Pulsed Driving Methods for Enhancing the Stability of

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Pulsed Driving Methods for Enhancing the Stability of Electrochemiluminescence Devices Eun-Song Ko, Jong Ik Lee, Hong Chul Lim, Ji-Eun Park, Seok Hwan Kong, Jong-In Hong, Moon Sung Kang, and Ik-Soo Shin ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00748 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Pulsed Driving Methods for Enhancing the Stability of Electrochemiluminescence Devices Eun-Song Ko,†, # Jong Ik Lee,‡, # Hong Chul Lim,¶ Ji-Eun Park,§ Seok Hwan Kong,‡ Jong-In Hong,¶ Moon Sung Kang, ‡,* and Ik-Soo Shin†,§,* †

Department of Chemistry, Soongsil University, Seoul 06978, Republic of Korea

‡

Department of Chemical Engineering, Soongsil University, Seoul 06978, Republic of Korea

¶

Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 08826,

Republic of Korea §

Department of ICMC convergence technology, Soongsil University, Seoul 06978, Republic of

Korea #

These authors contributed equally to this work

* Corresponding Authors [email protected] (I.-S.S.) and [email protected] (M.S.K.)

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ABSTRACT: As a new device platform comprising only electrochemiluminescence (ECL) luminophores and an electrolyte sandwiched between electrodes, ECL devices (ECLDs) promise to be cost efficient for large-area emissive applications. However, rapid degradation of luminescence, along with thermal decomposition of the electrochemical components, has proven a seemingly fundamental problem in ECLDs. To alleviate this issue, we investigated the influence of inserting a resting period during the operation of such devices by applying a squareshaped pulsed signal. The inserted resting period enhances the device stability, as it allows the effective reaction volume near the electrodes to be replenished with ECL luminophores, thus preventing undesired side reactions. Moreover, the application of a current pulsed signal, rather than a voltage pulse, leads to further enhancement of the device stability, attributable to even distribution of the redox reaction over the rough surface of the electrode under current control. Under controlled pulsed-current operation (100 µA at 10 Hz), the emission characteristics of an ECLD employing a neutral iridium(III) complex as the luminophore can be preserved for ~1 h.

Keywords Electrogenerated chemiluminescence, elctrochemiluminescence device, pulsed driving method, current driving method


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1.

INTRODUCTION Light emission can be achieved from the excited states of luminophores, particularly from

those produced by charge-transfer reactions between the oxidized and reduced forms of a chemical species.1-3 This process, commonly known as electrogenerated chemiluminescence or electrochemiluminescence (ECL), has been widely exploited in the fields of chemical analysis,4 immunoassays,5-6 and biosensors.7-8 Recently, ECL has been utilized in developing a new type of light-emitting device, referred to as an ECL device (ECLD).9-12 ECLDs, which have the potential to be disruptive technology to current light-emitting devices, can be readily prepared from solution processes over a large area and operated at low voltages below 10 V. By introducing a matrix polymer into an active ECL solution, solid-state mechanical integrity can also be imparted to the active layer of these devices, making ECLDs highly suitable for flexible, stretchable electronic applications.13-14 Moreover, the architecture of ECLDs can deviate from the conventional structure of two electrodes vertically sandwiching the emissive layer. Instead, simpler structures with the two electrodes placed in parallel are possible, which are suitable for fabricating devices through printing.9, 15 Despite the intriguing opportunities, the operation stability of ECLDs remains a considerable challenge. One approach to address this issue is to exploit an ECL luminophore with inherent structural stability in its ionic radical form.16 The addition of metal oxide nanoparticles into the ECL solution mixture is also suggested to provide an additional pathway for electron transfer that helps the balance between the redox reactions.17 Besides these approaches based on materials engineering, the luminance properties of ECLDs as well as their stability can be improved by applying different driving schemes. It should be noted that the driving methods for ECLDs can be different from those for conventional solid-state light-emitting diodes. This is


