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High-Performance Light-Emitting Electrochemical Cells by Electrolyte Design Jonas Mindemark,†,‡,§ Shi Tang,†,§ Jia Wang,† Nikolai Kaihovirta,† 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

‡

S Supporting Information *

ABSTRACT: Polymer light-emitting electrochemical cells (LECs) are inherently dependent on a suitable electrolyte for proper function. Here, we design and synthesize a series of alkyl carbonate-capped starbranched oligoether-based electrolytes with large electrochemical stability windows, facile ion release, and high compatibility with common light-emitting materials. LECs based on such designed electrolytes feature fast turn-on, a long operational lifetime of 1400 h at >100 cd m−2 and a record-high power conversion efficiency of 18.1 lm W−1, when equipped with an external outcoupling film.

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INTRODUCTION Light-emitting electrochemical cells (LECs) can feature lowvoltage light emission from simple single-layer devices based on solely solution-processable materials.1−5 Importantly, the LEC thereby circumvents many of the issues associated with the manufacturing of the more commonplace organic light-emitting diode (OLED),2 and as such promises to constitute an appealing alternative for a wide range of low-cost and/or large-area emissive applications.4,6−10 The enabling characteristic of the LEC, which separates it from the OLED, is the presence of mobile ions, i.e., an electrolyte, in the light-emitting active material.11−14 The LEC performance is as such intimately dependent on the properties of this electrolyte, and drawbacksin the form of slow turn-on, poor efficiency, limited stability, or a combination thereofhave commonly been attributed to the employment of an inadequate electrolyte.15−19 With this in mind, it is surprising that the number of investigated electrolytes is quite small. The pioneering LEC report utilized the salt LiCF3SO3 dissolved in high-molecularweight poly(ethylene oxide) (PEO) as the electrolyte, and this import from the Li-polymer battery field has remained a popular choice, although such electrolytes suffer from a limited electrochemical stability window18,19 and ambient-temperature crystallization.17 Reports on alternative LEC electrolytes, in the form of polymerizable ion-pair monomers20−22 and ion transporters,10,23,24 as well as ionic liquids,25−27 are in this context both interesting and motivated, although it should be noted that none of these novel electrolytes has as-of-yet delivered a fast, stable, and efficient LEC. Our group recently employed a nonpolymerizable, starbranched, hydroxyl-capped oligoether, termed trimethylolpropane ethoxylate (TMPE-OH), as the ion-solvating and ion© XXXX American Chemical Society

transporting medium (“the ion transporter”), into which we dissolved the salt LiCF3SO3. LECs based on this electrolyte featured a long operational lifetime but suffered from slow turnon.28 In a follow-up study, we methylated the hydroxyl endgroups to synthesize TMPE-OCH3 with the ambition to speed up the ion kinetics, and the corresponding LECs did, as desired, feature a much improved turn-on.29 Herein, we further elaborate on this concept and report on the synthesis of a group of TMPE-based ion transporters that were designed to fulfill a set of identified LEC-specific criteria. We find that LEC devices based on these new ion transporters feature fast onset, a record-high efficiency of >18 lm/W, and a long operational lifetime of 1400 h at >100 cd m−2.

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EXPERIMENTAL SECTION

Example Synthesis of TMPE-OC. TMPE-OH (Mn ∼ 450 g mol−1, 1.35 g) was dissolved in dry dichloromethane (20 mL). Octyl chloroformate (2.3 mL, 12 mmol) was added, and the solution was cooled in an ice bath. Pyridine (1.8 mL, 22 mmol) was added over a period of 20 min. After an additional 30 min, the ice bath was removed and the reaction was allowed to proceed at room temperature overnight. N,N-Dimethylethanolamine (0.45 mL, 4.5 mmol) was added to quench the reaction, and the reaction mixture was washed with deionized water (20 mL), 1 M hydrochloric acid (2 × 20 mL), and saturated aqueous NaHCO3 (20 mL). The organic phase was dried with MgSO4 and filtered and the solvent evaporated. Solvent residues were removed in a vacuum oven at ∼40 °C over P2O5 to yield 2.24 g of a slightly yellowish and slightly viscous liquid. The successful Received: December 15, 2015 Revised: April 1, 2016

