DC-Driven, Sub-2 V Solid-State Electrochemiluminescent Devices by

Article pubs.acs.org/cm

DC-Driven, Sub‑2 V Solid-State Electrochemiluminescent Devices by Incorporating Redox Coreactants into Emissive Ion Gels Hong Chul Moon,† Timothy P. Lodge,*,†,‡ and C. Daniel Frisbie*,† †

Department of Chemical Engineering & Materials Science University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ABSTRACT: We demonstrate a DC-driven solid-state electrochemiluminescent (ECL) device based on a solution-processable ion gel containing luminescent complexes and tetrabutylammonium oxalate (TBAOX) as a coreactant. The ECL gel, comprising a polystyrene-block-poly(methyl methacrylate)-block-polystyrene copolymer, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ionic liquid, tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate (Ru(bpy)3(PF6)2) and TBAOX, was cast as a film on indium tin oxide-coated glass (anode) by spin-coating, and a top electrode (cathode) was prepared by brush-painting Ag paint. Application of 1.6 V DC bias across the device resulted in the onset of light emission. The maximum luminance was achieved at 1:5 mol ratio of Ru(bpy)3(PF6)2 and TBAOX, and the turn-on voltage was independent of the composition. The simplicity of the ECL device and its low voltage operation characteristics make it potentially attractive as a display element for printed electronic circuits.

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Ru(bpy)32 + + e− → Ru(bpy)31 +

INTRODUCTION

(1)

Polymer gel electrolytes (PGEs) are promising materials for diverse applications such as gas separation membranes,1−3 ion conducting media in electrochromic devices,4,5 batteries,6,7 electrochemical transistors,8−21 and light-emitting devices.22,23 In particular, ion gels comprising room temperature ionic liquids (ILs) and ABA triblock copolymers with IL insoluble A blocks and a IL soluble B block are attractive PGE materials owing to their outstanding ionic conductivity, high capacitance, negligible vapor pressure, and good solution processability.24−26 Very recently, we have demonstrated solid-state electrochemiluminescent (ECL) light-emitting devices based on ECL luminophores such as the well-known Ru(II)-complex (Ru(bpy)3Cl2) added to ion gels consisting of polystyrene-blockpoly(methyl methacrylate)-block-polystyrene (SMS) and 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]).22 In contrast to conventional liquid-based ECL systems,27,28 we designed a rubbery ECL gel so that we could fabricate flexible, emissive devices by a simple two-step solution process. To operate the devices, we applied AC voltages to generate both Ru(bpy)31+ (reduced, by reaction 1) and Ru(bpy)33+ (oxidized, by reaction 2) near one single electrode. The spontaneous electron transfer reaction 4 (Eorxn > 0, ΔG = −nFEorxn < 0) consisting of two half-reactions (reactions 1 and 3) occurred by overlap of the concentration profiles of both redox species. As a result, the excited molecule Ru(bpy)32+,* was formed, which then emitted red-orange light (reaction 5). © 2014 American Chemical Society

E o red = −1.40 V vs SCE29

Ru(bpy)33 + + e− → Ru(bpy)32 +

E o red = +1.20 V vs SCE29 (2)

Ru(bpy)33 + + e− → Ru(bpy)32+, *

E o red ≈ −0.84 V vs SCE30 (3)

Ru(bpy)31 + + Ru(bpy)33 + →Ru(bpy)32+, * + Ru(bpy)32 + E o rxn ≈ + 0.56 V

Ru(bpy)32+, * → Ru(bpy)32 + + hν (2.04 eV ∼ 610 nm)

(4) (5)

Importantly, the development of an ECL device utilizing DC voltages is potentially more practical for printed electronics applications where DC power will be supplied by thin film batteries. Although there are a few reports on ECL in liquid cells with DC bias,31−33 DC-driven solid-state ECL devices based on ion gels have not been reported yet. Two strategies to solid-state ECL devices may be proposed: (1) the use of an extremely thin ion gel layer, or (2) the incorporation of a redox coreactant into the gel. In the first strategy, the oxidized and reduced species are generated at the anode and the cathode, respectively, when a DC voltage is applied. In order to generate the excited luminophores by reactions 1−5 (annihilation Received: July 8, 2014 Revised: August 30, 2014 Published: September 1, 2014 5358

dx.doi.org/10.1021/cm502491n | Chem. Mater. 2014, 26, 5358−5364

Chemistry of Materials

Article

Scheme 1. Synthetic Route for the Preparation of Tetrabutylammonium Oxalate (TBAOX) Soluble in Ion Gels