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mainly because the mass transport of luminophores (by migration, diffusion, or a combination of the two) is critical for operating an electrochemical cell. As an example, for the ECL annihilation pathway, the reduced and oxidized forms of a luminophore should encounter each other within the cell to form the excited states. The basic driving method is to apply a voltage continuously to the device. However, owing to the finite diffusivity/mobility of the components, the response time of ECLDs is typically long; the reduced species generated at one electrode has to navigate within the device and encounter the oxidized species generated at the other to form the luminophore excited state.9 A coreactant can be added into the ECL mixture to facilitate the formation of the luminophore excited state through additional electrochemical charge-transfer cycles.18 However, the lifetime of such devices is fundamentally restricted by the content of the coreactant, as it is continuously consumed during the electrochemical cycles. Recent ECLDs are operated based on voltages with alternating polarities. Such a driving scheme is often referred to as the AC-driving method; it should be noted that even though AC stands for alternating current, in fact, the reported AC-driven ECLDs are operated with alternating voltages rather than with alternating currents.10, 13, 15, 17 As the application of voltages with alternating polarities allows the formation of both the reduced and oxidized species of an ECL luminophores at the same electrode, emission can be achieved promptly with high intensity. The downside of both the fixed and alternating voltage driving methods, referred to as continuous driving methods, is the continuous application of potential to the electrochemical cell. Under the continuous application of electrical inputs, undesired side reactions are likely to occur at the electrode interface, deteriorating the luminance stability. In addition, any nonideality of the electrode surface (including surface roughness and the pinholes) results in the current flux associated with the


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redox reaction being distributed nonuniformly over the electrode and concentrated at the defects, causing local thermal stress.19 Herein, we attempted to drive ECLDs with pulsed input signals that comprise an active period and a resting period, referred to as pulsed driving methods, inspired from the classical strategy in attaining uniform electrochemical electrode reactions.20-21 Exploiting the old wisdom to this new type of light emitting device resulted in enhancement in its operational stability. Specifically, for this study, ECLDs based on the bis(2-phenylquinoline) iridium(III) picolinic acid ((pq)2Ir(pico)) luminophore were fabricated. A comparative study on an electrochemical cell operated with different types of pulsed signal (voltage-controlled and current-controlled pulses) at different frequencies, which has been rarely studied explicitly in the field. Driving the devices with a pulsed current rather than with a pulsed voltage was found to be more effective in enhancing the stability and lifetime of the ECLDs. This is because the redox reactions occur more evenly over a rough electrode surface when the density of injected charges at the electrode is controlled rather than the electric potential. The study not only suggests an optimal method for operating ECLDs, but it also reveals fundamental aspects for the operation of a new class of light-emitting devices.

2.

EXPERIMENTAL

2.1 Reagents and Materials 2-Chloroquinoline (99%), phenylboronic acid (for HPLC, ≥97.0%), potassium carbonate (K2CO3; ACS reagent, ≥99.0%), dichloromethance (DCM; for EP, ≥99.5%) tetrahydrofuran (THF; for HPLC, ≥99.9%), methanol (MeOH; for HPLC, ≥99.9%), 2-methoxyethanol (99%), picolinic acid (≥99.0%), sodium carbonate (Na2CO3; ACS reagent, ≥99.5%), 2-ethoxyethanol (99%), tetrabutylammonium hexafluorophosphate (TBAPF6; 98%), acetonitrile (anhydrous,


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99.8%), lithium perchlorate (LiClO4; 99.99%), and propylene carbonate (PC; anhydrous, 99.7%) were purchased from Sigma-Aldrich. Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4; 99%) and iridium(III) chloride (99.8%) were purchased from Alfa Aesar. All the purchased chemical reagents were used without further purification. Glass substrates patterned with indium tin oxide (ITO) transparent electrodes (10 Ω/sq) were purchased from SNI Co., Ltd.

