Perspective pubs.acs.org/journal/apchd5
Light-Emitting Devices Based on Electrochemiluminescence: Comparison to Traditional Light-Emitting Electrochemical Cells Seok Hwan Kong,† Jong Ik Lee,† Seunghan Kim, and Moon Sung Kang* Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea S Supporting Information *
ABSTRACT: Electrochemical processes can be exploited to operate lightemitting devices with unusual functionality. For example, light-emitting electrochemical cells (LECs) contain a small amount of electrolyte within the organic/polymer light-emitting active layer. The electrolyte in the active layer allows the multiple charge injection layers that are deemed critical for organic light-emitting diodes to be avoided. Very recently, an alternative lightemitting device platform based on electrochemical processes was also suggested. These devices rely on electrochemiluminescence (ECL), a lightemission process based on the charge transfer reaction between reduced and oxidized forms of luminophores. Although the ECL process has been extensively investigated in the field of analytical chemistry, its utilization in electronic devices is a new approach that offers unique opportunities. Despite the interesting opportunities, a good introduction to the subject is not available, particularly with a focus on electronic device applications. Moreover, the operation of ECL devices is often confused with that of LECs, even though they follow distinct working principles. This confusion occurs mainly because the active layers for both ECL devices and LECs contain light-emitting material and electrolyte material (although their compositions are completely different). Therefore, clarifying the difference between the two sister devices would be both necessary and useful. In particular, a comparison of the two sister devices would highlight the unique opportunities for ECL devices and inspire researchers to devise a novel lightemitting device platform, which is the primary goal of this perspective. KEYWORDS: light-emitting device, electrochemiluminescence, light-emitting electrochemical cells, electrolyte, luminophore
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units for next-generation computational systems that mimic the complicated signal processing between neurons.6−8 Likewise, light-emitting devices can also harness the electrochemical process for operation. A good example is a device that includes an electrolyte within the emissive layer, which is better known as a light-emitting electrochemical cell (LEC). The presence of the electrolyte within the active layer of the LEC facilitates the injection of electrons and holes from their respective electrodes. Therefore, LECs offer a unique device platform that is highly suitable to produce printed light sources while avoiding issues related to meticulous energy alignment between the constituting materials,9−14 unlike the case for organic light-emitting diodes (OLEDs) comprising purely nonionic electronic components. Recently, an alternative lightemitting device platform for LECs was demonstrated that also relies on electrochemical processes.15−18 The operation of these devices relies on electrogenerated chemiluminescence, or electrochemiluminescence (ECL). ECL is a light emission process obtained from an excited state of a luminophore that forms as a consequence of the electron transfer reaction between electrochemically active species.19−22 In fact, ECL has been widely used in analytical techniques, including diagnostics,23−26
xploiting electrochemical phenomena in electronics has led to exciting opportunities that could not be done solely using traditional semiconductor technology.1 This is not a surprising approach considering that both electrochemistry and electronics rely, in principle, on physical processes involving electrons. The reduction of an electrochemically active species resembles the process of injecting an electron into a semiconductor. Conversely, the oxidation of an electrochemically active species can be considered as an extraction of an electron from a semiconductor or, equivalently, as the injection of holes into the material. Dye-sensitized solar cells (DSSCs) are well-established photovoltaic devices whose operation is based on redox cycles of electrolytes in combination with the photoexcitation process of a dye.2 The development of DSSCs has opened alternating engineering principles for renewable energy technology and industry beyond the p−n-junction-type semiconductor solar cells. Thin-film transistors that employ electrolyte gate dielectrics are another interesting class of devices that offer unprecedented opportunities in printed, portable electronics that can be operated at low voltages.3−5 In these devices, the current density of the semiconductor films can be effectively modulated using the electrolyte gate dielectrics with exceptionally large capacitance upon the formation of an electric double layer at the semiconductor−electrolyte interface or through electrochemical doping/dedoping processes of a semiconductor channel. More recently, these devices have been exploited in building the basic © 2017 American Chemical Society
Received: Revised: Accepted: Published: 267
August 2, 2017 October 30, 2017 October 30, 2017 October 30, 2017 DOI: 10.1021/acsphotonics.7b00864 ACS Photonics 2018, 5, 267−277
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environmental assays,27−29 and biowarfare agent detection,30 because ECL is extremely sensitive to the electrochemical environment of the analyte solution. However, ECL was only recently launched into a light-emitting device platform, and it is expected to provide a versatile device platform for printed, flexible, and even stretchable optoelectronics. We refer to such devices as electrochemiluminescence devices (ECLDs), which are distinguished from LECs. Despite the similarity of the constituent materials that are included in both devices, the operation of ECLDs is completely different from that of LECs. This implies that the design and engineering principles as well as the future aspects of these two types of devices may be different. To improve upon recent accomplishments in ECLDs, therefore, the differences and similarities of these two types of devices must be clearly understood. Although there are already a handful of comprehensive textbooks and reviews for LECs,13,14,31 a description of this new device platform is difficult to find. Accordingly, this perspective provides a general description of ECLDs, including the component materials, their composition, device operation principles, and interesting features of the novel device platform. In particular, the description will be made by highlighting the differences with their sister devices, i.e., LECs, in order to deliver new opportunities for ECLDs effectively.
