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Alternating-Current-Driven, Color-Tunable Electrochemiluminescent Cells

Taiki Nobeshima , Masaru Nakakomi , Kazuki Nakamura , and Norihisa Kobayashi *

Electrochemiluminescence (ECL) is a light emission phenom-enon induced by electrochemical redox reactions. [ 1,2 ] In general, the excited state of a luminescent molecule is formed by elec-tron transfer between reduced and oxidized species of the mol-ecule that have been generated electrochemically, thus leading to luminescence. This reaction is often called annihilation ECL because the reduced and oxidized species vanish by charge neu-tralization after the ECL reaction. [ 3,4 ] In contrast, co-reactant ECL, another type of ECL mechanism, requires a co-reactant in addition to the emitting material. Co-reactant ECL has attracted ample attention as it can be applied in biosensor devices such as those used to detect amino acids, proteins, and DNA. [ 5–7 ]

Here, we focus on annihilation ECL to fabricate light-emit-ting devices, as this type of ECL has some important advan-tages. First of all, the structure of such ECL devices is quite simple: they are basically composed of a pair of transparent electrodes with the emitting material dissolved in the electrolyte solution between the electrodes. The basic architecture of the device enables the fabrication of large-area devices through the use of printing processes. Other advantages are the absence of a nanometer-thick thin fi lm and relatively few limitations on the choice of electrode materials. Several researchers have previ-ously reported ECL devices based on metal complexes, organic molecules, or conductive polymers. [ 8–10 ] Such ECL devices are also called light-emitting electrochemical cells (LECs). The fi rst report about solid-state LECs based on conductive polymers was in 1995 [ 10 ] and since then, LECs have generated interest as possible alternatives to organic light-emitting diodes (OLEDs). Unfortunately, however, LECs are not suitable for display appli-cations because LECs generally exhibit slow turn-on responses since they usually operate on direct current (DC), and collisions between the reduced and oxidized species are needed to drive the ECL reaction. In order for the redox species to collide with each other, they must diffuse from each electrode to the center of the reaction layer. Because of the relatively slow diffusion of redox species in the electrolyte solution and/or the slow electron transfer of the conducting polymer, the turn-on response of DC-driven ECL devices is generally not suitable for practical use.

© 2013 WILEY-VCH Verlag Gwileyonlinelibrary.com

DOI: 10.1002/adom.201200056

T. Nobeshima, M. Nakakomi, Dr. K. Nakamura, Prof. N. KobayashiDepartment of Image and Materials ScienceGraduate School of Advanced Integration ScienceChiba University1-33 Yayoi-cho , Inage-ku , Chiba , 263-8522 , Japan E-mail: koban@faculty.chiba-u.jp

We previously reported a facile method of improving the emission properties of an ECL device. The method is based on using an alternating current (AC) to operate the device. [ 11 ] During an AC cycle, oxidized and reduced species are gener-ated at the same electrode surface because the bias polarity reversibly switches between anode and cathode, resulting in the effi cient generation of annihilation ECL without long-distance diffusion of the relevant species. As a result, the AC-operation method facilitates not only a quick turn-on response but also high-intensity emission.

In this Communication, we report novel multicolor AC-ECL cells. Indeed, color-tunable light-emitting devices are of high practical interest, and multicolor LEDs based on single organic light-emitting diode cells have already been reported. [ 12,13 ] Although there have also been reports on methods of tuning the color of the light emitted from some ECL systems, the under-lying mechanisms are based on a change in chemical struc-ture, the components of the material, or the pH of the solu-tion, [ 14–16 ] meaning that ECL color-tuning has not been dem-onstrated in a single cell without pretreating the composition of the solution. Otherwise, the color of the light emitted from some co-reactant ECL systems has been tuned without pretreat-ment, [ 17,18 ] although the multicolor systems are unsuitable for long-term use. In the study reported here, we demonstrate AC-method-based ECL color-tuning in a single annihilation-ECL-based cell. The color of emission from the ECL cell was tuned by varying the applied frequency, which is a unique feature of the AC method. The amplitude was also considerably modu-lated to tune the ECL color; however, frequency switching was used to clarify color tenability and to effi ciently use the AC-ECL method (discussed later). The novel ECL cell contains rubrene (RUB) and 9,10-diphenylanthracene (DPA) as the redox-active luminescent materials and exhibits yellow and white multicolor light emission by the application of various AC frequencies.