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DOI: 10.1021/acs.chemmater.5b04847 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials conversion of the hydroxyl end-groups was confirmed by 1H NMR spectroscopy, as shown in Figure S1. Electrolyte Characterization. Cyclic voltammetry (CV) measurements were carried out with an Autolab PGSTAT302 potentiostat/ galvanostat (Eco Chemie) that was controlled with the GPES software. A Au-coated glass plate was the working electrode, a Pt rod was the counter electrode, a Ag wire was the quasi-reference electrode, and 0.1 M LiCF3SO3 and 0.2 M ion transporter in anhydrous acetonitrile was used as the electrolyte. The scan rate was 0.05 V/s. Directly after each CV measurement, a calibration scan was run with a small amount of ferrocene added to the electrolyte. All CV potentials are reported vs the ferrocene/ferrocenium ion (Fc/Fc+) reference. 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 measurements were performed under N2 atmosphere in a glovebox ([O2] < 1 ppm, [H2O] < 0.5 ppm). FTIR spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer equipped with an attenuated total reflectance (ATR) accessory. Viscosities were measured with rotational viscometry using a TA Instruments Advanced Rheometer AR2000 equipped with a cone-and-plate geometry (20 mm, 1° stainless steel cone). The ionic conductivity of the electrolytes was determined by electrochemical impedance spectroscopy on samples sandwiched between stainless steel electrodes separated by a 0.5 mm PTFE spacer. The measurements were performed on an Autolab PGSTAT302 potentiostat/galvanostat (Eco Chemie) with the FRA2 module, in the range 1 MHz−1 Hz using an amplitude of 10 mV. The ionic conductivity was calculated from the ionic resistance, obtained as the low-frequency intercept with the x-axis in a Nyquist plot. AFM measurements were performed in tapping mode using a MultiMode SPM microscope equipped with a Nanoscope IV Controller (Veeco Metrology). Device Fabrication and Characterization. Master solutions were prepared by separately dissolving the constituent materials in anhydrous tetrahydrofuran at a concentration of 6.5 mg/mL (SY) and 10 mg/mL (ion transporter and LiCF3SO3). The molar ratio between SY and the ion transporter was kept constant, which resulted in the following mass ratios for the active-material inks: SY:ion transporter:LiCF3SO3 = 1:x:0.03, x = 0.14 (TMPE-EC), x = 0.17 (TMPEBC), x = 0.20 (TMPE-OC). LECs were fabricated by spin-coating the active-material ink at 2000 rpm for 60 s onto carefully cleaned indium−tin−oxide (ITO) coated glass substrates (20 Ω/square, Thin Film Devices, U.S.A.). The dry thickness of the active material was 100 nm for all ion transporters. A set of four Al cathodes were deposited on top of the active material by thermal evaporation (at p < 5 × 10−4 Pa) through a shadow mask. The light-emission area, as defined by the size of the cathode, was 0.85 × 0.15 cm2. The devices were characterized using a computer-controlled source-measure unit (Agilent U2722A) and a calibrated photodiode, equipped with an eye-response filter (Hamamatsu Photonics), and connected to a data acquisition card (National Instruments USB-6009) via a current-tovoltage amplifier. All of the above procedures, except for the cleaning of the substrates, were carried out in two interconnected N2-filled glove boxes ([O2] < 1 ppm, [H2O] < 0.5 ppm).