guest organic materials that have been exploited in the OLED community are also known to exhibit ECL.42,47 Moreover, ECL from nanostructured materials such as inorganic semiconductor nanocrystals,48−51 metal nanoclusters,52 and carbon nanostructures53,54 has been demonstrated. The second component is the electrolyte that is necessary to form the oxidized and reduced states of the luminophore as well as the coreactants if present. For iTMC ECL luminophores, the luminophore and the counterion themselves can serve as ionic components for the corresponding processes. For other types of luminophores, the addition of electrolytes in the active layer is mandatory. In fact, electrolytes are added even to iTMC luminophores to enhance the ionic processes of the ECL. Typical electrolytes include many of the classic electrolyte systems, such as phosphates or borates in organic solvent or water.20 Room-temperature ionic liquids have also served as the ECL medium.15,17,55,56 The mixture of these ECL luminophores and electrolyte (and coreactant) is typically liquid. Perhaps the fluidic nature of the liquid has prevented wide use of ECL oriented toward electronic applications, which have traditionally preferred the solid-state device integrity. The new opportunities for the liquid active material as well as the recent methods to solidify the ECL active layer with solid-state integrity will be discussed below. We note that both of these components, the luminophore and the electrolyte, also constitute the active layer of LECs, the sister light-emitting devices. Consequently, the distinction between the operating methods of the two devices has been often overlooked. It should be pointed out that the relative compositions of the luminophore and the electrolyte for ECLDs and LECs are completely different. The major component of the active layer for ECLDs is the electrolyte, whereas that for the LECs is the luminophore. Thus, the active layer of the ECLD can be regarded as a small number of ECL luminophores moving within a vast liquid body of electrolyte; the luminescence of an ECLD is achieved when the oxidized and reduced species are encountered together in the electrolyte. Thus, the convective process of the ECL luminophore is critical in ECLD. Meanwhile, in LECs, only a small amount of electrolyte is added into the active layer. The role of the electrolyte in LECs is to assist in the injection of electrons and holes into the luminophore and the subsequent electrochemical doping process before they form excitons. Here, electrons and holes are delivered by a conductive process through the emissive material, rather than by a convective process. This difference in the composition causes huge distinctions between the device operation principles, features, and opportunities of ECLDs and LECs, which will be described below. To directly highlight the contrast, we prepared model ECLDs and LECs and compared the characteristics of these two different types of electrochemical devices (see the endnote for the experimental details57). We emphasize that the active layers of the two model devices contain the very same luminophore and electrolyte materials, but with different compositions. The luminophore was tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate ([Ru(bpy)3][PF6]2), and the electrolyte was 1-ethyl3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI]). The active layer for the ECLD contained more electrolyte than luminophore ([Ru(bpy) 3 ][PF 6 ]2 : [EMIM][TFSI] = 1:10 w/w), whereas that for the LEC contained more luminophore than electrolyte ([Ru(bpy)3][PF6]2:[EMIM][TFSI] = 10:1 w/w). In both devices, the active layer was sandwiched between metal and transparent oxide electrodes. By comparison of the device characteristics of these sister devices, their operation could be clearly understood.