2.2 Synthesis of (pq)2Ir(pico) Synthesis of 2-phenylquinoline. A mixture of 2-chloroquinoline (609 mg, 3.72 mmol), phenylboronic acid (1168 mg, 4.09 mmol), K2CO3 (1.54 g, 11.16 mmol), and Pd(PPh3)4 (0.13 g, 0.11 mmol) in THF (15 mL) and MeOH (15 mL) was refluxed for 16 h. After cooling to room temperature, the reaction mixture was extracted with dichloromethane, and the organic layer was washed with brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel using dichloromethane and hexane (1:3 v/v) to give a white powder of 2-phenylquinoline (590 mg, 86% yield). 1H NMR (300 MHz, CDCl3) δ 8.42 (d, 1H, J = 2.9 Hz), 8.03–7.90 (m, 2H), 7.69 (d, 1H, J = 8.7 Hz), 7.56–7.33 (m, 3H), 7.30 (dd, 1H, J = 8.7, 2.9 Hz), 3.92 (s, 3H). Synthesis of (pq)2Ir(µ-Cl)2Ir(pq)2 dimer. A mixture of 2-phenylquinoline (509 mg, 2.75 mmol) and iridium(III) chloride hydrate (328 mg, 1.10 mmol) in 2-ethoxyethanol (45 mL) and H2O (15 mL) was refluxed for 24 h.22 After cooling to room temperature, water (100 mL) was added to the reaction mixture. The mixture was stirred for 30 min at room temperature to obtain a yellow precipitate, which was then washed many times with water and dried under an IR lamp for 12 h to give (pq)2Ir(µ-Cl)2Ir(pq)2 dimer (500 mg, 76% yield). 1H NMR (300 MHz, DMSOd6) δ 9.76 (d, 2H, J = 2.5 Hz), 9.28 (d, 2H, J = 2.7 Hz), 8.20 (d, 2H, J = 9.1 Hz), 8.09 (d, 2H, J =


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9.1 Hz), 7.86–7.51 (m, 8H), 6.95–6.51 (m, 8H), 6.24 (d, 2H, J = 7.4 Hz), 5.64 (d, 2H, J = 7.4 Hz), 3.95 (d, 12H, J = 8.4 Hz). Synthesis of 2-phenylquinoline iridium(III) picolinic acid [(pq)2Ir(pico)]. A mixture of (pq)2Ir(µ-Cl)2Ir(pq)2 dimer (100 mg, 0.16 mmol), Na2CO3 (167 mg, 1.58 mmol), and picolinic acid (486 mg, 3.95 mmol) in 2-ethoxyethanol (10 mL) was refluxed for 16 h. After cooling to room temperature, the reaction mixture was concentrated under vacuum and extracted with dichloromethane. The organic phase was washed with water and brine, and dried over anhydrous Na2SO4. Volatiles were evaporated and the resulting crude product was purified by column chromatography on silica gel using dichloromethane to give a red powder of (pq)2Ir(pico) (42.6 mg, 75% yield). 1H NMR (300 MHz, DMSO-d6) δ 8.64 (d, 1H, J = 8.2 Hz), 8.58 (d, 1H, J = 8.7 Hz), 8.49 (m, 3H), 8.23 (d, 1H, J = 8.2 Hz), 8.07 (d, 1H, J = 7.9 Hz), 8.01 (d, 1H, J = 8.1 Hz), 7.95 (d, 1H, J = 7.6 Hz), 7.86 (t, 2H, J = 12.6 Hz), 7.61 (d, 1H, J = 8.2 Hz), 7.56 (d, 1H, J = 6.2 Hz), 7.50 (t, 2H, J = 13.6 Hz), 7.42 (t, 1H, J = 15.7 Hz), 7.23 (d, 1H, J = 8.8 Hz), 7.01 (m, 3H), 6.74 (t, 1H, J = 14.4 Hz), 6.63 (t, 2H, J = 16.3 Hz), 6.12 (d, 1H, J = 7.6 Hz),

2.3 ECLD Fabrication and Characterization ECLDs were fabricated through the following process under ambient conditions. First, glass substrates (2.5 cm × 1.5 cm) with ITO electrodes were ultrasonicated in trichloroethylene for 15 min and in acetone for another 15 min, and dried by N2 gas blowing. The substrates were then cleaned with UV-ozone (AC-6, Ahtech LTS) for 40 min. Thermal tape (Solaronix Co.) was applied to three edges of the ITO/glass substrate, which served as a spacer (60 µm), whereas no tape was applied to the remaining edge so that an ECL solution could be inserted through the opening. The resulting sandwiched substrates were heated at 150 °C for 10 min on a hot plate.


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Separately, a series of ECL solutions was prepared by dissolving 10 mg of (pq)2Ir (pico) in 1 mL of PC with different concentrations of LiClO4 (0, 50, 100, 300, 500, and 700 mM), followed by vigorous stirring for 12 h. Subsequently, 0.1 mL of the resulting ECL solution was injected between the sandwiched substrates through the opening. Electroluminescence from the ECLDs was analyzed under various operational conditions; static or pulsed (0.1–100 Hz) input of either voltage or current was applied to the devices using a potentiostat (CHI 650B). The intensity of the resulting ECL was collected using a low-voltage photomultiplier tube (PMT) module (H-6780, Hamamatsu) operated at 1.0 V. The absolute luminance of the emission intensity in cd/m2 was calibrated by comparison with the luminance intensity from a commercial red LED (λ = 660 nm) using the PMT and a PhotoResearch PR 655 spectroraidometer.