Figure 1. (a) Scheme of the fabrication of the DC-driven ECL device by a two-step solution process. (b) Chemical structures for all components in emissive ECL gels. The ECL gel contains 1:3:4:16 (wt %) of Ru(bpy)3(PF6)2, TBAOX, SMS (17k-86k-17k) and [EMI][TFSI].

the normal Ru(bpy)32+ (reaction 9) or oxidized Ru(bpy)33+ (reaction 10) near the anode spontaneously (Eorxn > 0, ΔG < 0), giving the reduced Ru(bpy)31+ or excited Ru(bpy)32+,*, respectively.34 In addition to the direct formation of Ru(bpy)32+,* by the coreactant pathway reaction 10, the generation of Ru(bpy)31+ allows the annihilation pathway (reaction 4) as well.

pathway), the oxidized and reduced species must diffuse from their respective electrodes to the bulk according to the concentration gradient. A thin gel is thus required to keep the necessary diffusion lengths small so that the fluxes remain large and the brightness of the device is enhanced. Although a thin ion gel layer (e.g., tens of nanometers thick) can be prepared by spin coating, the deposition of a top electrode using either brush-painting of Ag-paint or physical vapor deposition is very difficult, because the ion gel is soft and penetrated by the metal, resulting in electrical shorts. On the other hand, use of a redox coreactant (the second strategy) has two significant benefits. First, coproduction of both reduced and oxidized species becomes achievable near a single electrode using DC voltages, thus eliminating the need to use an extremely thin ion gel layer and AC voltages. For example, if oxalate (C2O42−) is employed as a coreactant in the Ru(bpy)32+ solution system, the intermediate CO2•−, derived from the oxidation of C2O42− (reaction 6) followed by the decomposition of the resulting C2O4•− (reaction 7), reacts with

C2O4•− + e− → C2O4 2 −

E o red = + 0.30 V vs SCE35

(6)

C2O4•− → CO2•− + CO2

(7)

CO2 + e− → CO2•−

(8)

E o red = − 2.20 V vs SCE36

Ru(bpy)32 + + CO2•− → Ru(bpy)31 + + CO2

E o rxn = +0.80 V (9)

Ru(bpy)33 + + CO2•− → Ru(bpy)32+, * + CO2 E o rxn ≈ + 1.36 V 5359

(10)



Article

Ru(bpy)3(PF6)2 instead of Ru(bpy)3Cl2 due to its higher solubility in the ionic liquid [EMI][TFSI]. Using this scheme, the ECL gel was successfully cast on the patterned ITO-coated glass (anode) as a film with a thickness of ∼35 μm, and a top electrode was deposited on the gel by brush-painting of Ag paint (cathode). Chemical structures for all components in the ECL gel are given in Figure 1b. Figure 2 displays the emission spectrum from the ECL device at +2.0 V. The device emitted red-orange light with λmax = 610

