. 21. Fang, J. F.; Yang, Y. L.; Edman, L. Appl. Phys. Lett. 2008, 93, 063503. 22. van Reenen, S.; Matyba, P.; Dzwilewski, A.; Janssen, R. A. J.; Edman, L.; Kemerink, M. J. Am. Chem. Soc. 2010, 132, 1377613781. 23. Fang, J.; Matyba, P.; Edman, L. Adv. Funct. Mater. 2009, 19, 2671-2676. 24. Li, X. Y.; AlTal, F.; Liu, G. J.; Gao, J. Appl. Phys. Lett. 2013, 103, 243304. 25. Lee, C. L.; Cheng, C. Y.; Su, H. C. Org. Electron. 2014, 15, 711-720. 26. Lin, F. D.; Wang, Y. J.; Lonergan, M. J. Appl. Phys. 2008, 104, 3517. 27. Gao, J.; Dane, J. Appl. Phys. Lett. 2003, 83, 3027-3029. 28. Fang, J.; Matyba, P.; Robinson, N. D.; Edman, L. J. Am. Chem. Soc. 2008, 130, 4562-4568. 29. Shao, Y.; Bazan, G. C.; Heeger, A. J. Adv. Mater. 2007, 19, 365-370. 30. Shao, Y.; Gong, X.; Heeger, A. J.; Liu, M.; Jen, A. K. Y. Adv. Mater. 2009, 21, 1972-1975. 31. Yang, C. H.; Sun, Q. J.; Qiao, J.; Li, Y. F. J. Phys. Chem. B 2003, 107, 12981-12988. 32. Sakanoue, T.; Sawabe, K.; Yomogida, Y.; Takenobu, T.; Seki, S.; Ono, S. Appl. Phys. Lett. 2012, 100, 263301. 33. Kaihovirta, N.; Larsen, C.; Edman, L. ACS Appl. Mater. Interfaces 2014, 6, 2940-2947. 34. Yu, Z.; Wang, M.; Lei, G.; Liu, J.; Li, L.; Pei, Q. J. Phys. Chem. Lett. 2011, 2, 367-372. 35. Asadpoordarvish, A.; Sandstrom, A.; Tang, S.; Granstrom, J.; Edman, L. Appl. Phys. Lett. 2012, 100, 193508. 36. Pei, Q. B.; Yang, Y.; Yu, G.; Zhang, C.; Heeger, A. J. J. Am. Chem. Soc. 1996, 118, 3922-3929. 37. Sandström, A.; Matyba, P.; Edman, L. Appl. Phys. Lett. 2010, 96, 053303. 38. Tang, S.; Edman, L. J. Phys. Chem. Lett. 2010, 1, 27272732. 39. Cao, Y.; Pei, Q. B.; Andersson, M. R.; Yu, G.; Heeger, A. J. J. Electrochem. Soc. 1997, 144, L317-L320. 40. Edman, L.; Pauchard, M.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2004, 95, 4357-4361. 41. Yu, Z. B.; Sun, M. L.; Pei, Q. B. J. Phys. Chem. B 2009, 113, 8481-8486. 42. Cooper, D. R.; Booth, C. Polymer 1977, 18, 164-170. 43. Becke, A. D. Phys. Rev. A 1988, 38, 3098-3100. 44. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785789. 45. Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. 46. Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654-3665. 47. Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257-2261. 48. Spångberg, D.; Hermansson, K. Chem. Phys. 2004, 300, 165-176. 49. Ohtaki, H.; Radnai, T. Chem. Rev. 1993, 93, 1157-1204. 50. Brandell, D.; Priimägi, P.; Kasemägi, H.; Aabloo, A. Electrochim. Acta 2011, 57, 228-236. 51. Steiner, T. Angew. Chem. Int. Ed. 2002, 41, 48-76. 52. Liu, Y.; Liu, M. S.; Jen, A. K. Y. Acta Polym. 1999, 50, 105-108. 53. Marcus, R. A. J. Chem. Phys. 1957, 26, 872-877. 54. Santos, L. F.; Carvalho, L. M.; Guimarães, F. E. G.; Gonçalves, D.; Faria, R. M. Synth. Met. 2001, 121, 1697-1698. 55. Matyba, P.; Andersson, M. R.; Edman, L. Org. Electron. 2008, 9, 699-710. 56. Sandstrom, A.; Matyba, P.; Inganas, O.; Edman, L. J. Am. Chem. Soc. 2010, 132, 6646-6647.

57. Kaufman, J. H.; Kanazawa, K. K.: Street, G. B. Phys. Rev. Lett. 1984, 53, 2461-2464. 58. Pei, Q.; Inganäs, O. J. Phys. Chem. 1992, 96, 10507-10514. 59. Wagberg, T.; Hania, P. R.; Robinson, N. D.; Shin, J. H.; Matyba, P.; Edman, L. Adv. Mater. 2008, 20, 1744-1749.



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