RUB and DPA are well-known light-emitting molecules that exhibit yellow and blue emission, respectively, upon photoexci-tation. [ 19–22 ] The mechanism of annihilation ECL for RUB and DPA is represented as follows:

R + e− → R•− (reduction on cathode) ( 1 )

R − e− → R•+ (oxidation on anode) ( 2 )

R•−+ R•+→ R∗+ R (radical annihilation and excited state ormation)f

( 3 )

R∗ → R + h< (light emission) ( 4 )

where R can be either RUB or DPA.

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Figure 1. CVs for solutions containing RUB (a), DPA (b), and both RUB and DPA (c). The left and right panels display CVs measured for cathodic and anodic potentials, respectively.

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Figure 2. ECL spectra for an ECL cell containing RUB (a) under applica-tion of 10 V sine-wave AC bias at 1000 Hz, and for a cell containing DPA (b) under application of a 10 V, 500 Hz sine-wave AC bias. Insets show operating cells.

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We fi rst investigated the electrochemical properties of RUB and DPA by acquiring the cyclic voltammograms (CVs) for these materials ( Figure 1 , for experimental details, see the Experimental Section). In the CV for RUB (Figure 1 a), two reversible redox couples are observed at −1.40 and +1.00 V vs. Ag/Ag + . The CV for DPA (Figure 1 b) also shows two redox couples at −1.87 and +1.43 V, although the oxidation of DPA at +1.43 V seemed irreversible. The redox behavior of DPA was dependent on the scan rate of the potential: the CV produced using the higher scan rate showed a better reversible redox couple for DPA (see Figure S1 in the Supporting Information). Since the scan rate of the potential in AC-ECL is much faster than in these CV measurements, the lifetime of a DPA cation radical does not signifi cantly affect ECL properties. The CVs thus indicate that RUB and DPA can undergo both reduction and oxidation, which enables annihilation ECL when an AC voltage is applied.

ECL cells were then fabricated by sandwiching an electro-lyte solution containing either RUB or DPA between a pair of indium tin oxide (ITO) glass electrodes to demonstrate AC ECL. Figure 2 shows the ECL spectra for the ECL cells under the application of sine-wave AC. The cell containing RUB showed yellow ECL with an emission peak at around 560 nm (Figure 2 a), while the cell containing DPA showed blue ECL with a peak at around 430 nm (Figure 2 b). The ECL spectra are almost identical to the photoluminescence and the corre-sponding DC-operated ECL spectra for these materials, indi-cating that applying AC bias effectively generates ECL of RUB and DPA. Figure 3 shows the plots of the AC-frequency depend-ence of the ECL intensities obtained when a 10 V sine-wave AC bias was applied to the ECL cells containing RUB (Figure 3 a) and DPA (Figure 3 b). The frequency was decreased from high to low values. The ECL from RUB was observed at frequencies below 1800 Hz at a peak intensity around 1100 Hz. The ECL from DPA, on the other hand, was observed below 1100 Hz, showing a peak at around 500 Hz. A maximum intensity was observed in the applied frequency range because the emitters and/or the media had degraded because of the high electrode potential, which resulted in the formation of an electric double layer (EDL) at low frequencies (also discussed later). Neither ECL cell exhibited emission at frequencies higher than 2000 Hz. Interestingly, the frequency range within which ECL was gener-ated differed signifi cantly for RUB and DPA, indicating that the ECL intensities could be controlled by changing the frequency

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Optical Mater. 2013, 1, 144–149

of the AC bias applied to the cell. We there-fore analyzed the color of the light emitted from the ECL cells by varying the frequency of AC bias applied to an ECL cell containing both RUB and DPA.

The CVs for the cell whose electrolyte solu-tion contained both RUB and DPA exhib-ited four redox couples, which were much the same as those observed for the cells containing either RUB or DPA (Figure 1 c). However, an irreversible anodic peak was observed at +1.21 V in the CV for the cell containing the mixed solution, and the peak is attributed to the peroxidation of RUB (as a similar peak was observed in the CV for the

cell containing RUB when the potential was swept to +1.60 V). The results indicate that the electrochemical properties of RUB and DPA were completely independent in the mixed solu-tion, suggesting the possibility of ECL color-tuning. From the frequency-dependent ECL (Figure 3 ), we expected an ECL for RUB alone from 1800 to 1200 Hz and an ECL from both RUB and DPA, or only DPA, below 1100 Hz when 10 V AC was applied to the ECL cell containing both RUB and DPA. When the frequency dependence of the ECL intensities from RUB (at 560 nm) and DPA (at 430 nm) in the mixed ECL cell was monitored ( Figure 4 ), ECL at around 560 nm was observed below 1800 Hz, while ECL at 430 nm was observed below 800 Hz. Thus, we achieved control over the ECL emissions by varying the frequency of the AC bias applied to the mixed ECL cell.