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To test these hypotheses, a series of alkyl carbonate-capped TMPE derivatives were synthesized by acylation of the hydroxyl end-groups with alkyl chloroformate reagents, as depicted in Scheme 1. Depending on the length of the alkyl Scheme 1. Schematic of the Synthesis of Alkyl CarbonateCapped TMPE Ion Transporters

end-group, the new ion transporters were termed ethyl carbonate-capped TMPE (TMPE-EC), butyl carbonate-capped TMPE (TMPE-BC), and octyl carbonate-capped TMPE (TMPE-OC). Electrolytes were prepared by dissolving LiCF3SO3 salt in each of the different ion transporters, and the electrochemical stability window (ESW) of these electrolytes was measured with the aid of cyclic voltammetry (CV). As displayed in Figures 1a and S2 and as summarized in Table S1, we find that the new ion-transporter electrolytes consistently feature a significantly expanded ESW in comparison to previously studied TMPE-OH, TMPE-OCH3, and PEO-based equivalents.29 A more detailed analysis of the CV data reveals that the alkyl carbonate capping results in an expansion of the anodic stability by ∼0.5 eV, whereas the cathodic stability remains essentially intact. We attribute the latter to that the cathodic stability is now not limited by the ion transporter but by the CF3SO3 anion.15,16 Polyether-based ion transporters are known to dissolve salts such as LiCF3SO3 through a multidentate ether oxygen coordination of the cation, whereas the anion is left effectively “free”. In order to investigate whether the addition of a carbonate group affects the Li-ion coordination, we turned to IR spectroscopy. Figure 1b presents the carbonyl (CO) stretch mode of a TMPE-OC:LiCF3SO3 electrolyte, where the emergence of a shoulder at ∼1723 cm−1 following the addition of the salt reveals that a fraction of the Li cations are coordinated also to a carbonyl group, i.e., featuring a mixed ether/carbonate coordination. This conclusion is further supported by the NMR data presented in Figures S3 and S4. We anticipate that the mixed coordination will lead to a weaker binding of the Li cation to its parent ion transporter and that the release and action of the cations during, e.g., the LEC turnon, as a consequence, will be more facile and fast. We also measured the ionic conductivity of the new electrolytes and find that it is lowest (2.1 × 10−6 S cm−1) for the largest ion transporter TMPE-OC and highest (6.1 × 10−6

RESULTS AND DISCUSSION

Inspired by the recent good results obtained with the TMPEbased ion-transporter platform29 and by the reported larger electrochemical stability of organic carbonates compared to corresponding ether-based compounds,30 we set out to design a carbonate-capped TMPE ion transporter. An additional design criterion considered that the LEC performance frequently has been reported to suffer from that the electrolyte and the light emitter tend to phase separate during operation.27,31,32 We anticipated that the latter process could be alleviated through the inclusion of alkyl chains on the end-groups of the ion transporter in order to render it more hydrophobic and thereby more compatible with a hydrophobic light-emitting compound. B


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Chemistry of Materials

Figure 1. (a) Cyclic voltammetry traces of TMPE-OCH3 (top) and TMPE-OC (bottom). (b) IR spectrum of a TMPE-OC:LiCF3SO3 electrolyte, with the two dotted black lines representing Voigtian line shape fits and the dashed red line representing the cumulative fit. (c) (10 μm × 10 μm) AFM images of the surface morphology of {SY + ion transporter + LiCF3SO3} active materials before (left) and after (right) annealing at 110 °C for 90 min, with the ion transporter being TMPE-OCH3 (top) and TMPE-OC (bottom).

Figure 2. (a) Turn-on kinetics during the first min of operation and (b) the long-term stability of ITO/{SY + ion-transporter + LiCF3SO3}/Al sandwich-cell LECs, with the device structure disclosed in the inset of (a). The devices were driven by j = 7.7 mA cm−2. (c) The long-term stability of a TMPE-OC device driven by a prebias of 7.7 mA cm−2 for 40 min and thereafter by 1.9 mA cm−2. (d) The temporal evolution of the efficiency for a TMPE-OC device equipped with a light-outcoupling film. The device was driven by a prebias of 7.7 mA cm−2 for 40 min and thereafter by 0.77 mA cm−2.