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BRIEF DESCRIPTION OF THE TWO ECL PATHWAYS Typically, ECL is classified into two categories. One is annihilation ECL, which involves the electron transfer reaction between an oxidized species and a reduced species of an ECL luminophore. With the benchmark ECL luminophore tris(2,2′bipyridine)ruthenium(II) (Ru(bpy)32+) as an example, annihilation ECL occurs when its reduced form (Ru(bpy)3+) and its oxidized form (Ru(bpy)33+) react to form an excited state (Ru(bpy)32+*) that eventually emits light upon its relaxation to the ground state (Ru(bpy)32+). The other one is coreactant ECL, which involves an additional contribution of a third species, a coreactant such as tripropylamine (TPrA), oxalate (C2O42−), or peroxydisulfate (S2O82−), in forming the excited state of the ECL luminophore. For Ru(bpy)32+ as an example, coreactant ECL occurs when excited Ru(bpy)32+* is formed via the reaction between oxidized Ru(bpy)33+ and strongly reductive TPrA• or CO2•− generated from TPrA or C2O42−, respectively.32−35 Such a process is particularly known as “oxidative−reductive” coreactant ECL. Likewise, “reductive−oxidative” coreactant ECL occurs when reduced Ru(bpy)3+ undergoes a charge transfer reaction with an oxidizing agent such as SO4•−.36,37 Since the luminescence is strongly affected by the amount of coreactant, which is often the analyte of interest, coreactant ECL is the process that is predominantly utilized in the analytical techniques.38 Details of these ECL processes have been described thoroughly in many other reviews.20,21
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ACTIVE LAYERS FOR ECLD Regardless of the type of ECL, the active layer of the ECLD basically contains the following two critical components. The first component is, of course, the ECL luminophore that emits light. A typical ECL luminophore includes an ionic transition metal complex (iTMC) that contains ruthenium (Ru) or iridium (Ir) as the metal core.19,39,40 The above-mentioned Ru(bpy)32+ is an archetypal ECL iTMC. Aromatic organic molecules, such as rubrene and diphenylanthracene,16,41,42 and conjugated polymers, such as poly(phenylene vinylene) derivatives43,44 and polythiophenes,45,46 can also yield ECL. Combinations of host− 268
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DEVICE OPERATION PRINCIPLES The light emission process for a basic annihilation-type ECLD (where coreactants are not included in the active layer) is first described. When a constant electric field is applied between the two electrodes of the device, the ionic species in the active layer, i.e., [EMIM]+, [TFSI]−, and [PF6]−, form an electric double layer (EDL) at the electrode−active layer interface to compensate for the electric potential generated by the field (Figure 1a). Even
the ECL can be coproduced from a single electrode (Figure 2a,b). We refer to this type of device as a DC-driven ECLD with a
Figure 2. (a) Schematic description of the operation of a DC-driven ECLD based on the coreactant ECL mechanism of Ru(bpy)32+ in the presence of oxalate (C2O42−): ① oxidation of the ECL luminophore at the anode (Ru(bpy)32+ → Ru(bpy)33+ + e−); ② oxidation of the coreactant at the anode (C2O42− → C2O4•− + e−; C2O4•− → CO2•− + CO2); ③ charge transfer reaction (Ru(bpy)33+ + CO2•− → Ru(bpy)2+* + CO2); ④ light emission (Ru(bpy)32+* → Ru(bpy)2+ + hν). Here only the anode of the device is shown. (b) Operation of a DC-driven ECLD based on the coreactant ECL mechanism of Ru(bpy)32+ in the presence of peroxydisulfate (S2O82−): ① reduction of the ECL luminophore at the cathode (Ru(bpy)32+ + e− → Ru(bpy)3+); ② reduction of the coreactant at the cathode (S2O82− + e− → SO4•− + SO42−); ③ charge transfer reaction (Ru(bpy)3+ + SO4•− → Ru(bpy)32+* + SO42−); ④ light emission (Ru(bpy)32+* → Ru(bpy)2+ + hν). Here only the cathode of the device is shown.
Figure 1. (a) Schematic description of the operation of a DC-driven ECLD based on the annihilation ECL mechanism of Ru(bpy)32+: ① reduction of the ECL luminophore at the cathode (Ru(bpy)32+ + e− → Ru(bpy)3+); ② oxidation of the ECL luminophore at the anode (Ru(bpy)32+ → Ru(bpy)33+ + e−); ③ charge transfer reaction (Ru(bpy)3+ + Ru(bpy)33+ → Ru(bpy)32+* + Ru(bpy)32+); ④ light emission (Ru(bpy)32+* → Ru(bpy)32+ + hν). (b) Series of photographs of the model ECLD under a constant DC bias (3 V).
