Light-Emitting Devices Based on Electrochemiluminescence

Subscriber access provided by READING UNIV

Perspective

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 ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00864 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

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 156-743, Seoul, Korea

‡

These authors contributed equally to this work

* [email protected]

KEY WORDS Light-emitting device, electrochemiluminescence, light-emitting electrochemical cells, electrolyte, luminophore

ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Electrochemical processes can be exploited to operate light-emitting devices with unusual functionality. For example, light-emitting electrochemical cells (LEC) contain a small amount of electrolyte within the organic/polymer light-emitting active layer. The electrolytes in the active layer allow avoiding the multiple charge injection layers that are deemed critical for organic light-emitting diodes (OLEDs). Very recently, an alternative light-emitting device platform based on electrochemical processes was also suggested. These devices rely on electrochemiluminescence (ECL), a light-emission 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 device operation of the ECL device is often confused with that of LECs, even if they follow distinct working principles. This confusion is mainly because the active layer for both ECL devices and LECs contain light-emitting material and electrolyte material (even if their composition is 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 the ECL devices and inspire researchers to devise a novel light-emitting device platform, which is the primary goal of this article.


Page 2 of 35

Page 3 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Exploiting the 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 the electrochemistry and the 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 the electron from a semiconductor or equivalently as the injection of holes into the material. Dye-sensitized solar cells (DSSCs) are well-established photovoltaic devices that operate based on redox cycles of electrolytes in combination with the photo-excitation process of a dye.2 The development of DSSCs has opened alternating engineering principles for renewable energy technology and industry beyond the pn-junction-type semiconductor solar cells. Thin-film transistors that employ electrolyte gate dielectrics are another interesting class of devices that offers 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/de-doping processes of a semiconductor channel. More recently, these devices have been exploited in building the basic 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 LECs facilitates the injection of electrons and holes from their respective electrodes. Therefore, LECs offer a unique device platform that is highly suitable


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 35

to produce printed light sources while avoiding meticulous energy alignment issues between the constituting materials,9-14 unlike the case for organic light-emitting diodes (OLEDs) comprised of purely non-ionic, electronic components. Recently, an alternative light-emitting device platform for LECs was demonstrated, which 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 the electrochemically active species.19-22 In fact, ECL has been a widely used in analytical techniques, including diagnostics,23-26 environmental assays,27-29 or biowarfare agent detection.30 This is because ECL is extremely sensitive to the electrochemical environment of the analytic solution. However, ECL was only recently launched into a lightemitting 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 difference as well as the similarity between these two 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 article 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 difference with its sister device, i.e., LEC, in order to deliver new opportunities for ECLDs effectively.


Page 5 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Brief Description of the Two ECL Pathways Typically, ECL is classified into two categories. One is the annihilation ECL that involves the electron-transfer reaction between an oxidized 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, the annihilation ECL occurs when its reduced form (Ru(bpy)31+) and its oxidized form (Ru(bpy)33+) react and 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 the coreactant ECL that involves an additional contribution of a third species, the 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 between the oxidized Ru(bpy)33+ and the strongly reductive TPrA• or CO2•- generated from TPrA or C2O42-, respectively.32-35 Such a process is particularly known as an “oxidative-reductive” coreactant ECL. Likewise, the “reductiveoxidative” coreactant ECL also occurs if the reduced Ru(bpy)31+ 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, the 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

Active Layers for ECLD Regardless of the ECL type, 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


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ru(bpy)32+ is an archetypal ECL iTMC. Aromatic organic molecules, such as rubrene or diphenyl anthracene,16,

41,42

or conjugated polymers, such as polyphenylene vinylene

derivatives43,44 and polythiophenes45,46, can also yield ECL. Combinations of host-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 nanocrystals48-51, metal nanoclusters52, and carbon nanostructures53,54 have 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 the iTMC ECL luminophore, the luminophore and the counter ion 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 usage of the ECL oriented to electronic applications, which have traditionally preferred the solid-state device integrity of the devices. The new opportunities from the liquid active material as well as the recent methods to solidify the ECL active layer with the 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 the LECs, the sister light-emitting devices. Consequently, the distinct operating method of the two devices has been often overlooked. It should be pointed out that the relative composition between the luminophore and the electrolyte for the ECLDs and LECs is completely different. The major component of the active layer for the ECLDs is the electrolyte, whereas that for the LECs is the luminophore. Thus, the active layer of the ECLD


