Quest for an Appropriate Electrolyte for High-Performance Light

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Quest for an Appropriate Electrolyte for High-Performance Light-Emitting Electrochemical Cells Shi Tang and Ludvig Edman* The Organic Photonics and Electronics Group, Department of Physics, Umeå University, SE-901 87 Umeå, Sweden

ABSTRACT The electrolyte plays a critical role in the operation of light-emitting electrochemical cells (LECs), and the LEC operational stability and efficiency can be severely limited by electrolyte-induced side reactions. Here we employ a trimethylolpropane-LiCF3SO3 electrolyte with a particularly wide electrochemical stability window and for-the-purpose appropriate transport properties to minimize cathodic side reactions and to keep the emission zone free from electrolyte species during steady-state operation. We utilize an optimized blend of this electrolyte and the conjugated polymer superyellow as the active material sandwiched between air-stable electrodes, and we demonstrate that such LEC devices can exhibit a notably respectable operational lifetime (57 days of uninterrupted operation at a brightness of >100 cd/m2) and a high efficiency (>10 lm/W at >100 cd/m2). SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

ight-emitting electrochemical cells (LECs) are defined by the existence of mobile ions (i.e., an electrolyte) within the active layer.1-14 The role of these ions during the operation of LECs has been intensely debated,15-20 but recent experimental results strongly suggest that LECs based on fluorescent conjugated polymers (CPs) operate under the realm of in situ electrochemical doping.21,22 When a voltage equal to or larger than the “band-gap potential” of the CP is applied to the electrodes of the device, the mobile ions redistribute so that the CP is p-type doped next to the anode and n-type doped next to the cathode. These highly conducting doped regions grow in size with time and eventually make contact in the bulk of the active layer under the formation of a light-emitting p-n junction. The tantalizing in situ formation of a p-n junction brings a number of appealing advantages in comparison with competing emissive technologies, such as organic light-emitting diodes (OLEDs), notably that both electrons and holes can be efficiently injected in a balanced fashion from air-stable and solution-processed electrode materials23,24 and that the device operation is remarkably insensitive to the thickness of the active layer.25-28 However, there is also typically a price to pay, and in the case of LECs, the most significant drawback in comparison to, for example, OLEDs has been a nonadequate operational lifetime. We have recently been able to identify a number of culprits that negatively affect the operational stability of LEC, and it has become clear that the electrolyte frequently plays a significant role in various detrimental side-reaction processes.29,30 First, we have established that the reduction stability of commonly employed electrolytes is inadequate and that the thermodynamically favored cathodic reaction as a consequence commonly is (irreversible) reduction of the electrolyte instead of the desired (reversible) n-type doping of

the CP.29 This type of “electrochemical side reaction” is manifested in that the light emission takes place very close to the cathode and, even more seriously, that a layer comprising side-reaction residues forms next to the cathode.29 Second, we have found evidence that a “chemical side reaction” can take place at the position of the light-emitting p-n junction, and it seems feasible that its origin is a detrimental interaction between the excitons (the excited CP) formed in the lightemission zone and immobile and remaining electrolyte species at the same spatial location.30 Both the electrochemical and the chemical side reactions will eventually result in the formation of an electronically insulating region within the active material, with the consequence that the current and the light-emission intensity will drop to zero; the LEC device has at this point reached the end of its operational lifetime. In this Letter, we have set out to alleviate the extent of these two side reactions and attain improved device stability by identifying and employing a carefully chosen electrolyte with appropriate electrochemical and transport properties. Specifically, we expect that such an “ideal” electrolyte should fulfill the following criteria: (i) The electrochemical stability window (ESW) of the electrolyte is expanded in comparison with previously utilized electrolytes, primarily on the cathodic side, so that it ideally encompasses both the n- and p-type doping potentials of the employed CP. (ii) All electrolyte species are mobile, and their concentration is selected such that the lightemission zone (the p-n junction) is void of electrolyte species during steady-state operation.

L

r 2010 American Chemical Society

Received Date: August 3, 2010 Accepted Date: August 30, 2010 Published on Web Date: September 02, 2010

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DOI: 10.1021/jz1010797 |J. Phys. Chem. Lett. 2010, 1, 2727–2732

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Figure 1. (a) Chemical structure of the constituents in the {TMPE þ LiCF3SO3} electrolyte. (b) Cyclic voltammetry data for a {TMPE þ LiCF3SO3} electrolyte (solid black line) and a “conventional” {PEO þ LiCF3SO3} electrolyte (dashed red line).

Figure 2. Schematic illustrating (a) a pristine device, where all Liþ cations are coordinated to TMPE molecules; (b) the initial drift of TMPE:Liþ cationic coordination complexes toward the cathode and CF3SO3- anions toward the anode; and (c) the steady-state p-n junction structure, where it is notable that the junction is free from both ions and TMPE molecules during light emission.