Second, low voltage operation becomes possible with the incorporation of a coreactant. For the standard annihilation pathway (reaction 4), both reduced and oxidized species are necessary to make the excited Ru(bpy)32+,*. According to the Eored values of reactions 1 and 2, at least 2.60 V must be applied for simultaneous production of both reduced and oxidized luminophores, Ru(bpy)31+ and Ru(bpy)33+, without a coreactant. On the other hand, just +1.20 V (vs SCE) is sufficient for the Ru(bpy)32+/oxalate system, because the voltage for Ru(bpy)32+ oxidation (+1.20 V vs SCE) is high enough for oxidation of oxalate (+0.30 V vs SCE)35 and creation of CO2•−. In other words, once CO2•− is formed at +0.30 V (vs SCE), the reduced Ru(bpy)31+ can be generated by the energetically favorable reaction 9. Then, when Ru(bpy)33+ is formed at +1.20 V (vs SCE), Ru(bpy)32+,* can be produced by either the annihilation pathway (reaction 4) or coreactant pathway (reaction 10), resulting in a lower device operating voltage, in principle as low as 1.20 V. Oxalate can be viewed as a consumable fuel for the device, providing reducing power and cutting the overall operating voltage approximately in half. Here we have employed the coreactant strategy to demonstrate a solid-state DC-driven ECL device based on a solution processable, emissive ECL gel. We chose oxalate (C2O42−) as a coreactant for the reason described above, namely the highly negative reduction potential (Eored) of CO2•− formed upon oxidation of oxalate at a low voltage.35−41 One challenge is that commercially available oxalates such as oxalic acid, sodium oxalate and ammonium oxalate are insoluble in ion gels, so we substituted the hydrogens in oxalic acid with tetrabutylammonium groups.36,37 The resulting tetrabutylammonium oxalate (TBAOX) is soluble in ion gels (Scheme 1) and can be used as a coreactant in our ion gel system. As a result, we were able to prepare a homogeneous ECL gel comprising a 1:3:4:16 (wt %) blend of tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate (Ru(bpy) 3 (PF 6 ) 2 ), TBAOX, polystyrene-block-poly(methyl methacrylate)-blockpolystyrene (SMS) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) on patterned ITO-coated glass (anode). A top electrode (cathode) was deposited on the gel using brush-painting of Ag paint. The resulting ECL device turned on and emitted red-orange light (λmax = 610 nm) at only +1.60 V. When the mole ratio of Ru(bpy)3(PF6)2 to TBAOX was varied with a fixed amount of Ru(bpy)3(PF6)2, the luminance increased as the amount of TBAOX increased, reaching saturation at a 1:5 mol ratio of Ru(bpy)3(PF6)2 and TBAOX. On the other hand, the turn-on voltage was essentially independent (1.60 V ± 0.04 V) of the gel composition. Overall, this work demonstrates a convenient strategy for obtaining a simple, low voltage, DC-driven, emissive device that is compatible with solution processing or printing.

Figure 2. Emission spectrum of the ECL device in which λmax = 610 nm corresponding to the emission from the 3MLCT excited state of Ru(bpy)32+. Inset is a photograph of the ECL device in ON-state at +2.0 V.

nm (2.04 eV) corresponding to the emission from the triplet metal-to-ligand charge transfer (3MLCT) excited state of Ru(bpy)32+,*,42 which is consistent with the process in ACdriven ECL devices based on Ru(bpy)3Cl2.22 The inset of Figure 2 shows the ECL device in its ON-state. The red-orange light was clearly observed within the area (ca. 2 mm × 2 mm) sandwiched between ITO (anode) and Ag (cathode) electrodes. Figure 3a depicts the current−voltage and current− luminance characteristics of the ECL device. In contrast to conventional organic light emitting diodes (OLEDs) or polymer light emitting diodes (PLEDs), the current density profile of the device indicated two distinct peaks at +1.29 V and +1.90 V as voltage increased. To understand the origin of these two peaks, we prepared a TBAOX only device without the Ru(II) complex. When we recorded its current−voltage characteristic (Figure 3b), only a single peak at +1.29 V was observed. Thus, we conclude that the peak at +1.29 V corresponds to the oxidation of TBAOX (reaction 6) and the peak at +1.90 V arises from the oxidation of Ru(bpy)3(PF6)2 (reaction 2). In addition, the luminance−voltage characteristic of the ECL device given in Figure 3a indicates that the device did not emit light before +1.60 V where Ru(bpy)31+ and CO2•− were both present. When the applied voltage reached +1.60 V, the oxidation of Ru(bpy)32+ (or preformed Ru(bpy)31+) to Ru(bpy)33+ began, and correspondingly electron transfer reactions between CO2•− and Ru(bpy)33+ (reaction 10) or between Ru(bpy)31+ and Ru(bpy)33+ commenced (reaction 4). Thus, at +1.6 V, Ru(bpy)32+,* was generated by a combination of the coreactant and the annihilation pathways. The turn-ON voltage was determined when the luminance reached 1 Cd/m2.