Page 6 of 35

Page 7 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

can be thought as a small number of ECL luminophores moving within a vast liquid body of electrolyte; the luminescence for 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 electrolytes is added into the active layer. The role of the electrolytes in the 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 conductive process through the emissive material, rather than the convective process. This difference in the composition causes huge distinctions in the device operation principles, features, and opportunities between 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 details.57 We emphasize that the active layer of both model devices contains 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-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI]). The active layer for the ECLD contained more electrolyte than luminophore

([Ru(bpy)3][PF6]2:[EMIM][TFSI] = 10:1 in wt%), whereas that for LEC

contained more luminophore than electrolyte ([Ru(bpy)3][PF6]2:[EMIM][TFSI] = 1:10 in wt%). The active layer for both devices was sandwiched between the metal and transparent oxide electrodes. By comparing the device characteristics of these sister devices, their operation could be clearly understood. Device Operation Principles


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The light emission process for a basic annihilation-type ECLD is first described (coreactants are not included in the active layer). 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]-, would form an electric double layer (EDL) at both the electrode/active layer interface to compensate for the electric potential generated by the field (Figure 1a). Even if Ru(bpy)32+ is also an ionic species, its motion would be much slower than that of other ionic species and therefore would weakly contribute to forming an EDL. In addition to the EDL formation, a reduction and oxidation of the ECL luminophore can also occur at both electrodes, particularly when the electric potential at the cathode/anode is larger than the reductive/oxidative potential of the luminophore. Consequently, Ru(bpy)31+ will be produced from the cathode, and Ru(bpy)33+ will be generated from the anode. For the ECL to occur, the resulting Ru(bpy)31+ 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 the electric potential is dominantly compensated by the EDL formation and the charge transfer reaction at the interfaces, the bulk of the active layer would remain charge-neutral. Then, diffusion along the concentration gradient of Ru(bpy)31+ and Ru(bpy)33+ would be the main origin of their mass transport. Since this process is a rather slow process, the lightemission from the ECLD would occur slowly after the device is biased. Also, either the Ru(bpy)31+ or Ru(bpy)33+ can be degraded along the long diffusion process before encountering the counterpart. Thus, the light emission under the application of a constant bias between the electrodes would not only be slow, but also inefficient. We refer to this type of operation as the DC-driven ECLD. 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 is slowly evolving.


Page 8 of 35

Page 9 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

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 the ECL can be co-produced from a single electrode (Figure 2a and 2b). We refer to this type of device as the DC-driven ECLD with a 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, the oxidation of both Ru(bpy)32+ and C2O42- occurs at the anode to yield Ru(bpy)33+ and CO2•-, respectively. The radical anion (CO2•-) 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 allows avoiding issues due to the slow transport of the reactants for ECL, thus resulting in a 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)31+ would have to transport the bulk of the active layer to result in the annihilation ECL, which would be slow and inefficient as described above. Therefore, the overall light emission for an ECL system included with coreactants would be dominated by the coreactant pathway at the vicinity of one of the two electrodes (thereby, only the electrochemical reactions occurring at a single electrode is depicted in Figure 2). Another benefit of employing 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 to 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 the long-term operation of the ECLD from this approach is fundamentally restricted.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 the AC-driven ECLD. By alternating the polarity of the voltages applied to the electrode, plentiful of both the reduced and oxidized species of ECL luminophore can be generated at the same electrode with a given timeinterval. In this way, the slow response of the device arising from the limited mass transport could be alleviated.59-61 Moreover, the entire annihilation ECL pathway (including the reduction and oxidation of 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 a ±3 V alternating driving voltage (at 500 Hz). Once the polarity of the applied potential is altered, the ECL could occur within a millisecond. Also, the luminance of the device is more intense than that operated under a constant voltage, which can be verified with photographic images in the figure. Interestingly, the AC-driven 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 an increasing frequency in a 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. At the high frequency regime, however, an increase in the operating frequency would not lead to an enhancement in the ECL. At such high frequencies, the EDL would not be formed promptly in a time scale similar to the AC cycle. Therefore, the effective potential at the electrodes would be reduced, and the corresponding reduction and oxidation reactions of the ECL luminophores at the respective electrodes would not be as effective as they were at low frequencies. This