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Chemistry of Materials Table 1. Performance Metrics of ITO/{SY + ion transporter + LiCF3SO3}/Al Sandwich-Cell LECsa ion transporter

turn-on time to >300 cd m−2 (s)b

TMPE-EC TMPE-BC TMPE-OC

8 (10) 12 (14) 9 (12)

TMPE-OCH3e TMPE-OHe

16 (47) 1740 (2000)

current efficacy (cd A−1)b,c

power conversion efficacy (lm W−1)b,c

lifetime at >300 cd m−2 (h)

8.3 (7.9) 10.4 (10.1) 10.6 (10.3) 14.6 (14.0)d 8.4 (7.5) 7.1 (6.4)

8.1 (7.9) 10.7 (10.3) 10.7 (10.4) 18.1 (17.2)d 7.1 (6.3) 5.9 (5.2)

155 312 355 130 130

a The devices were driven by j = 7.7 mA cm−2, unless noted differently. bResult from the champion device. The average from ≥4 independent measurements is presented in parentheses. cMeasured at maximum luminance. dRecorded on a device equipped with an outcoupling film. The device was driven by a prebias of 7.7 mA cm−2 for 40 min, and thereafter by 0.77 mA cm−2. eData from a previous study.29

S cm−1) for the smallest ion transporter TMPE-EC. These values are significantly lower than for both TMPE-OH and TMPE-OCH3-based electrolytes comprising similar concentrations of salt. We find that the same trend applies for the viscosity, as it also decreases with increasing ion transporter size. Interestingly, this implies that Walden’s rule does not apply for these electrolytes. Tabulated values on the ionic conductivity and the viscosity are included in Table S1. The active material in an LEC also contains a light-emitting compound, and in this study we have utilized an amorphous conjugated polymer termed Super Yellow (SY).33,34 Figure 1c shows the surface morphology of {SY + ion transporter + LiCF3SO3} active-material films based on the “old” ion transporter TMPE-OCH3 and the “new” ion transporter TMPE-OC, before and after a high-temperature annealing step. The nonannealed films are highly smooth, but after annealing distinct micrometer-sized islands appear in both films, which we interpret as phase separation between the light emitter and the electrolyte. It is notable that this phase separation is less prominent in the film based on the new ion transporter, as manifested in the lower RMS and peak values in Figure 1c, and that the trend is that phase separation decreases with increasing length of the alkyl chain on the ion transporter (see also Figure S7). This implies that the addition of the alkyl end-group does indeed render the electrolyte more compatible with a hydrophobic light emitter, such as SY, and that the efficiency of this compatibilization is dependent on the length of the alkyl chain. In this context it is relevant to point out that LEC devices frequently are exposed to high temperatures during the drying of solvent-cast films and because of selfheating during operation (only a fraction of the injected carriers result in photons exiting the device; the remainders are lost as heat). We now turn our attention to the performance of LEC devices based on the new ion transporters. The employed device structure is included as an inset in Figure 2a. Summarizing data for both the new and old TMPE-based LECs are presented in Table 1, and we mention that the all devices featured uniform light emission to the eye. Importantly, we find that all three new ion transporters feature a fast turn-on of ∼10 s to 300 cd m−2 during galvanostatic driving at j = 7.7 mA cm−2 (Figure 2a), which is a significant improvement in comparison to earlier TMPE-based LECs. Interestingly, this improvement in the turn-on kinetics was attained despite the fact that the new electrolytes exhibit a lower ionic conductivity than the old electrolytes (see Table S1). In this context, we call attention to that the turn-on of an LEC can depend on a number of factors in addition to the ionic conductivity,17,26,35,36 including the phase morphology (an intimate blending of the electrolyte and light-emitter is preferable to bring the ions in