2.4 Electrochemical Analysis Electrochemical analysis of the resulting (pq)2Ir(pico) solutions was performed using a CH Instruments 650B Electrochemical Analyzer (CH Instruments, Inc.). Conventional cyclic voltammetry (CV) and chronocoulometry (CC) of the (pq)2Ir(pico) solutions was used to investigate the basic electrochemical redox properties of the luminophore. The electrochemical measurements were conducted using a Ag/Ag+ (3 M AgNO3) reference electrode. A Pt wire was used as the counter electrode and a planar Pt electrode (diameter = 3 mm) was used as the working electrode. Before measurements, the working electrode was polished with 0.05 M alumina (Buehler) on a felt pad following sonication in a 1:1 mixed solution of deionized water and absolute ethanol for 5 min. Then, the resulting electrode was blown with N2 gas for 1 min.


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Prior to the experiments, all the electrochemical solutions were purged with ultrapure N2 for at least 15 min. An additional CV experiment was conducted using a Pt ultramicroelectrode (Pt UME, radius = 25 µm) to investigate the diffusion coefficient (Do) of (pq)2Ir(pico) in solution with different concentrations of electrolyte. When a disk-shaped UME is used, Do for (pq)2Ir(pico) can be obtained from the limiting current (Ilim) of the linear sweep voltammogram using the following equation.23 Ilim = 4nFrC*Do

(1)

where F is the Faraday constant, r is the electrode radius, and C* is the bulk concentration of the electroactive species.

2.5 Calculation of Potential Difference The effective potential difference between the electrode and the bulk ECL solution was estimated according a method developed by Kobayashi et al.10 when a voltage-controlled pulsed signal was applied. First, a low voltage of 1 V was applied to a reference cell containing only the electrolyte (without the ECL luminophore) and the current through the cell was monitored. Because the operation voltage is sufficiently low, the cell current of the cell under such conditions can be assumed to be non-faradaic in origin. Therefore, by integrating the current signal collected over time, the charge density accumulated at 1 V upon forming the electric double layer was calculated (4.41 µC/cm2). Multiplying the integrated value by 2.5 allowed estimation of the charge density that should be obtained at 2.5 V (11.01 µC/cm2). This value was set as the reference charge density accumulated at 2.5 V. Comparison of this reference value with the actual charge density estimated by integrating the current signal of a pulse-driven ECLD


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over a given time yielded the relative ratio between the effective potential applied to the cell and the reference potential of 2.5 V. In contrast, when a current-controlled pulsed signal was applied to the cell, the potentiostat used to apply the input current signal was used to record the effective potential of the cell directly.

3.

RESULTS AND DISCUSSION

Scheme 1. Schematic descriptions of ECLD operation by a continuous driving method (a) and a pulsed driving method (b). The ECL luminophores within the reaction volumes near the electrodes would eventually be depleted under the continuous application of electrical potential. In contrast, pulsed driving of the ECLD with a resting period allows the reaction volumes to be replenished with ECL luminophores.

Scheme 1a describes the basic operation principle of an ECLD based on the annihilation pathway and the associated issues arising from the sluggish motions of ECL luminophores in the cell. When an electrical voltage is applied between the two electrodes, electric double layers are


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formed at the two electrode/solution interfaces, and then charge-transfer reactions of the ECL luminophores occur at the interfaces (reduction at the cathode and oxidation at the anode). For the resulting reduced form (blue spheres, Scheme 1a) and oxidized form (red spheres, Scheme 1a) of the ECL luminophore to encounter each other, they must diffuse/migrate to the opposite electrode. If the diffusivity/mobility of the ECL luminophore is low, not only are encounters between the reduced and oxidized forms of the ECL luminophore delayed, but also the replenishment of pristine ECL luminophores (grey spheres, Scheme 1a; the redox active species at the respective electrodes) within the effective reaction volume near the electrode surface will occur slowly. As the voltage is continuously applied between the electrodes over the entire period of operation, the shortage of reactants at the surface of each electrode increases the chance of undesired side reactions, which eventually degrades the stability of the cells over time. We hypothesized that the application of a pulsed input signal comprising a series of active periods (t1 < t < t2) and resting periods (t2 < t < t3) can help enhance the stability of ECLDs. The insertion of a resting period between active periods provides time for the reaction zone to be replenished with fresh ECL luminophores and thus, suppresses undesired side reactions (Scheme 1b). Such a method is a well-established strategy for preparing a uniform film on a substrate (electrode) through electrolytic deposition.20,