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RESULTS AND DISCUSSION Figure 1a outlines the fabrication process. One of the significant advantages of this DC-driven ECL device is facile fabrication by a two-step solution process because of its simple structure consisting of two electrodes sandwiching an emissive ECL gel.22 To prepare homogeneous ion gels by a solution process, all components including the coreactant must be soluble in a common solvent. Therefore, we synthesized TBAOX using a simple acid−base reaction between oxalic acid and tetrabutylammonium hydroxide (Scheme 1).36,37 For the redox luminophore, in contrast to previous work,22 we prepared 5360



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Figure 3. (a) Current−voltage (○) and luminance−voltage (■) characteristics of the ECL device. The composition of Ru(bpy)3(PF6)2:TBAOX:SMS:[EMI][TFSI] in the ECL gel is 1:3:4:16 (wt %). (b) Comparison of current−voltage characteristics of the devices with TBAOX only (●) and with both of TBAOX and Ru(bpy)3(PF6)2 (○).

Figure 4. (a) Current−voltage characteristics of the ECL device employed in Figure 3a at different scan numbers. The arrows point to the peak arising from the oxidation of TBAOX. The profiles of the 10th and the 20th scans are shifted vertically. (b) Luminance−voltage characteristics of the ECL device at different scan numbers. The dotted line corresponds to the luminance of 1 Cd/m2, a benchmark to determine the turn-on voltage.

The luminance was enhanced and eventually saturated as voltage increased further. We also estimated the quantum efficiency (η) of the ECL device by η = # of emitted photons/# of injected electrons, which gives η = 0.04% for the present ECL device. It is noted that such a low quantum efficiency is typical of ECL.43,44 To understand the device behavior in detail, we performed repeated voltage sweeps. Figure 4a displays the current density profiles for the ECL device at different scan numbers. Although the current profile for the second scan clearly showed two peaks corresponding to the oxidation of oxalate at +1.29 V and Ru(bpy)32+ at +1.90 V, the peak due to oxalate oxidation decreased and eventually disappeared as the scan number increased, implying that the oxalate species near the electrode were fully consumed. Namely, there was no generation of the intermediate CO2•− from the direct oxidation of oxalate corresponding to reactions 6 and 7. Nonetheless, the ECL device still turned on at +1.60 V and emitted light even after the voltage was swept 20 times (see Figure 4b). As oxalate is exhausted near the electrode, oxalate in the bulk diffuses toward the electrode. On the other hand, Ru(bpy)33+ diffuses from the electrode to the bulk due to the concentration gradient, and the concentration profiles of oxalate and Ru(bpy)33+ overlap.

Because Eored (+1.20 V vs SCE)29 for reaction 2 is higher than Eored (+0.30 V vs SCE)35 for reaction 6, electron transfer from oxalate to Ru(bpy)33+ (reaction 11) occurs spontaneously (Eorxn > 0, ΔG < 0). From the resulting C2O4•−, CO2•− can be formed by reaction 7. As a result, even though there is little oxalate directly oxidized near the electrode, the excited Ru(bpy)32+,* can still be produced and light emission is observed. Ru(bpy)33 + + C2O4 2 − → Ru(bpy)32 + + C2O4•− E o rxn = + 0.90 V

(11)

The overall reaction mechanisms occurring in the ECL gel are summarized in Figure 5. Two reaction routes for the production of the strong reducing agent, CO2•−, can be proposed: (1) direct oxidation of oxalate (reaction 6) near the electrode (Figure 5a) and (2) electron transfer reaction (reaction 11) between oxidized Ru(bpy)33+ and oxalate (Figure 5b). These two routes (Figures 5a and 5b) may contribute to the formation of CO2•− simultaneously. However, when the oxalate concentration near the electrode is high enough in the early stage, the mechanism shown in Figure 5a followed by 5361



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Figure 5. Two possible routes occurring in the ECL gel based on the production of a strong reducing agent, CO2•−: (a) direct oxidation near the electrode and (b) electron transfer reaction between oxalate and Ru(bpy)33+. The reaction mechanisms for the generation of Ru(bpy)32+,*: (c) coreactant pathway and (d) annihilation pathway. All reduction potentials are given vs SCE.