Page 10 of 35

Page 11 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

suppresses the rate of the annihilation reaction between the reduced and oxidized species of the ECL luminophores, and it reduces the ECL intensity at higher frequencies. Overall, there will be an optimal frequency that yields the maximal luminescent intensity for the AC-driven ECLDs. Figure 3c shows the frequency-dependent luminance behavior of the model ECLD collected at ±3 V alternating driving voltages. These unique characteristics of the AC-driven ECLDs were exploited to design a frequency-dependent color tunable device, which will be introduced with more details in the following section. For all cases above, we note that the electrons and holes in the ECLD are delivered to the emission zone of the device based on 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 the LECs. The operation of the LECs is explained by a combination of the following two mechanisms, which in separate, describes the device operation under two extremes of applied electric field. For LECs under very low electric field, the presence of cations and anions constituting EDL near the electrode assists the injection of electrons and holes, respectively (Figure 4a). Since the applied electric potential is mostly compensated at the EDLs at both electrodes, the potential within the bulk of the active layer is null, and the injected electrons and holes are transported via diffusion before they encounter to form excitons and generate light. This explanation is referred to as 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 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 p-type doped regimes. This explanation is referred to as 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


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

conductive process. Sequential hopping of the electrons and holes between the 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 that they provide a conductive pathway 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 the typical LECs 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,69,70,71 which includes i) a pre-biasing step to form 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 the ECLD because the ions (not only the cations and anions of the electrolyte but also the reduced and oxidized forms of ECL luminophore) should move convectively within the active layer for light emission. Table 1 summarizes the key features of the ECLD and LECs as well as the experimental results that were obtained from our test devices. The active layer of an ECLD (whether it is liquid or gel-type 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 transport within the active layer through convective processes taking the ionic forms of the luminophore as a shuttle before resulting in the 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 Supporting Information).


Page 12 of 35

Page 13 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

The slow device response owing 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 by 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 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 for attaining more efficient solid state light emission. For LECs, the injected electrons and holes from the respective electrodes would transport within the active layer through sequential hopping processes between the neighboring luminophores before recombination. Although a few reports of AC-driven LECs are available72-73, LECs are typically driven by a DC bias.

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 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 mechanical bending as much as the flexible substrate and electrodes could withstand. Moreover, one can utilize microfluidic channels to introduce different ECL solutions between transparent electrodes to yield multi-color microfluidic ECLDs (Figure 5b).42,75 These microfluidic devices can be continuously fed with fresh ECL


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

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 electrodes. Because reduction/oxidation of a given ECL luminophore can occur only when a potential larger 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 least onset potential would be selectively attained. Whereas at a high electrode potential (large enough to attain luminescence from multiple ECL luminophores in the mixture), a mixed color from multiple luminophores would be attained with its relative mixed ratio changing with the potential applied to the cell. For example, Hogan and Francis et al.76,77 have demonstrated tuning the emission color for an electrochemical cell containing a mixture of red ([Ru(bpy)3(L)]2+, where L=N4,N4’-bis((2S)-1methoxy-1-oxopropan-2-yl)-2,2’-bipyridyl-4,4’-dicarboxamide),

green

(Ir(ppy)3,

where

ppy=2-phenylpyridine), and blue (Ir(df-ppy)2(ptp), where ptp=2-(3-phenyl-1H-1,2,4-triazol5-yl) pyridinato) ECL luminophores added with correactant (TPrA). In the presence of a coreactant that can serve as the reducing agent, the onset of ECL upon increasing the electrode potential of the anode followed the order of the oxidative potential of the respective luminophores. In their work, ECL from the green luminophores (the one with the lowest oxidative 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 color. Here, the relative contribution of the emission from the different luminophores was