coreactant. For example, when oxalate (C2O42−), a benchmark coreactant for Ru(bpy)32+ ECL, is added into the ECL active layer, Ru(bpy)32+* can be produced near the anode of the device.58 Specifically, oxidation of both Ru(bpy)32+ and C2O42− occurs at the anode to yield Ru(bpy)33+ and CO2•−, respectively. The CO2•− radical anion then serves as the reducing agent to convert Ru(bpy)33+ to the excited Ru(bpy)32+*, which eventually releases light near the anode. The production of the excited species from a single electrode avoids the issues due to the slow transport of the reactants for ECL, thus resulting in relatively prompt light emission. We note that reduction of the luminophore may occur at the counter electrode (i.e., the cathode in the example above). However, the as-produced Ru(bpy)3+ would have to be transported through the bulk of the active layer to result in annihilation ECL, which would be slow and inefficient as described above. Therefore, the overall light emission for an ECL system including coreactants would be dominated by the coreactant pathway at the vicinity of one of the two electrodes (therefore, only the electrochemical reactions occurring at a single electrode are depicted in Figure 2). Another benefit of incorporating the coreactant into the ECL active layer is to reduce the operating voltage of the device,58 because the reduction (or oxidation) of the coreactant is typically attained at a lower electrode potential compared with that of the ECL luminophore. However, the weakness of this approach is that the redox reaction involving the coreactant is not fully reversible. Often, the reaction yields gaseous products such as CO2. Once the added coreactants are consumed, the coproduction of the reduced and oxidized species would be terminated. This indicates that attaining long-term operation of the ECLD by this approach is fundamentally restricted. A clever way to circumvent this problem is to drive the device with alternating voltages (Figure 3a). We refer to this type of device as an AC-driven ECLD. By altering the polarity of the voltage applied to the electrode, plenty of both the reduced and oxidized species of the ECL luminophore can be generated at the
though Ru(bpy)32+ is also an ionic species, its motion would be much slower than that of other ionic species, and therefore, it would weakly contribute to forming the EDL. In addition to the EDL formation, reduction and oxidation of the ECL luminophore can also occur at the two electrodes, particularly when the electric potential at the cathode/anode is larger than the reductive/oxidative potential of the luminophore. Consequently, Ru(bpy)3+ will be produced at the cathode and Ru(bpy)33+ will be generated at the anode. For the ECL to occur, the resulting Ru(bpy)3+ should encounter Ru(bpy)33+ within the active layer. This indicates that the mass transport of these electrochemically active species is critical. If one could assume that EDL formation and the charge transfer reaction at the interface dominantly compensate for the electric potential, the bulk of the active layer would remain charge-neutral. Then diffusion along the concentration gradient of Ru(bpy)3+ and Ru(bpy)33+ would be the main origin of their mass transport. Since this process is rather slow, the light emission from the ECLD would occur slowly after the device is biased. Also, either the Ru(bpy)3+ or Ru(bpy)33+ can be degraded during the long diffusion process before encountering the counterpart. Thus, the light emission under the application of a constant bias between the electrodes would be not only slow but also inefficient. We refer to this type of operation as DC-driven. Figure 1b displays a series of snapshots taken from a model DC-driven ECLD at 3 V. The images show that the emissive process of the device containing a 60 μm-thick active layer slowly evolves. One way to improve the device response of these ECLDs is to add coreactants into the active layer. In the presence of redox coreactants, both the reduced and oxidized species necessary for 269
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to reduced ECL intensity at higher frequencies. Overall, there is an optimal frequency that yields the maximal luminescence intensity for AC-driven ECLDs. Figure 3c shows the frequencydependent luminance behavior of the model ECLD collected at an alternating driving voltage of ±3 V. These unique characteristics of AC-driven ECLDs were exploited to design a frequencydependent color-tunable device, which will be introduced with more details in the following section. For all of the cases above, we note that the electrons and holes in the ECLD are delivered to the emission zone of the device by means of a convective process via the reduced and oxidized forms of the luminophore, respectively. This is fundamentally different from what occurs for the electrons and holes in LECs. The operation of LECs is explained by a combination of the following two mechanisms, which separately describe the device operation under two extremes of applied electric field. For LECs under a very low electric field, the presence of cations and anions constituting the EDL near the electrode assists the injection of electrons and holes, respectively (Figure 4a). Since the EDLs at Figure 3. Schematic description of the operation of an AC-driven ECLD based on the annihilation ECL mechanism of Ru(bpy)32+: ① reduction of the ECL luminophore (Ru(bpy)32+ + e− → Ru(bpy)3+); ② oxidation of the ECL luminophore (Ru(bpy)32+ → Ru(bpy)33+ + e−); ③ charge transfer reaction (Ru(bpy)3+ + Ru(bpy)33+ → Ru(bpy)32+* + Ru(bpy)32+); ④ light emission (Ru(bpy)32+* → Ru(bpy)32+ + hν). (b) Transient optical response of the model ECLD under an AC bias (500 Hz, ±3 V). The inset shows a photograph of the ECLD under operation. (c) Operational frequency-dependent luminance of the model ECLD (±3 V).