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Figure 3. Dependence of normalized ECL intensities of RUB cells (a) and DPA cells (b) on frequency of 10-V sine-wave AC bias applied to cells. Intensities were monitored at 560 nm for RUB ECL and 430 nm for DPA ECL.

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Obviously, the difference between the frequency ranges at which ECL emission was observed for the cell containing RUB and DPA plays a key role in tuning the color of the light emitted from the ECL cell. We presumed that this difference depends on the gap in potential between reduction and oxida-tion of the isolated materials. From the reduction and oxidation peak potentials of each material (Figure 1 ), the gaps in potential were calculated as 2.40 and 3.30 V for RUB and DPA, respec-tively. ECL was generated at higher frequency for the ECL cell containing only RUB (which exhibited a relatively small gap in potential) than it was for the ECL cell containing only DPA. This result can be explained by taking into account the mecha-nism of formation of an electric double layer (EDL) when AC bias was applied to the ECL cell.

Generally, EDL formation results in a difference in potential between electrode and bulk solution and accordingly, electro-chemical redox reactions occur at the electrode. [ 23 ] Since the EDL is formed rapidly (i.e., within a few milliseconds), most researchers studying conventional DC-ECL have ignored the process by which EDLs form as it is largely irrelevant. How-ever, this process cannot be ignored in AC ECL at high fre-quency because EDLs form on a time scale similar to that of AC cycles. When a high-frequency AC bias is applied to an ECL cell (e.g., 10 kHz AC), the time span required for each AC

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Figure 4. Dependence of ECL intensities measured at 430 and 560 nm from ECL mixed cells (containing both RUB and DPA) on frequency of 10 V sine-wave AC bias applied to cells.

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half-cycle is too short for a complete EDL to form. We therefore estimated the maximum difference in potential induced by the EDL during each half-cycle by comparing the amount of charge required for EDL formation between the application of DC bias and sine-wave AC bias (for details of the calculation, see the Experimental Section).

Figure 5 shows that the maximum difference in potential between the electrodes and the bulk solution increased with decreasing frequency of the AC bias applied to the ECL cell. When 10 V sine-wave AC bias was applied at 1800 Hz to the cell, a difference in potential of ca. 2.7 V was generated in the EDL within a half-cycle (ca. 0.28 ms). This difference in poten-tial is large enough to initiate the redox reaction of RUB. How-ever, ECL was not generated from DPA at 1800 Hz because the difference in potential did not facilitate the redox reaction of DPA. Rather, ECL generation from DPA required a lower fre-quency than ECL generation from RUB because a longer time is necessary to generate the larger difference in EDL potential required to initiate the redox reaction of DPA. The difference in potential at 1100 Hz, which was the highest frequency leading to light emission from DPA (Figure 3 ), was calculated to be ca. 3.7 V, which is suffi ciently large to facilitate the redox reaction of DPA in a two-electrode cell. The results thus clearly indicate that the gap in the potential of luminescent molecules strongly correlates with the range of AC bias frequencies applied to the ECL cell.

Upon closer examination, the estimated difference in poten-tial at the highest frequency leading to ECL did not completely coincide with the gap in the potential of the molecules. This fi nding indicates that a higher difference in potential is required for EDL than to achieve the redox reaction in a two-electrode cell and that this high difference in potential should be main-tained over a certain period of time to generate ECL when AC bias is applied to the cell. The maximum ECL intensity shown in Figure 3 may also be explained in terms of the difference in potential induced by EDL formation. For example, a difference of over 6 V was generated between the electrodes and the bulk solution when 10 V AC bias was applied below 500 Hz to the cell. Such a difference in potential is too high to maintain the stability of the materials and cells in most electrochemical sys-tems, resulting in the degradation of RUB, DPA, and N -methyl-2-pyrrolidone (NMP), which is commonly used as the solvent.

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As a result, the intensity of the ECL from DPA decreased, and ECL was not generated from RUB below 500 Hz. RUB, in par-ticular, was easily degraded by the electrochemical reaction, especially in the presence of oxygen, because the high voltage applied to the cell caused the peroxidation of RUB (as indi-cated by the anodic peak at 1.21 V in Figure 1 c). Maximum ECL intensity was therefore observed at the applied AC frequency shown in Figures 3 and 4 . However, ECL generation was suc-cessfully switched on and off for each luminophore by control-ling the potential of the EDL. Namely, the off state of the ECL was controlled by applying an AC frequency much higher than the one that was just incapable of generating redox reactions of ECL molecules, and the on state was controlled by applying an AC frequency low enough to generate ECL.