Toward this end, we have screened and tested a large number of different electrolytes, and a particularly promising candidate turned out to be the salt LiCF3SO3 dissolved in trimethylolpropane ethoxylate (TMPE). The chemical structure of the {TMPEþLiCF3SO3} electrolyte is presented in Figure 1a, and its ESW is indicated in Figure 1b by the solid black line. Specifically, the (irreversible) redox peaks with onsets at -1.9 and þ0.6 V versus Fc/Fcþ span the comparatively wide voltage range within which the {TMPEþLiCF3SO3} electrolyte is electrochemically inert. A commonly utilized electrolyte in LEC devices has been LiCF3SO3 dissolved in poly(ethylene oxide) (PEO),1,27,31,32 and its ESW is indicated by the dashed red line in Figure 1b for comparison. It is clear that {TMPEþLiCF3SO3} represents a significant stability improvement (by ∼0.2 V) over {PEOþLiCF3SO3} on the important cathodic side. In this context, we reiterate that the ESW here is defined to be the voltage range within which a species is electrochemically inert and that potential reversible redox reactions involving the species accordingly also can limit its ESW. For instance, it could very well be that the cathodic ESW limit of one or both of the electrolytes in Figure 1b could originate in a potentially reversible plating of Li. Moreover, the CV scans displayed in Figure 1b were performed on the electrolytes, as dissolved in


acetonitrile, but side reactions involving acetonitirile are here excluded based on previous experiments that demonstrated that acetonitrile is electrochemically inert in the probed voltage range.33 The {TMPEþLiCF3SO3} electrolyte is a viscous liquid at room temperature, but when blended with a majority-component solid-state CP, here superyellow (SY),31,34-37 a free-standing film suitable for use as the active layer in an LEC device can be fabricated. The anticipated ideal operation of an LEC device based on this CP-electrolyte combination is schematically depicted in Figure 2. The Liþ cations within the pristine active layer will coordinate strongly to the electronegative ether oxygens of the TMPE compound (Figure 2a). When a voltage is applied to the pristine device, the cationic TMPE:Liþ complexes will drift toward the cathode, whereas the “free” CF3SO3- anions will drift toward the anode (Figure 2b). If the applied voltage exceeds the band gap potential of SY, then p-type and n-type doping are formed at the anode and cathode, respectively (Figure 2b), and eventually, a lightemitting p-n junction is created in the bulk of the active layer (Figure 2c). In this study, we have selected to utilize an active material mass ratio of {SY:TMPE:LiCF3SO3} = {1/0.1/0.03} for two

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Figure 4. Galvanostatic data recorded on an ITO/PEDOT-PSS/ {SYþTMPEþLiCF3SO3}/Al sandwich cell with an active material mass ratio of {1:0.1:0.03}. The device was prebiased at j = 7.7 mA/cm2 and thereafter continuously driven at j = 1.9 mA/cm2 for the remainder of its lifetime.

Figure 3. Device data recorded on an ITO/PEDOT-PSS/{SYþTMPEþ LiCF3SO3}/Al sandwich cell with an active material mass ratio of {1:0.1:0.03}. The scan rate was 0.02 V/s.

principal reasons: (i) It corresponds to approximately one TMPE molecule per Liþ cation, so that no TMPE molecules are left noncoordinated and effectively noncharged (Figure 2a). (ii) All ions in the active material are expected to become locked up in the doped SY regions so that the light-emitting p-n junction is effectively void from ions (as well as cationcoordinated TMPE solvent molecules) during steady-state operation, as depicted in Figure 2c; see ref 38 for a detailed discussion on the ion-optimization topic. Moreover, an atomic force microscopy study on such an electrolyte-dilute {SYþ TMPEþLiCF3SO3} film reveals a perfectly smooth surface (data not shown), which is obviously desirable from a device fabrication perspective. We now shift our attention to the performance of LEC devices comprising the {SYþTMPEþLiCF3SO3} active layer sandwiched between an ITO/PEDOT-PSS anode and an Al cathode. Figure 3a shows a typical current densitybrightness-voltage graph recorded at a scan rate of 0.02 V/s. The turn-on voltage (defined to be the voltage at which the brightness exceeds 1 cd/m2) for the pristine device is 3.1 V, and the brightness reaches 14 000 cd/m2 already at V = 5.3 V. The efficiency data for the same device during the voltage scanning is presented in Figure 3b. The maximum current efficacy is 10.7 cd/A (at a brightness of 1330 cd/m2), and the maximum power conversion efficacy (PCE) is 7.7 lm/W (at a brightness of 1100 cd/m2). The performance of LEC devices is in general dependent on the scan rate (or the biasing history)39-43 because of the limited mobility of the ions in the solid-state active layer, and an even lower