close proximity to the doping sites),29 the strength of the ion solvation (a too strong solvation by the ion transporter will render the ion release during LEC turn-on difficult),29 and ion association. 37 Accordingly, we tentatively attribute the improved turn-on kinetics of the new ion-transporter LECs to our above observations of an improved phase morphology and a weakened coordination of the Li cation. Figure 2b presents the long-term performance of a TMPEOC LEC during galvanostatic driving (corresponding data for the TMPE-BC and TMPE-EC devices are shown in Figure S8). Once again, we find that the shift to the new ion transporter results in a notably improved performance (see Table 1), with the largest TMPE-OC being the best with a 355 h operational lifetime at a high luminance of >300 cd m−2. By using a driving protocol including a prebias at a higher current density, we obtained an operational lifetime of 1400 h at >100 cd m−2 for the same type of device (see Figure 2c). We note that these data were recorded on nonencapsulated devices operating in a N2-filled glovebox and that recent results suggest that an even better stability can be obtained from properly encapsulated devices operating under ambient air.38 We attribute the prolonged stability of the new iontransporter LECs to (i) an improved phase compatibility between the electrolyte and the light-emitter and (ii) the expanded ESW for the new ion transporters. As the stability of an LEC has been demonstrated to be sensitively dependent on the ion concentration and the concomitant doping concentration, a homogeneous distribution of the ions in a wellblended active material will result in a more stable operation; for a discussion on this topic, see ref 38. The expanded ESW of the electrolyte will in turn suppress electrolyte-induced lifetimelimiting side reactions.18,19 The same two effects are also anticipated to lead to an improved efficiency, with a homogeneous optimized doping concentration in a well-mixed system leading to low exciton quenching,38−40 whereas a decreased amount of electrolyte side-reaction residues at the electrode interfaces is expected to result in a lowered drive voltage.18 It is only the former effect that is manifested in the current efficacy, and the markedly improved value for this parameter (to >10 cd A−1) attained with the two larger of the new ion transporters is thus taken as an indication of that an improved phase morphology indeed can result in a better harvesting of the excitons. In this context, we mention that we have measured the photoluminescence quantum efficiency of all of the active materials based on different ion transporters but that the detected small variations (see Figure S9) allowed us to exclude it as a cause of the change in the current efficacy. The most important efficiency parameter is, however, the power conversion efficacy (PCE), and both the TMPE-BC and D


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Chemistry of Materials

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TMPE-OC devices feature an impressive PCE value well above 10 lm W−1 at a high luminance of 800 cd m−2. This is not only a significant improvement over previous TMPE-based LECs (see Table 1) but also compared to nominally similar LECs with electrolytes comprising PEO-41 or ionic liquid-based electrolytes.27 In addition, a further improvement could be attained by equipping the device with a light-outcoupling film, as detailed in ref 42. The operation of such a device is displayed in Figure 2d, and to the best of our knowledge the obtained PCE value for our champion device of 18.1 lm W−1 (at a luminance of 112 cd m−2) is the highest for a polymer LEC to date.

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CONCLUSIONS To conclude, we report on the design and synthesis of a series of star-branched oligoether-based ion transporters with alkyl carbonate end-groups, which considers the specific requirements of the electrolyte in LEC devices. We find that such optimized LECs feature a fast turn-on, a long operational lifetime of >350 h at >300 cd m−2 and 1400 h at >100 cd m−2, and a record-high efficiency of >18 lm W−1. We anticipate that the demonstrated connection between the synthesis of an LECfit electrolyte and the improved device performance should inspire further work in this overlooked field and mention that the realization of an LEC electrolyte with improved cathodic stability will represent an important and much-desired breakthrough.

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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.5b04847. Additional experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

(J.M. and S.T.) These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Swedish Foundation for Strategic Research, the Swedish Research Council, the Swedish Energy Agency, Kempestiftelserna, and the Knut and Alice Wallenberg Foundation.

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REFERENCES

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