Applying low-amplitude AC bias may be an alternative method of driving ECL that could maintain the stability of ECL cells. Actually, when 4 V square-wave AC bias was used to measure ECL, the ECL intensity increased with decreasing applied frequency and did not show any maximum intensity (see Figure S2 in the Supporting Information). This result indi-cates that the ECL cell to which a 4 V square-wave AC bias was applied degraded less than the ECL cell to which a 10 V sine-wave AC bias was applied. However, when a 4 V square-wave AC bias was applied, the highest frequency for ECL generation shifted to lower values for each ECL cell containing RUB or DPA, and the difference in the range of emission frequencies for each molecule also became smaller (change in frequency = Δ f =300 Hz) than when a 10 V sine-wave AC bias was applied ( Δ f = 1000 Hz). Further, the yellow emission from RUB was insuffi ciently strong when the 4 V square-wave AC bias was applied.

However, ECL color could be tuned by changing the AC amplitude at 1 Hz (see Figure S3 in the Supporting Informa-tion). In this method, a smaller voltage, such as around 2.8 V

Figure 6. Photographs and concomitant ECL spectra for ECL mixed cells during operation when a 10 V sine-wave AC bias was applied at a frequency of either 1000 Hz (A,C) or 300 Hz (B,D) to the cells. Results demonstrate frequency-controlled color-tuning of devices.

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for yellow ECL generation and around 3.4 V for white ECL generation, was applied at a driving frequency of 1 Hz to tune the color of the light emitted from the ECL cells. How-ever, a smaller AC driving voltage required lower-frequency application in order to com-pletely charge the EDL and generate ECL. Although continuous emission was not observed at a driving frequency of 1 Hz, two blinks per second should be observed, which is not suffi cient for practical use. Therefore, a 10 V sine-wave AC bias was used to tune the ECL color in this study.

Comparing the frequency dependences of the ECL intensities shown in Figures 3 and 4 reveals that the highest frequency for ECL generation from DPA differed from that of only DPA (1100 Hz) and from that of the ECL from both RUB and DPA (800 Hz). It also reveals that the highest frequency for ECL generation from RUB (1800 Hz) was con-stant for both cells. However, unlike the ECL intensity of the cell containing only RUB, that of the cell containing both RUB and DPA decreased considerably to a frequency in the

© 2013 WILEY-VCH Verlag GAdv. Optical Mater. 2013, 1, 144–149

range 1100–900 Hz. Such behavior could result from electron transfer between RUB •− and DPA •+ and/or RUB •+ and DPA •− , therefore inhibiting ECL generation. The intensity of the ECL from RUB further showed a secondary peak at 700 Hz. Inter-estingly, ECL from RUB in the mixed ECL cell was observed below 500 Hz, even though the ECL cell with RUB was the only cell that did not exhibit ECL at these frequencies. The enhance-ment of RUB ECL in the mixed cell could be due to the transfer of excitation energy from DPA to RUB. Since the absorption band for RUB from ca. 400 to 560 nm considerably overlaps the emission band of DPA, the excitation energy of DPA can be effectively transferred to the ground states of RUB. Evidence of such energy transfer is clearly shown in the photoexcited fl uorescence spectra of DPA/RUB mixtures (Figure S4 in the Supporting Information). Specifi cally, a decrease in DPA fl uo-rescence and an increase in RUB fl uorescence were observed when DPA molecules were photoexcited (375 nm), supporting the excitation energy transfer from DPA to RUB. Both blue ECL from DPA and yellow ECL from RUB were observed in the low frequency range (700–300 Hz). Therefore, the concentration of DPA in the mixture solution was 4 times higher than that of RUB in order to generate white light emission.

Finally, we demonstrated ECL color-tuning by applying dif-ferent AC frequencies. Figure 6 shows photographs and ECL spectra for the mixed cell to which either 1000 or 300 Hz AC bias was applied. When a 1000 Hz AC bias was applied, yellow light was emitted from RUB (Figure 6 a). The ECL became white when the frequency of the AC bias was switched to 300 Hz (Figure 6 b). From the ECL spectrum measured with an AC bias at 300 Hz, both RUB and DPA contributed to ECL generation, resulting in white light emission in accordance with additive color mixing. Although the ECL color-tuning was reproducible, the cell showed only limited stability. The main limitation of the cell was its short lifetime, which is related to the stability

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of the luminescent molecules and the high operating voltage. Therefore, further studies to improve the device stability are currently in progress, including those directed towards applica-tion of lower-voltage AC, and the use of ECL molecules that are more stable under electrochemical reaction conditions and an ambient environment.