Page 15 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

found to change with the potential applied to the cell. Therefore, different mixed colors of three luminophores could be attained. We note that the energy transfer between the different luminophores can be also 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. Color tuning can be also achieved from AC-driven ECLD containing a mixture of ECL luminophores. Kobayashi et al.16 demonstrated that the color of the 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 the maximal light emission, as discussed above, an ECLD with a mixture of ECL 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 a purely yellow emission at high operating frequencies, whereas the device operating at low frequencies yielded a white emission based on mixed luminescence from both the DPA and rubrene. In this work, color tuning was explained based on the frequency-dependent effective potential applied to the cell under an AC bias. As described above, the EDL under an AC bias may form in a time scale comparable or even slower 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 in the low-frequency conditions. When multiple ECL luminophores are present in the active layer, the luminophore with a larger bandgap (the offset between the oxidation and reduction potential), i.e., the DPA, could only luminesce at lower frequencies, whereas the other ECL luminophores with a smaller bandgap, i.e., the rubrene, could luminesce even at higher frequencies.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Another interesting opportunity for the ECLDs is that they can be coupled with other electrochemical processes to design innovative multi-functional 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, because, under an AC bias, both the reductive and the 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 and Lee et al.18 demonstrated a dual function electrochemical device that can serve as a light-emitting device under AC-voltage driving conditions and as an energy storage device under a DC-voltage driving condition by using pseudo-capacitive electrodes for ECLDs (Figure 7b). We note that the interesting concepts introduced above have been mostly demonstrated from a device containing liquid component, i.e., the ECL solution. However, for practical light-emitting 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, Itoh et al.15, 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


Page 16 of 35

Page 17 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

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 Lodge and Frisbie et al.17 The group added a small amount ( 3hr. Finally, Al electrode (150 nm) was thermally evaporated. 58. H. C. Moon; T. P. Lodge; C. D. Frisbie, DC-Driven, Sub-2 V Solid-State Electrochemiluminescent Devices by Incorporating Redox Coreactants into Emissive Ion Gels. Chem. Mater. 2014, 26, 5358-5364. 59. T. Nobeshima; K. Nakamura; N. Kobayashi, Reaction Mechanism and Improved Performance of Solution-Based Electrochemiluminescence Cell Driven by Alternating Current. Jpn. J. Appl. Phys. 2013, 52, 05DC18. 60. T. Nobeshima; T. Morimoto; K. Nakamura; N. Kobayashi, Advantage of an ACDriven Electrochemiluminescent Cell Containing a Ru(bpy)32+ Complex for Quick Response and High Efficiency. J. Mater. Chem. 2010, 20, 10630-10633. 61. T. Nobeshima; K. Nakamura; N. Kobayashi, Electrochemical Materials for Novel Light Emitting Device and Dual Mode Display. J. Photopolym. Sci. Technol. 2013, 26, 397402. 62. J. Halls; S. Graham; N. Tessler; R. Friend, Electric Field Distribution in Polymer Light-emitting Electrochemical Cells. Phys. Rev. Lett. 2000, 85, 421. 63. N. Tessler; S. Graham; R. Friend, Ionic Space-Charge Effects in Polymer LightEmitting Diodes. Phys. Rev. B 1998, 57, 12951-12963. 64. M. Buda; G. Kalyuzhny; A. J. Bard, Thin-Film Solid-State Electroluminescent Devices Based on tris (2, 2 ‘-Bipyridine) Ruthenium (II) Complexes. J. Am. Chem. Soc. 2002, 124, 6090-6098. 65. Q. Pei; Y. Yang; G. Yu; C. Zhang; A. J. Heeger, Polymer Light-emitting Electrochemical Cells: in situ Formation of a Light-Emitting p−n Junction. J. Am. Chem. Soc. 1996, 118, 3922-3929. 66. H. Rudmann; S. Shimada; M. F. Rubner, Operational Mechanism of Light-Emitting Devices Based on Ru (II) Complexes: Evidence for Electrochemical Junction Formation. J. Appl. Phys. 2003, 94, 115-122. 67. S. van Reenen; P. Matyba; A. Dzwilewski; R. A. Janssen; L. Edman; M. Kemerink, A Unifying Model for the Operation of Light-emitting Electrochemical Cells. J. Am. Chem. Soc. 2010, 132, 13776-13781. 68. T. Hu; L. He; L. Duan; Y. Qiu, Solid-State Light-emitting Electrochemical Cells Based on Ionic Iridium (III) Complexes. J. Mater. Chem. 2012, 22, 4206-4215. 69. J. Gao; G. Yu; A. J. Heeger, Polymer Light-emitting Electrochemical Cells with Frozen pin Junction. Appl. Phys. Lett. 1997, 71, 1293-1295. 70. J.-H. Shin; S. Xiao; Å. Fransson; L. Edman, Polymer Light-emitting Electrochemical Cells: Frozen-Junction Operation of an “Ionic liquid” Device. Appl. Phys. Lett. 2005, 87, 043506-043509.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