same electrode with a given time interval. In this way, the slow response of the device arising from the limited mass transport can be alleviated.59−61 Moreover, the entire annihilation ECL pathway (including the reduction and oxidation of the ECL luminophore as well as the charge transfer reaction and light emission process) are in principle recyclable. Therefore, the fundamental restriction arising from the continuous consumption of the reactants can be avoided. Figure 3b shows the transient optical response of the model ECDL applied with an alternating driving voltage of ±3 V (at 500 Hz). Once the polarity of the applied potential is altered, the ECL can occur within a millisecond. Also, the luminance of the device is more intense than that operated under a constant voltage, as can be verified from the photographic image in the figure. Interestingly, the ACdriven ECLD exhibits frequency-dependent luminescence, yielding a maximum intensity at a specific frequency; with increasing AC frequency, the ECL intensity initially increases, but it then peaks and eventually decreases.16 The initial increase in the ECL with increasing frequency in the low-frequency regime can be understood from the increased chance of the annihilation reaction between the reduced and oxidized species of the ECL luminophore near the electrode, since more reduced and oxidized species would be generated within a given time interval at increased AC frequencies. In the high-frequency regime, however, an increase in the operating frequency does not lead to an enhancement in the ECL. At such high frequencies, the EDL cannot be formed promptly on a time scale similar to the that of the AC cycle. Therefore, the effective potential at the electrodes is reduced, and the corresponding reduction and oxidation reactions of the ECL luminophores at the respective electrodes are not as effective as they were at low frequencies. This suppresses the rate of the annihilation reaction between the reduced and oxidized species of the ECL luminophores, leading
Figure 4. (a, b) Schematic descriptions of LEC operation based on (a) the ED model and (b) the ECD model. Conduction of electrons is depicted by blue arrows and that of holes by white arrows. (c) Evolution of the luminance (red) and the built-in driving voltage (black) for the model LEC with time under a constant current density of 1 mA/mm2. (d) Photograph of the model LEC under operation (6 V).
both electrodes mostly compensate for the applied electric potential, the potential within the bulk of the active layer is null, and the injected electrons and holes are transported via diffusion before they encounter each other to form excitons and generate light. This explanation is called the electrodynamic (ED) model.11,62,63 When LECs are under a high electric field, the cations and anions within the active layer lead to the growth of 270
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Table 1. Comparison of ECLD and LEC Characteristics and Experimental Data from Our Test Devices active layer composition (wt %) delivery of electrons and holes effective operation method response time max. intensity
ECLD
LEC
electrolyte (major) [EMIM][TFSI] (10) luminophore (minor) [Ru(bpy)3][(PF6)2] (1) convective transport (luminophores are used as a shuttle) AC 500 Hz, ±3 V short (s) 1 min strong >4000 cd/m2
the case in LECs, where only a small amount of electrolyte is added in a solid-state luminophore film; the electrolyte in LECs can be considered as an additive that promotes electrochemical processes to attain more efficient solid-state light emission. For LECs, the injected electrons and holes from the respective electrodes are transported within the active layer through sequential hopping processes between the neighboring luminophores before recombination. Although a few reports of ACdriven LECs are available,72,73 LECs are typically driven by a DC bias.