24-26

The strategy can be applied to resolve

problems in this new class of light emitting devices that also rely on a series of electrochemical reactions. The optimal resting period will depend on the various kinetic processes involved in the ECL process, including electric double layer formation, delivery of the reactants, chargetransfer reactions at the interface, and removal of the products from the electrode. The resulting device performance will also be influenced by whether the pulsed input signal is applied through controlled voltage (i.e., pulsed-voltage method) or through controlled current (i.e., pulsed-current


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method) because redox reactions on the rough surface of the electrodes may undergo different kinetic pathways. Therefore, the optimal operating conditions yielding enhanced stability of ECLDs should be determined.

Figure 1. (a) Synthetic procedure for preparing (pq)2Ir(pico). (b) Photoluminescence emission spectrum of (pq)2Ir(pico) (10 µM in acetonitrile, λmax = 571 nm). (c) Cyclic voltammogram of (pq)2Ir(pico) (2 mM in acetonitrile, 0.1 M TBAPF6 supporting electrolyte). Scan rate = 100 mV/s.

Figure 1a shows the chemical structure of (pq)2Ir(pico) and its synthetic scheme, which was modified from the literature.27 Briefly, cyclometalated iridium dimers (pq)2Ir(µ-Cl)2Ir(pq)2 were synthesized by refluxing IrCl3·xH2O with 2-phenylquinoline. By refluxing a solution of (pq)2Ir(µ-Cl)2Ir(pq)2 in 2-ethoxyethanol with an excess amount of picolinic acid, orange-colored


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(pq)2Ir(pico) was obtained. This luminophore is known to yield a photoluminescence (PL) quantum yield (ΦPL = 0.27) larger than that of the benchmark tris(bipyridine)ruthenium(II) (Ru(bpy)32+, ΦPL = 0.05). Figure 1b shows the PL spectrum of (pq)2Ir(pico) in solution, yielding orange-red emission (λPL = 571 nm corresponding to HOMO-LUMO energy gap of 2.17 eV). More importantly, the ECL quantum yield (ΦECL) of (pq)2Ir(pico) can reach up to 0.88, which is 18 times larger than that of Ru(bpy)32+.27 The high annihilation ΦECL of (pq)2Ir(pico) is attributed to its stable and reversible oxidation and reduction processes.27 Figure 1c shows a cyclic voltammogram of (pq)2Ir(pico) collected at a scan rate of 100 mV/s. The onset of the (pq)2Ir(pico) oxidation reaction occurred at 0.62 V ((pq)2Ir(pico) → (pq)2Ir(pico)+ + e-), whereas the onset of the reduction reaction occurred at -1.90 V ((pq)2Ir(pico) + e- → (pq)2Ir(pico)-). The half-wave potentials for oxidation and reduction of (pq)2Ir(pico) were 0.70 and -2.00 V (vs. Ag/Ag+), respectively, and both the electrochemical processes were reversible at a scan rate of 100 mV/s. ECLDs were fabricated by sandwiching a solution containing (pq)2Ir(pico) and LiClO4 in PC between two glass substrates with transparent ITO contacts. First, we compared the luminescence performance of ECLDs that were operated using continuous and pulsed voltages with a magnitude of 2.50 V. The magnitude of the applied voltage corresponds to the offset between the onset potentials for the oxidation and reduction processes of (pq)2Ir(pico) obtained from the cyclic voltammogram (∆E = Eox(onset) – Ered(onset) = 0.62 – (–1.90) = 2.52 V). Thus, it should be sufficient to generate electrochemical redox reactions at the respective electrodes while suppressing the application of an undesired overpotential to the cell. Under an applied voltage of 2.5 V, the internal energy of the cell can be as high as 2.34 eV (–∆H = –∆G – T∆S =


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2.50 – 0.16 = 2.34 eV),3, 28-31 which is energetically sufficient to generate triplet excitation of (pq)2Ir(pico) (2.17 eV).