supplied from thin film batteries. Such simple, solution processable, solid-state DC-driven ECL devices based on emissive ion gels are potentially promising components for printed electronics applications. Improvement of the brightness of this device is desirable. The use of alternative ionic liquids showing viscosity lower than that of [EMI][TFSI] (η ∼ 34 cP at 20 °C)45 could be a simple solution, because it is likely that mass transfer of redox species (i.e., a luminophore and a coreactant) in the ion conductive channel of the gel is the limiting factor. Our ongoing work also aims to develop a regenerable alternative coreactant instead of the consumable oxalate for practical applications.

pathways in Figures 5c and 5d may be dominant for light emission. On the other hand, as oxalate near the electrode is depleted, the production of CO2•− is mainly caused by reaction 11 (Figure 5b). Namely, once CO2•−, a crucial component in this system, is generated by either mechanism in Figure 5a or 5b, the production of the excited Ru(bpy)3 2+,* by a combination of the coreactant pathway (Figure 5c) and the annihilation pathway (Figure 5d) is essentially the same. We also investigated the dependence of luminance and turnon voltage on the ECL gel composition. The composition of Ru(bpy)3(PF6)2:SMS:[EMI][TFSI] was fixed in the weight ratio 1:4:16, and then we varied the mole ratio of Ru(bpy)3(PF6)2 to TBAOX. Figure 6a gives the luminance profiles at different compositions. The ECL device could not emit light without TBAOX, whereas light emission was detected even with the addition of the small amount (5 times less than that of Ru(bpy)3(PF6)2) of TBAOX although the intensity was lower than the 1 Cd/m2 benchmark for turn-on voltage. When we added equimolar TBAOX, distinct light emission was observed from the device. As the amount of TBAOX was increased further, the maximum luminance was enhanced and eventually saturated at 1:5 mol ratio of Ru(bpy)3(PF6)2:TBAOX. On the other hand, the turn-on voltage was quite similar (1.60 V ± 0.04 V) independent of the gel composition (Figure 6b).

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Sigma-Aldrich except Ag paint (Leitsilber 200 silver paint, Ted Pella) and used as received. Tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate (Ru(bpy)3(PF6)2) was prepared using an anion exchange reaction between tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3Cl2) and excess ammonium hexafluorophosphate (NH4PF6). Tetrabutylammonium oxalate (TBAOX) was prepared by the reaction between oxalic acid and aqueous tetrabutylammonium hydroxide solution according to the literature.36,37 Polystyrene-block-poly(methyl methacrylate)-block-polystyrene SMS (17k-86k-17k) triblock copolymer and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) were prepared previously by a two-step atom transfer radical polymerization (ATRP)24 and anion exchange reaction,46 respectively. It is noted that the ion gel composed of 20 wt % SMS (17k-86k-17k) and 80 wt % [EMI][TFSI] exhibited both excellent ionic conductivity (∼2 mS/cm) and moderate storage modulus (∼3 kPa) which are sufficient for use in ECL devices.24 The indium−tin oxide (ITO)-coated glass (sheet resistance 8−12 Ω/sq, CG-51IN-S107, Delta Technologies Ltd.) was sequentially sonicated in acetone (5 min), methanol (5 min) and isopropanol (5 min), followed by UV/ozone treatment for 10 min before use.

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SUMMARY In summary, we have demonstrated the first solid-state electrochemiluminescent (ECL) device utilizing DC voltages, based on an emissive ion gel. By incorporating oxalate as a coreactant, sub-2 V operating voltage was achieved. The coreactant strategy offers the opportunity to apply ECL devices to printed electronics such as flexible displays with power 5362



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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Russell J. Holmes for access to his device characterization equipment, Dr. Sipei Zhang for providing the SMS block copolymer, and Ankit Mahajan for the measurement of the gel thickness. T.P.L. and C.D.F. acknowledge financial support from the Air Force Office of Scientific Research under FA9550-12-1-0067.