electrochemically doped n-type and p-type regimes in the active layer from the respective electrodes (Figure 4b). Light emission occurs in the intrinsic regime located between the n-type and ptype doped regimes. This explanation is called the electrochemical doping (ECD) model.9,12,64−66 A number of references are available to describe the two models.14,67,68 Whether the operation follows the ED model or the ECD model or even a combination of the two, the nature of the charge transport in the LECs is a conductive process. Sequential hopping of electrons and holes between neighboring luminophores is necessary along the concentration gradient or electric field, which is distinct from the convective charge transport in the ECLD described above. It can be viewed that the luminophores in the LECs are virtually immobile and provide conductive pathways for electrons and holes to hop. Since the electrical response of the cations and anions in the LECs is a slow process, the intensity of a typical LEC evolves with time. Figure 4c shows the transient response of the model LEC under a condition yielding a constant current density, which demonstrates that obtaining the maximal intensity of the LECs is delayed. Figure 4d shows a photograph of the model LEC after a bias of 6 V was applied to the device. We note that this slow device response has been resolved by employing a frozen junction approach,69−71 which includes (i) a prebiasing step to form a favorably distributed ion profile for LEC operation and (ii) a subsequent freezing step to fix the distribution of the ions before operating the device. This approach, however, is not applicable to ECLDs because the ions (not only the cations and anions of the electrolyte but also the reduced and oxidized forms of the ECL luminophore) should move convectively within the active layer for light emission. Table 1 summarizes the key features of ECLDs and LECs as well as the experimental results that were obtained from our test devices. The active layer of an ECLD (whether it is a liquid or geltype solid) contains more electrolyte than ECL luminophore. In such a case, electrons and holes are injected into the active layer through reduction and oxidation reactions at the respective electrodes, and the as-injected electrons and holes are transported within the active layer through convective processes, taking the ionic forms of the luminophore as a shuttle, before undergoing radiative recombination. Whether an ECLD contains a liquid-state active layer or a gel-type solid-state active layer, the significance of the convective charge transport of the luminophore should be emphasized, since the luminophore has to drift within the electrolytic medium for both types of active layer (see the Supporting Information). The slow device response due to the slow convective process can be circumvented by operating the device using an AC bias, and thus, recent devices with prompt response were driven in such a way.17,18,55 We comment that increasing the concentration of the luminophore may enhance the light intensity of an ECLD. However, the maximal concentration would be eventually limited by the solubility of the luminophore in the given electrolyte. This is not
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NEW OPPORTUNITIES FOR ECLDS
ECLDs provide unique opportunities to develop innovative light-emitting devices. First of all, exploiting the liquid nature of the ECL solution, deformable light-emitting devices could be simply fabricated by encapsulating the ECL solution between flexible substrates (Figure 5a).74 A prototype flexible device was
Figure 5. (a) Schematic description of a flexible ECLD containing a liquid emissive layer. Images were redrawn from ref 74. (b) (top) Schematic description of a microfluidic ECLD and demonstration of a multicolor ECLD using blue, yellow, and red ECL luminophores. (middle) Series of photographs showing the degradation of the microfluidic ECLD with time. (bottom) Series of photographs showing the recovery of the microfluidic ECLD after injection of a fresh ECL solution into the microfluidic device. Reprinted with permission from ref 42. Copyright 2014 Elsevier B.V. 271
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Figure 6. (a) Color tuning in an ECLD containing a mixture of two ECL luminophores (Ir(ppy)3 and [Ru(bpy)2(L)]2+) and a coreactant (TPA). Upon application of different electrode potentials, the mixture yields different colors based on different contributions of the two luminophores in luminescence. Reprinted with permission from ref 76. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) 3D diagram presenting the applied potential vs ECL emission relation for a color-tunable ECLD containing a mixture of three ECL luminophores ([Ru(bpy)2(dm-bpy-dc)]2+, Ir(ppy)3, and Ir(df-ppy)2(ptp)). Reprinted from ref 77. Copyright 2014 American Chemical Society. (c) Color tuning in an AC-driven ECLD containing a mixture of two ECL luminophores (rubrene (RUB) and diphenylanthracene (DPA)). Because different ECL luminophores have different optimal frequencies resulting in maximal light emission, the ECLD emitted different colors at different operating frequencies. Reproduced with permission from ref 16. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
from multiple luminophores would be attained, with its relative mixed ratio changing with the potential applied to the cell. For example, Hogan, Francis, and co-workers76,77 have demonstrated tuning of the emission color for an electrochemical cell containing a mixture of red ([Ru(bpy)3(L)]2+, where L = N4,N4′-bis((2S)-1-methoxy-1-oxopropan-2-yl)-2,2′-bipyridyl4,4′-dicarboxamide), green (Ir(ppy)3, where ppy = 2-phenylpyridine), and blue (Ir(df-ppy)2(ptp), where ptp = 2-(3-phenyl1H-1,2,4-triazol-5-yl)pyridinato) ECL luminophores with an added coreactant (TPrA). In the presence of a coreactant that could serve as the reducing agent, the onset of ECL upon increasing the electrode potential of the anode followed the order of the oxidation potentials of the respective luminophores. In their work, ECL from the green luminophore (the one with the lowest oxidation potential) was found at low applied potentials. As the applied potential was increased, ECL from the red and blue luminophores was sequentially added to yield mixed colors. Here the relative contributions of the emissions from the different luminophores were found to change with the potential applied to the cell. Therefore, different mixed colors of the three luminophores could be attained. We note that energy transfer between the different luminophores can also be attained when a mixture of ECL luminophores with the appropriate energy alignment is used as the emissive layer.17,47 Accordingly, controlling the energy transfer between multiple ECL luminophores provides an additional route to tune the color of the cell.