Figure 2. (a) Temporal luminance response of ECLDs under the application of constant and pulsed (60 Hz) voltages. (b) Temporal luminance responses of pulse-driven ECLDs with various operational frequencies (1, 30, 60, and 100 Hz). (c) Typical current profile of an ECLD under the application of a pulsed-voltage signal (2.5 V at 60 Hz). The inset displays data over an extended period of time. (d) Potential difference between the electrode and the bulk ECL solution induced during the application of pulsed voltages at various operational frequencies.

Figure 2a shows temporal luminance profiles for ECLDs under application of constant (black) and pulsed (orange at 60 Hz) inputs of voltages. The application of a constant voltage yielded an abrupt enhancement in the emission of the device, followed by rapid degradation. In contrast, ECLDs operated with a pulsed voltage yielded more stable and long-lasting luminance. Under the application of a constant voltage, the concentration of (pq)2Ir(pico) near the electrode will be


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depleted quickly, unless (pq)2Ir(pico) from the bulk solution can be delivered to the electrode promptly. We conjecture that the depletion of (pq)2Ir(pico) near the electrode leads to undesired secondary reactions at the electrode, which eventually degrade the light emission and stability of the device. In contrast, when the resting period is provided during the pulsed-operated mode, fresh (pq)2Ir(pico) can replenish the effective reaction volume near the electrode. As the chargetransfer process involving both oxidation and reduction of the luminophore is the most favorable reaction at the electrode, the replenishment of (pq)2Ir(pico) suppresses undesired secondary reactions and enhances the stability of the luminance from the device. Figure 2b shows the luminance profiles of ECLDs operated at various pulse frequencies (1– 100 Hz, 50% duty cycle). The associated current profiles of the devices are plotted in Figure 2c and Figure S1. Note that finite current is observed, even when 0 V is applied to the cell during the resting period. This finite current is referred here as the reverse current. Interestingly, the magnitude and the shape of the reverse current is comparable to the current level observed during the active period. The result indicates that oxidation and reaction of the luminophores also occur during the resting period of operation, just as much as they occur during the active period. Consequently, ECL would still be obtained during the resting period, and rather a small fluctuation in the ECL intensity would be observed from ECLDs under the application of a pulsed voltage.12 Note that the suppression in the fluctuation of ECL intensity is not significant, when a current-controlled pulsed input signal is applied, as shown below. The stability of the luminance improved as the frequency increased; the luminance intensity was reduced rapidly to 27.3% of the maximum emission at 1 Hz, but 94.2% of the maximum emission was maintained when operated at 100 Hz. The enhanced stability at higher pulse frequencies can be attributed to the two factors, both of which suppress undesired side reaction at


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the electrode. First, the application of a shorter active period to the ECL layer under higher pulse frequencies only provides time for the most kinetically favored reaction (i.e., oxidation and reduction of the ECL luminophore) to occur selectively at the electrode. Second, the effective potential at the electrode is reduced at higher frequencies, which helps reduce the rate of unintended reactions at the electrode. Because ionic motion cannot trace the short transient of the electric field at high frequencies, electric double layer formation would be incomplete during each pulse cycle at high frequencies and thus, the effective potential induced by the electric double layer during the pulsed voltage would be reduced. This scenario can be supported by measuring the potential difference induced at the interface of the electrodes and the solution in the ECLD at various frequencies of pulsed voltages, according to the method suggested by Kobayashi et al.10 As shown in Figure 2d, the potential difference at the interface of the electrodes and the solution decreases as the frequency of the pulsed voltage increases. When a 2.50 V square wave pulse was applied at 0.1 Hz, a difference of 2.49 V was measured during a half-cycle. However, at higher frequencies, e.g., 100 Hz, the measured potential difference was only 1.90 V. Although the estimated potential difference may not be the true potential at the interfaces, this result qualitatively supports that the application of a pulsed voltage at high frequencies suppresses unnecessary overvoltages and thus, enhances device stability. Based on the combinatorial influences of these two factors, a half-life of 1300 s can be obtained for ECLDs operated with pulsed-voltage input at 60 Hz. However, the enhanced stability of the device at high pulse frequencies was accompanied by a decrease in the overall luminance. When the rate of electric double layer formation is limited, the resulting reduction in the effective potential at the electrode not only suppresses undesired side reactions, which is critical for enhancing the stability, but inevitably, the rates of the intended reduction and oxidation reactions


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at the electrode are also suppressed. Based on Figure 2d, the effective potential of 1.9 V at 100 Hz is lower than the energy required for triplet excitation of (pq)2Ir(pico).