turn-on voltage equal to the band gap potential of SY can be attained at a lower scan rate (or for a recently prebiased device). Here we want to point out that even though the cathodic stability potential of the {TMPEþLiCF3SO3} electrolyte represents an improvement over other studied electrolytes, it is still situated below the n-type doping potential of SY.44 The thermodynamic preferred cathodic reaction in these LEC devices is thus reduction of the electrolyte. We have previously shown that to minimize the extent of this undesired electrochemical side reaction, it is advisable to operate the devices at a high prebias during the doping formation process.29,38,44 One convenient way of accomplishing a high prebias in an LEC device is to drive it at constant current (in galvanostatic mode) because the effective device resistance decreases strongly during the initial doping process. Here we have utilized the galvanostatic approach but also employed a prebias stage during which we have employed an even higher current and, as a consequence, a higher voltage for a limited period of time (until the brightness reached ∼250 cd/m2). Figure 4 presents the device data for a typical sandwich cell driven in galvanostatic mode. The initial prebias was j = 7.7 mA/cm2, whereafter the device was driven continuously at j = 1.9 mA/cm2 until the brightness dropped below 100 cd/m2, at which point the device was pronounced “dead”. With this definition, the operational lifetime of the device was 1150 h, corresponding to 48 days of uninterrupted operation. (See Figure 4a.) The peak efficiency of the same device is also

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impressive because the current efficacy and PCE reached 8.6 cd/A and 8.6 lm/W, respectively. (See Figure 4b.) We find that the applied operation protocol resulted in a high drive voltage of V > 10 V during the first few seconds of operation and suggest, in line with previous observations,29 that this high prebias voltage minimizes the extent of undesired electrochemical side reactions. It is notable that both the operational lifetime and the efficiency presented in Figure 4b represent a distinct improvement over similar SY-based LEC devices utilizing PEO-based electrolytes.31,44 We find it highly plausible that this improvement can be attributed to the enhanced reduction stability of the {TMPEþLiCF3SO3} electrolyte (Figure 1b) and the fact that the mobility of all species within the electrolyte (both the ions and the ionic solvent) allows the formation of an emission zone completely free from anything but the emissive CP. (See Figure 2c.) We also again emphasize that a further enhancement in device stability, and presumably device efficiency, can be anticipated for electrolytes with a further expanded reduction stability, which ideally should encompass the reduction potential of the CP. Rudmann and Rubner introduced the concept of including an inert and glassy polymer into the active material of a smallmolecule-based LEC for improved device performance,45,46 and Shao and coworkers subsequently showed that the concept was applicable for CP-based LECs as well.47 Inspired by these findings, we have added high-molecular-weight PS to the active layer of our devices so that the mass ratio between the constituent compounds was {SY:PS:TMPE:LiCF3SO3} = {1:0.25:0.1:0.03}. The device data for two such nominally identical sandwich cells during long-term operation in galvanostatic mode are presented in Figure 5. The device shown in Figure 5a exhibited an impressive operational lifetime of 1375 h, corresponding to more than 57 days of uninterrupted operation at a peak PCE value of 5.2 lm/W, whereas the device in Figure 5b exhibited an operational lifetime of 770 h but at a higher peak PCE value of 10.2 lm/W. We choose to include these two sets of data to illustrate that we find a variation in the device-to-device performance but also that the baseline performance of these devices is rather impressive. In fact, we have performed similar long-term measurements on six independent devices, and the operational lifetime within this set was found to vary between 770 and 1375 h, whereas the peak PCE value ranged between 5.2 and 10.2 lm/W. To the best of our knowledge, the device presented in Figure 5a, with an operational lifetime of 1375 h, represents the most stabile CP-based LEC reported up-to-date. Shao and coworkers47 have demonstrated devices comprising an ionic liquid as the electrolyte with operational lifetimes on the order of hundred hours, and Sandstr€ om et al.44 and Fang et al.38 have recently reported devices based on PEO-based electrolytes with lifetimes in the range between 600 and 1000 h. We also note that Costa et al.48,49 and Rudmann et al.45 have demonstrated small-molecule-based LEC devices with (a differently defined) operational lifetime on the order of 1000 h. The achievement of an efficiency above 10 lm/W (Figure 5b) is also a notable feat because a screening of the


Figure 5. Galvanostatic data recorded on two different ITO/ PEDOT-PSS/{SYþPSþTMPEþLiCF3SO3}/Al devices, with an active material mass ratio of {1:0.25:0.1:0.03}. The devices were prebiased at j = 7.7 mA/cm2 and thereafter driven continuously at j = 1.9 mA/cm2.