prepared by sealing an ECL solution based on rubrene (yellow), 9,10-diphenylanthracene (DPA) (blue), or Ru(bpy)3Cl2 (orange) with electrolyte between two flexible substrates with transparent electrodes. The resulting devices and their performance were tolerant against as much mechanical bending as the flexible substrate and electrodes could withstand. Moreover, one can utilize microfluidic channels to introduce different ECL solutions between transparent electrodes to yield multicolor microfluidic ECLDs (Figure 5b).42,75 These microfluidic devices can be continuously fed with fresh ECL solutions. Therefore, issues resulting from the degradation of the ECL luminophore upon undergoing repetitive redox reactions can be avoided. Perhaps precise mixing of multiple ECL solutions may be attainable with a microfluidic structure that would lead to attaining gradual tuning of emission colors from a given set of ECL luminophores emitting distinct colors. Active tuning of the color for the ECLD can be achieved by using a mixed solution of ECL luminophores as the emissive layer and controlling the potential applied to the electrodes. Because reduction/oxidation of a given ECL luminophore can occur only when a potential higher than its specific reduction/oxidation potential is applied, the ECL of different luminophores should onset at different electrode potentials (Figure 6a,b). At a low electrode potential, the ECL from the luminophore with the lowest onset potential would be selectively attained, whereas at a high electrode potential (high enough to attain luminescence from multiple ECL luminophores in the mixture), a mixed color 272
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Figure 7. Dual-functional ECLDs. (a) Reflective/emissive dual-mode display containing both electrochromic and ECL materials in a single cell. The device operates in its reflective mode under a DC bias and in its emissive mode under an AC bias. Reproduced with permission from ref 61. Copyright 2013 The Society of Photopolymer Science and Technology (SPST). (b) Energy storage/light-emitting dual-mode device containing an ECL active layer sandwiched between pseudocapacitive electrodes. The device operates in its energy storage mode under a DC bias and in its emissive mode under an AC bias. Reproduced with permission from ref 18. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Figure 8. Solid-state ECLDs with a gel-type active layer. (a) Gelation of the ECL active layer achieved using oxide nanoparticles as a filler. Reprinted with permission from ref 15. Copyright 2009 The Electrochemical Society. (b) Gelation of the ECL active layer achieved by exploiting a physically crosslinked self-assembled network of a triblock copolymer and demonstration of a patterned, flexible ECLD. Reprinted from ref 17. Copyright 2014 American Chemical Society. (c) Gelation of the ECL active layer achieved by exploiting a physically cross-linked self-assembled network of a block copolymer in ECL solution and demonstration of a sticker-type ECLD. Reprinted with permission from ref 55. Copyright 2016 Nature Publishing Group.