Figure 3. (a) Temporal luminance responses of pulsed-voltage-driven ECLDs (2.5 V, 60 Hz) with various concentrations of the supporting electrolyte (LiCiO4, 50–700 mM). (b) Linear sweep voltammograms of (pq)2Ir(pico) in various concentrations of the supporting electrolyte (LiCiO4). (c) Summary of diffusion coefficients extracted from the linear sweep voltammograms. (d) Temporal luminance response of pulsed-voltage-driven ECLDs (2.5 V, 60 Hz) with 300 mM of the supporting electrolyte (LiCiO4) obtained over an extended period of time.

In this scenario, the luminance of the ECLD may be enhanced by retaining the effective potential drop at higher frequencies. We hypothesized that this effect can be achieved by increasing the concentration of the supporting electrolyte, which should facilitate the formation of an electric double layer. Figure 3a shows the luminance profiles of ECLDs fabricated with


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various concentrations (50–700 mM) of the supporting electrolyte (LiClO4). For these measurements, the pulse frequency was fixed at 60 Hz (2.5 V), yielding stable performance (Figure 3a). The ECLDs exhibited an increase in luminance with increasing LiClO4 concentrations at concentrations less than 300 mM. However, the experimental results did not follow this trend at LiClO4 concentrations greater than 300 mM. Instead, LiClO4 concentrations of more than 300 mM obstruct the mass transport of (pq)2Ir(pico), likely owing to changes in the viscosity of the solution.32 This behavior was supported by directly estimating the diffusion coefficient (Do) of (pq)2Ir(pico) in solution by carrying out CV using a UME as the working electrode; the Do values were obtained from the limiting current values of the linear sweep voltammograms (Figure 3b). Figure 3c summarizes Do values for (pq)2Ir(pico) in different concentrations of LiClO4. The decrease in Do with increasing LiClO4 concentrations indicates that the viscosity of the ECL solution increases. The increase in the viscosity of the ECL solutions confirmed directly using a viscometer is plotted Figure S2. Overall, the results indicate that increasing the concentration of the supporting electrolyte does make positive influence on obtaining stronger luminance from ECLDs. However, the benefits are effective only when the addition of salts does not increase the viscosity of the solution substantially, since the operation of ECLD relies on direct transport of the luminophore. Using the optimal LiClO4 concentration (300 mM), the pulsed-voltage-driven ECLD yielded a half-life of 1300 s when the device was operated with pulsed-voltage (2.5 V) input at 60 Hz (Figure 3d).


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Scheme 2. Schematic descriptions of ECLD operation under (a) the voltage-controlled method and (b) the current-controlled method.

Application of an electrical input to an electrochemical cell by the current-control method is expected to yield a different outcome than that achieved by the voltage-control method. When the input signal is applied using the voltage-controlled method, the current density on the electrode is not evenly distributed over the entire electrode owing to the finite roughness of the electrode surface; the current density is mostly focused on the edges of protruded regions on the rough surface (Scheme 2a). Such local variation in the current density on the electrode surface causes local thermal stress on the electrode surface, which in turn facilitates unintended side reactions at the regions of highest thermal stress. Consequently, the stability of the electrochemical cell deteriorates. When the input signal is applied using the current-controlled method (so that the number—but not the potential—of charges is controlled), the electric potential, which is determined by the local concentration of electrochemically active species according to the Nernst equation, is not distributed evenly over the rough electrode surface. Under such conditions, any locations with high electric potentials result in faster charge-transfer reactions at the electrode/solution interface and thus, the local concentration of electrochemically active species decreases, which in turn lowers the local electrical potential (Scheme 2b).


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Following this feedback loop, the local variation in electric potential evens out. Consequently, stability issues arising from local thermal stress can be alleviated. We note that the currentcontrolled method of applying input signals to an electrochemical cell has been widely used in electrodeposition, i.e., galvanic electrodepostion.20,

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The thicknesses of films with excellent

uniformity can be controlled easily through galvanic electrodeposition.