literature implies that solely one previous CP-based LEC device has exhibited a PCE value exceeding 10 lm/W.50 To summarize, we have identified an electrolyte with attractive properties for LEC applications because all electrolyte species are mobile and because the cathodic stability of the electrolyte is expanded in comparison with up-to-now commonly employed alternatives. We anticipated that detrimental electrolyte-induced side reactions would be strongly suppressed in LEC devices comprising an optimized blend of this electrolyte and a CP as the active material, and we were also able to verify indirectly this hypothesis by demonstrating yellow-green-emitting LEC devices with an unprecedented combination of good operational stability and high efficiency. Moreover, by introducing a minor amount of PS into such an optimized active material blend, highly efficient LEC devices that emitted significant light for almost 2 months of uninterrupted operation were realized.

METHODS SUMMARY Cyclic voltammetry (CV) measurements were carried out with a Au-coated glass substrate as the working electrode, a Pt rod as the counter electrode, and a Ag wire as the quasireference electrode. The electrolyte comprised LiCF3SO3 (Alfa Aesar) and either 2 M PEO (Mw = 400, Polysciences) or 2 M TMPE (Mw = 450, Aldrich) dissolved in CH3CN (anhydrous, Aldrich); note that the molarity of PEO corresponds to the number of moles of ethylene oxide repeat units per liter of CH3CN. A freshly prepared electrolyte solution, a pristine working electrode, and carefully cleaned counter and pseudoreference electrodes were invariably used for the CV measurements.

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the Swedish Research Council (Vetenskapsrådet) for financial support. L.E. is a “Royal Swedish Academy of Sciences Research Fellow” supported by a grant from the Knut and Alice Wallenberg Foundation.

The CV sweeps were driven and measured by an Autolab PGSTAT302 potentiostat. Directly after each (cathodic or anodic) CV scan, a calibration scan was run with a small amount of bis-(η-cyclopentadienyl)iron(II) (ferrocene, g98%; Fluka) added to the electrolyte (∼10-4 M ferrocene concentration in CH3CN). All potentials in the CV measurements are reported versus the ferrocene/ferrocenium ion (Fc/Fcþ) reference redox system. The onset potentials for oxidation and reduction were calculated as the intersection of the baseline with the tangent of the current at the half-maximum of the peak. All sample preparation and CV measurements were performed under an inert atmosphere in a N2-filled glovebox. AFM images were recorded using a MultiMode SPM microscope with a Nanoscope IV Controller (Veeco Metrology) operating under ambient conditions. For device fabrication, the following materials were utilized for the active material: A decyloxyphenyl substituted poly(1,4-phenylene vinylene) CP termed SY (Merck), an iontransport material TMPE, a salt LiCF3SO3, and, in some experiments, an inert polymer PS (Mw = 9 105, Aldrich). All materials were used as received, with the exception of the LiCF3SO3 salt, which was dried in a vacuum oven at T = 473 K before use. The materials were dissolved separately in anhydrous tetrahydrofuran:SY in a concentration of 5 mg/ mL, and TMPE, LiCF3SO3, and PS in a concentration of 10 mg/mL. These master solutions were thereafter mixed together in an appropriate amount so that a desired active material ratio (as specified in the main text) was attained; the blend solution was thereafter stirred for 6 h at T = 323 K on a magnetic hot plate. LEC devices were fabricated by sequentially spin-coating ITO-coated glass substrates (1.5 1.5 cm2, 20 ohms/square; Thin Film Devices, Anaheim, CA) with poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS, Aldrich; thickness: d = 50 nm) and active material (d = 100 nm). On top of the stack, Al cathodes were deposited by thermal evaporation through a shadow mask at p < 2 10-6 mBar. The devices were driven by, and current was measured with, a Keithley 2400 source-meter.The brightness was measured using a calibrated photodiode with an eye response filter (Hamamatsu Photonics) connected though a current-to-voltage amplifier to a HP 34401A meter. The brightness values were typically double-checked during a measurement with a brightness meter (CS-100A, KONICA). All of the above device preparation procedures and measurements, except the cleaning of the substrates and the deposition of PEDOT-PSS, were carried out in two interconnected N2-filled glove boxes ([O2] < 3 ppm, [H2O] < 0.5 ppm).

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: ludvig.edman@ physics.umu.se.

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ACKNOWLEDGMENT We thank Dr. Jia Wang and Andreas Sandstr€ om at Umeå University for assistance with the CV measurements and the LEC characterization, respectively. We are grateful to Kempestiftelserna, Carl Tryggers Stiftelse, and


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