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REFERENCES

(1) Gu, Y.; Lodge, T. P. Macromolecules 2011, 44, 1732. (2) Gu, Y.; Cussler, E. L.; Lodge, T. P. J. Membr. Sci. 2012, 423−424, 20. (3) Voss, B. A.; Bara, J. E.; Gin, D. L.; Noble, R. D. Chem. Mater. 2009, 21, 3027. (4) Bohnke, O.; Rousselot, C.; Gillet, P. A.; Truche, C. J. Electrochem. Soc. 1992, 139, 1862. (5) Deepa, M.; Awadhia, A.; Bhandari, S. Phys. Chem. Chem. Phys. 2009, 11, 5674. (6) Meyer, W. H. Adv. Mater. 1998, 10, 439. (7) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (8) Bhat, S. N.; Pietro, R. D.; Sirringhaus, H. Chem. Mater. 2012, 24, 4060. (9) Ozel, T.; Gaur, A.; Rogers, J. A.; Shim, M. Nano Lett. 2005, 5, 905. (10) Cho, J. H.; Lee, J.; Xia, Y.; Kim, B.; He, Y.; Renn, M. J.; Lodge, T. P.; Frisbie, C. D. Nat. Mater. 2008, 7, 900. (11) Cho, J. H.; Lee, J.; He, Y.; Kim, B.; Lodge, T. P.; Frisbie, C. D. Adv. Mater. 2008, 20, 686. (12) Lee, J.; Panzer, M. J.; He, Y.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129, 4532. (13) Hong, K.; Kim, S. H.; Lee, K. H.; Frisbie, C. D. Adv. Mater. 2013, 25, 3413. (14) Kergoat, L.; Herlogsson, L.; Piro, B.; Pham, M. C.; Horowitz, G.; Crispin, X.; Berggren, M. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 8394. (15) Ha, M.; Zhang, W.; Braga, D.; Renn, M. J.; Kim, C. H.; Frisbie, C. D. ACS Appl. Mater. Interfaces 2013, 5, 13198. (16) Lee, K. H.; Zhang, S.; Gu, Y.; Lodge, T. P.; Frisbie, C. D. ACS Appl. Mater. Interfaces 2013, 5, 9522. (17) Kim, S. H.; Hong, K.; Lee, K. H.; Frisbie, C. D. ACS Appl. Mater. Interfaces 2013, 5, 6580. (18) Lee, S. W.; Lee, H. J.; Choi, J. H.; Koh, W. G.; Myoung, J. M.; Hur, J. H.; Park, J. J.; Cho, J. H.; Jeong, U. Nano Lett. 2010, 10, 347. (19) Shin, M.; Song, J. H.; Lim, G.-H.; Lim, B.; Park, J.-J.; Jeong, U. Adv. Mater. 2014, 26, 3706. (20) Kim, B. J.; Jang, H.; Lee, S.-K.; Hong, B. H.; Ahn, J.-H.; Cho, J. H. Nano Lett. 2010, 10, 3464. (21) Lee, S.-K.; Kim, B. J.; Jang, H.; Yoon, S. C.; Lee, C.; Hong, B. H.; Rogers, J. A.; Cho, J. H.; Ahn, J.-H. Nano Lett. 2011, 11, 4642. (22) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc. 2014, 136, 3705. (23) Itoh, N. J. Electrochem. Soc. 2009, 156, J37. (24) Zhang, S.; Lee, K. H.; Frisbie, C. D.; Lodge, T. P. Macromolecules 2011, 44, 940. (25) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (26) Lodge, T. P. Science 2008, 321, 50. (27) Nobeshima, T.; Morimoto, T.; Nakamura, K.; Kobayashi, N. J. Mater. Chem. 2010, 20, 10630. (28) Nobeshima, T.; Nakakomi, M.; Nakamura, K.; Kobayashi, N. Adv. Optical Mater. 2013, 1, 144.

Figure 6. (a) Dependence of the luminance on the mole ratio between TBAOX and Ru(bpy)3(PF6)2 in the ECL gel where the composition of Ru(bpy)3(PF6)2:SMS:[EMI][TFSI] is fixed as 1:4:16 (wt %). The dotted line corresponding to the luminance of 1 Cd/m2 is a benchmark to determine the turn-on voltage. (b) Plots of maximum luminance (●) and turn-on voltage (□) versus mole ratio of TBAOX relative to Ru(bpy)3(PF6)2.