Figure 4. (a) Temporal luminance responses of pulsed-current-driven ECLDs at various operational frequencies (100 µA at 0.1, 1, 5, 10, and 60 Hz). (b) Typical transient profile of the electric potential of a pulsed-current-driven ECLD (10 Hz). The inset shows the transient profile plotted over an extended period of time. (c) Potential difference between the electrode and the ECL solution induced during the application of a pulsed current at various operational frequencies. (d) Comparison of temporal luminance response of a pulsed-voltage-driven ECLD (2.5 V, 60 Hz) and a pulsed-current-driven ECLD (100 µA, 10 Hz). .


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Figure 4a shows the temporal luminance profiles of ECLDs operated under the pulsed input of current at various frequencies (0.1–60 Hz). The magnitude of the current pulse was 100 µA, which corresponds to the saturated current level of the device operated at a constant voltage of 2.5 V from the previous experiment. Interestingly, the luminance characteristics of the pulsedcurrent-driven ECLDs appeared different from those of the pulsed-voltage-driven devices. First, the intensity of ECL emission under pulsed-current operation exhibited pronounced fluctuations; a higher emission intensity was obtained when entering the active period and a lower emission intensity was obtained when entering the resting period. The difference between the two stages can be understood from the nature of the current-controlled pulse input signal. Unlike the voltage-controlled pulse, the charge injection should strictly be zero during the resting period of the current-controlled pulse, because the input current is zero during this period. Despite the finite effective potential applied during the resting period comparable to that during the active period, as shown in Figure 4b and Figure S3, the fluctuation in the ECL intensity is substantial. Second, a different frequency dependence was observed for the luminance of the ECLDs driven by a pulsed current. When the frequency of the applied pulse increased from 0.1 to 10 Hz, the luminance of pulsed-current-driven ECLDs increased. The increase in luminance with the increasing operational frequency of the pulsed input results from the reduced duration of the active period at higher frequencies, which allows only the kinetically favorable redox reactions to occur at the electrodes. However, pulse frequencies greater than 10 Hz degraded the luminance rapidly, such that luminance was hardly observable at >100 Hz. Similar to the explanation given above, slow formation of the electric double layer prevented the application of sufficiently high potentials at the electrodes, and therefore, the rate of the intended redox reactions at the electrodes, as well as the resulting emission intensity, was suppressed at high


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pulse frequencies. Figure 4c shows the potential difference between the electrodes and the bulk solution measured during the application of a pulsed current. The potential induced by pulsedcurrent operation was 2.33 V at 0.1 Hz, which is energetically sufficient for triplet excitation of (pq)2Ir(pico) (2.17 eV). Increasing the frequency gradually decreased the potential, which eventually become smaller than the energy necessary to excite (pq)2Ir(pico). Finally, the stability of pulsed-current-driven ECLDs was superior to that of pulsed-voltage-driven ECLDs. Figure 4d compares the transient luminance profiles of ECLDs operated with optimal pulsed-current (100 µA at 10 Hz) and pulsed-voltage (2.5 V at 60 Hz) driving methods. The half-life of the luminance was 3600 s (1 h) when the device was operated with pulsed-current input, about 2.3 times longer than that achieved with pulsed-voltage input.

4.

CONCLUSION The influence of different driving methods on the operation of ECLDs was examined.

Operating the device by pulsing the input signal helped enhance the device stability compared with that achieved by the continuous application of potential. We attribute the enhancement in stability to the insertion of a resting period during operation, which helps replenish the concentration of ECL luminophores near the electrodes that could be depleted in devices operated under the continuous application of potential. We also examined the influence of the control mode of the pulsed signal explicitly (voltage-controlled or current-controlled pulsed signal) on operating the device. Owing to the smoothing effect of distributed local potentials realized under current-controlled operation, enhanced device stability was obtained using pulsedcurrent driving methods. We believe that the fundamental understanding gained from this work


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will provide basic guidelines for developing optimal device operation methods for ECLDs with enhanced stability.

ASSOCIATED CONTENT Supporting Information Profiles of ECLDs operated with various controlled-voltage or controlled-current pulses. Additional supplementary figures are available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Technology Innovation Program (10051665) funded by the Ministry of Trade, Industry & Energy, Korea and the Korea Display Research Corporation (KDRC) for the development of future device technology for the display industry.


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TABLE OF CONTENTS


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Pulsed Driving Methods for Enhancing the Stability of

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