luminophores would emit different colors at different operating frequencies. For example, an AC-driven ECLD containing a mixture of DPA (blue) and rubrene (yellow) yielded purely yellow emission at high operating frequencies, whereas the device operating at low frequencies yielded white emission based on mixed luminescence from both the DPA and rubrene. In this work, color tuning was explained in terms of the frequency-
Color tuning can be also achieved from AC-driven ECLDs containing a mixture of ECL luminophores. Kobayashi et al.16 demonstrated that the color of a ECL luminophore mixture could be tuned by applying an AC voltage with different frequencies (Figure 6c). Because different ECL luminophores have different optimal frequencies resulting in maximal light emission, as discussed above, an ECLD with a mixture of ECL 273
DOI: 10.1021/acsphotonics.7b00864 ACS Photonics 2018, 5, 267−277
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sticker-type displays on various metallic substrates through simple transfer and lamination processes. Furthermore, the devices could retain their performance under largely bent conditions. Finally, ECLDs exhibit features that are highly suitable for the development of printed light sources. It is obvious that the ECL solution (or even the precursor of the ECL gel) can be applied to various solution processes, including printing methods. A prototype mask-printed ECLD has been demonstrated by Itoh15 using a gel-type active layer (Figure 9a). Moreover, the
dependent effective potential applied to the cell under an AC bias. As described above, the EDL under an AC bias may form on a time scale comparable to or even longer than that of the AC cycle. The effective potential at the electrode under such high operating frequencies would not be sufficient to promote the reduction/oxidation reactions at the electrode as it was under the low-frequency conditions. When multiple ECL luminophores are present in the active layer, the luminophore with a larger band gap (the offset between the oxidation and reduction potentials), i.e., DPA, could only luminesce at lower frequencies, whereas the other ECL luminophore with a smaller band gap, i.e., rubrene, could luminesce even at higher frequencies. Another interesting opportunity for the ECLDs is that they can be coupled with other electrochemical processes to design innovative multifunctional devices. Kobayashi et al.61 also demonstrated a dual reflective/emissive mode display by modifying one of the two electrodes of an ECLD with an electrochromic material (Figure 7a). The application of a DC bias operated the device in its reflective mode; under a DC bias, the color of the electrochromic electrode could be changed via reduction (or oxidation), while emission based on ECL did not occur because only the oxidative (or reductive) species of the luminophore could be generated at the emissive electrode. Meanwhile, the emissive mode of the device could be realized with an AC bias, under which both the reductive and oxidative species of the ECL luminophore could be generated at the emissive electrode. Such dual-mode devices can be utilized in advanced displays or smart windows that can also function as lighting. Meanwhile, Hong, Lee, and co-workers18 demonstrated a dual-function electrochemical device that can serve as a lightemitting device under AC-voltage driving conditions and as an energy storage device under DC-voltage driving conditions by using pseudocapacitive electrodes for ECLDs (Figure 7b). We note that the interesting concepts introduced above have been mostly demonstrated from devices containing a liquid component, i.e., the ECL solution. However, for practical lightemitting device applications of ECL, realizing a solid-state device platform is critical. A simple but powerful method to achieve this is by inserting an additive into the ECL active layer that leads to the formation of a gel network. By forming the gel, one can mostly preserve the electrochemical properties of the ECL solution while realizing the solid-state physical integrity of the device. For example, Itoh15,56 devised a gel comprising an emissive ECL solution (a mixture of [Ru(d8-bpy3)][PF6]2 and [AMIM][TFSI] (where [AMIM] = 1-allyl-3-butylimidazolium) and oxide nanoparticles as a gelation filler (Figure 8a). Because the given emissive gel did not contain any volatile organic solvents, degradation in the device performance associated with the removal of the solvent could be avoided. An alternative strategy to form a solid-state ECL device was suggested by Moon, Lodge, and Frisbie,17 who added a small amount (3 h. Finally, an Al electrode (150 nm) was thermally evaporated. (58) Moon, H. C.; Lodge, T. P.; Frisbie, C. D. DC-Driven, Sub-2 V Solid-State Electrochemiluminescent Devices by Incorporating Redox Coreactants into Emissive Ion Gels. Chem. Mater. 2014, 26, 5358−5364. (59) Nobeshima, T.; Nakamura, K.; Kobayashi, N. Reaction Mechanism and Improved Performance of Solution-Based Electrochemiluminescence Cell Driven by Alternating Current. Jpn. J. Appl. Phys. 2013, 52, 05DC18. (60) Nobeshima, T.; Morimoto, T.; Nakamura, K.; Kobayashi, N. Advantage of an AC-Driven Electrochemiluminescent Cell Containing a Ru(bpy)32+ Complex for Quick Response and High Efficiency. J. Mater. Chem. 2010, 20, 10630−10633. (61) Nobeshima, T.; Nakamura, K.; Kobayashi, N. Electrochemical Materials for Novel Light Emitting Device and Dual Mode Display. J. Photopolym. Sci. Technol. 2013, 26, 397−402. 276
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