Preparation of Patterned ITO-Coated Glass. Patterned ITOcoated glass was prepared using photolithography. First, the photoresist (S1813, Shipley) was spin-coated onto ITO-coated glass at 3000 rpm for 60 s followed by soft-baking at 105 °C for 5 min. Then, UV-irradiation (Oriel flood exposure system) was conducted on the photoresist film for 5 s with a mask, and the exposed area was washed by a solution of 351 Developer:H2O (1:5 by volume). Then, the resulting film was baked at 120 °C for 5 min. The ITO-coated glass was etched in a solution consisting of 5% nitric acid (20 mL), 37% hydrochloric acid (220 mL) and water (180 mL) at 80 °C for 3 min. Finally, the photoresist on the substrate was fully removed by washing with acetone. ECL Device Fabrication and Characterization. ECL devices consisting of ITO, ECL gel and Ag were fabricated in ambient air using a two-step solution process. A dichloromethane (DCM) solution of Ru(bpy) 3 (PF 6 ) 2 :TBAOX:SMS(17−86−17):[EMI][TFSI]:DCM (1:3:4:16:40 wt %) was spin coated onto the patterned ITO-coated glass. The thickness of the ECL gel was ∼35 μm determined by scanning electron microscopy (Quanta 200 3D, FEI). The resulting ECL film was dried at 50 °C for 1 h and residual solvent was removed fully under reduced pressure. Then, the Ag electrode was prepared by brush-painting the Ag paint. The spectrum of emitted light from the ECL gel was recorded on a fiber-coupled HR4000 Ocean Optics spectrometer. The current density−voltage-luminance characteristics for the ECL device were obtained using a calibrated Hamamatsu S3584−08 photodetector and a HP4155C parameter analyzer. Voltages were applied such that the positive supply terminal was connected to the ITO electrode. 5363



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(29) Richter, M. M. Chem. Rev. 2004, 104, 3003. (30) The reduction potential (Eored ≈ −0.84 V vs SCE) for the reaction Ru(bpy)33+ + e− → Ru(bpy)32+,* was calculated by Eored (Ru(bpy)33+/ Ru(bpy)32+, +1.20 V (vs SCE)) − E* (excitation energy from Ru(bpy)32+ to Ru(bpy)32+,*, 2.04 eV). The lower reduction potential of Ru(bpy)33+/Ru(bpy)32+,* couple is due to greater difficulty for introducing an electron into the LUMO (for Ru(bpy)32+,*) than into the unfilled HOMO (for Ru(bpy)32+). See ref 34. (31) Igarashi, R.; Nosaka, Y.; Miyama, H.; Kaneko, M.; Yokoyama, M. J. Electrochem. Soc. 1988, 135, 2987. (32) Håkansson, M.; Jiang, Q.; Spehar, A.-M.; Suomi, J.; Kotiranta, M.; Kulmala, S. Anal. Chim. Acta 2005, 541, 171. (33) Nobeshima, T.; Nakamura, K.; Kobayashi, N. Jpn. J. Appl. Phys. 2013, 52, 05DC18. (34) Bard, A. J. Electrogenerated Chemiluminescence; Marcel Dekker: New York, 2004. (35) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. (36) Chang, M.-M.; Saji, T.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 5399. (37) Qi, H.; Chang, J.; Abdelwahed, S. H.; Thakur, K.; Rathore, R.; Bard, A. J. J. Am. Chem. Soc. 2012, 134, 16265. (38) Walton, D. J.; Phull, S. S.; Bates, D. M.; Lorimer, J. P.; Mason, T. J. Electrochim. Acta 1993, 38, 307. (39) Ege, D.; Becker, W. G.; Bard, A. J. Anal. Chem. 1984, 56, 2413. (40) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987, 59, 670. (41) Rubinstein, I.; Martin, C. R.; Bard, A. J. Anal. Chem. 1983, 55, 1580. (42) Juris, A.; Balzani, V. Coord. Chem. Rev. 1988, 84, 85. (43) Itoh, N. Materials 2010, 3, 3729. (44) Pighin, A.; Conway, B. E. J. Electrochem. Soc. 1975, 122, 619. (45) Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Inorg. Chem. 1996, 35, 1168. (46) Susan, M. A. B. H.; Kaneko, T.; Noda, A.; Watanabe, M. J. Am. Chem. Soc. 2005, 127, 4976.

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DC-Driven, Sub-2 V Solid-State Electrochemiluminescent Devices by

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