In summary, we have successfully demonstrated the tuning of ECL color by modulating the frequency applied to a single AC-ECL cell containing two different luminescent molecules. A key factor in tuning the ECL color is the differ-ence between the frequency ranges required for generating ECL from the molecules, which appears to be closely related to the gap in the redox potential of the molecules. When the polarity of the electrodes in this AC-ECL system is switched quickly, the difference in potential induced by EDL forma-tion is affected by the frequency of the AC bias applied to the cell. A frequency-controlled color-tunable ECL device was achieved because each ECL molecule has a different fre-quency range in which the difference in potential generates a redox reaction at the electrodes. Further studies to improve the color tunability and the stability of the cell are currently in progress. We hope that our investigations into ECL light-emitting devices will stimulate further development of these promising devices.

Experimental Section Materials : 5,6,11,12-Tetraphenylnaphthacene (trivial name: rubrene

(RUB)) (Tokyo Chemical Industry), 9,10-diphenylanthracene (DPA) (Tokyo Chemical Industry), and tetra- n -butylammonium perchlorate (TBAP) (Kanto Chemical) were used as received. N -Methyl-2-pyrrolidone (NMP) (Kanto Chemical), and N , N -dimethylformamide (DMF) (Kanto Chemical) were placed as received in a glove box fi lled with dry argon gas and were used to prepare solutions in the glove box.

Electrochemical measurement : Cyclic voltammetry was performed using an electrochemical analyzer (Model 440A, CH Instruments, Austin, TX) and a three-electrode cell equipped with a Pt disc electrode as the working electrode (3 mm in diameter), a Pt wire electrode as the counter electrode, and a Ag/Ag + reference electrode. DMF solutions of RUB (1 m m ) or DPA (4 m m ) containing TBAP (100 m m ) as the supporting electrolyte were used for CV measurements. A solution containing both RUB (1 m m ) and DPA (4 m m ) in DMF was also prepared and used for another CV measurement. The measurements were conducted in the glove box. The CV measurements were performed at 100 mV s –1 .

Electrochemiluminescence (ECL) Spectroscopy : TBAP-containing (100 m m ) NMP solutions of RUB (5 m m ) or DPA (20 m m ) were prepared. A solution containing both RUB (5 m m ) and DPA (20 m m ) was also prepared. The solutions were placed between a pair of indium tin oxide (ITO) electrodes with spacers at an interelectrode distance of 200 μ m to fabricate the ECL cells. The ECL cells contained an effective electrode area of 10 mm × 10 mm and were not sealed. The ECL cells were prepared in the glove box. A signal function generator (SG-4115, Iwatsu Test Instruments) was used to apply AC voltages to the ECL cells. The ECL optical spectra were measured using a photonic multichannel analyzer (PMA C10027, Hamamatsu Photonics). All experiments were conducted under ambient conditions (temperature: 25 °C, humidity: 50–60%, oxygen concentration: 20%).

Calculation of the Difference in Potential in the Electric Double Layer : The difference in potential between the electrode and the bulk solution was estimated by measuring the current of a blank cell to which various voltages were applied. A solution of TBAP in NMP was prepared without redox-active materials and was placed between a pair of ITO electrodes with spacers to fabricate a blank cell in the glove box. The blank cell

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was identical to the ECL cell (i.e., it had an effective electrode area of 10 mm × 10 mm and an interelectrode distance of 200 μ m). The function generator was used to apply AC and DC voltages to the blank cell, and the current response was measured using an oscilloscope (WaveJet 314, Teledyne LeCroy) in air. The total amount of charge required in order to completely form the electric double layer (EDL) was calculated from the monitored current of the blank cell to which 1 V DC had been applied, as the current of the blank cell consisted of only nonfaradaic current owing to EDL formation. Since the amount of charge is proportional to the applied voltage, the calculated amount of charge was multiplied by ten to calculate the difference in potential in the EDL when a 10 V AC bias was applied to the cell. The difference in potential generated in the EDL during each AC half-cycle was estimated using the ratio of the amount of charge measured when a 10 V AC bias was applied at various frequencies to the blank cell, to the standard amount of charge.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientifi c Research (B) (No. 24350090) and Challenging Exploratory Research (No. 22655060) from the Japan Society for the Promotion of Science (JSPS), the Advanced School for Organic Electronics under the Chiba University Global COE Program, Venture Business Laboratory project of Chiba University, the Futaba Electronics Memorial Foundation and JSPS Research Fellowships for Young Scientists.

Received: December 10, 2012Published online: February 15, 2013

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