71. S. Tang; L. Edman, On-Demand Photochemical Stabilization of Doping in Lightemitting Electrochemical Cells. Electrochim. Acta 2011, 56, 10473-10478. 72. C. Yin; Y.-z. Zhao; C.-z. Yang; S.-y. Zhang, Single-Ion Transport Light-Emitting Electrochemical Cells: Designation and Analysis of the Fast Transient Light-Emitting Responses. Chem. Mater. 2000, 12, 1853-1856. 73. J. D. Slinker; J. Rivnay; J. A. DeFranco; D. A. Bernards; A. A. Gorodetsky; S. T. Parker; M. P. Cox; R. Rohl; G. G. Malliaras; S. Flores-Torres; H. D. Abruña, Direct 120V, 60Hz Operation of an Organic Light Emitting Device. J. Appl. Phys. 2006, 99, 074502. 74. R. Okumura; S. Takamatsu; E. Iwase; K. Matsumoto; I. Shimoyama In Solution Electrochemiluminescent Microfluidic Cell for Flexible and Stretchable Display, IEEE Int. Conf. Micro Electro Mech. Syst., 22nd, IEEE: 2009; pp 947-950. 75. T. Kasahara; S. Matsunami; T. Edura; R. Ishimatsu; J. Oshima; M. Tsuwaki; T. Imato; S. Shoji; C. Adachi; J. Mizuno, Multi-Color Microfluidic Electrochemiluminescence Cells. Sens. Actuators, A 2014, 214, 225-229. 76. E. H. Doeven; E. M. Zammit; G. J. Barbante; C. F. Hogan; N. W. Barnett; P. S. Francis, Selective Excitation of Concomitant Electrochemiluminophores: Tuning Emission Color by Electrode Potential. Angew. Chem. 2012, 124, 4430-4433. 77. E. H. Doeven; G. J. Barbante; E. Kerr; C. F. Hogan; J. A. Endler; P. S. Francis, Red– Green–Blue Electrogenerated Chemiluminescence Utilizing a Digital Camera as Detector. Anal. Chem. 2014, 86, 2727-2732.


Page 24 of 35

Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Table 1. Comparison of ECLD and LEC characteristics and experimental data from our test devices. ECLD Active layer composition (wt%)

Delivery of electrons and holes

Electrolyte (major) Luminophore (minor)

LEC

[EMIM][TFSI] (10) [Ru(bpy)3][(PF6)2] (1)

Convective transport (luminophores are used as shuttle)

Electrolyte (minor) Luminophore (major)

[EMIM][TFSI] (1) [Ru(bpy)3][(PF6)2] (10)

Conductive transport (sequential hopping occurs between neighboring luminophores)

Effective operation method

AC

500 Hz, ±3V

DC

6V

Response time

Short (s)

1 min

Max. intensity

Weak

700 cd/m2

Strong

>4000 cd/m2


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) Schematic description of a DC-driven ECLD operation based on 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+ – e- → Ru(bpy)33+); 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) A series of photographs of the model ECLD under a constant DC bias (3 V).


Page 26 of 35

Page 27 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 2. (a) Schematic description of a DC-driven ECLD operation based on 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+ – e- → Ru(bpy)33+); ② Oxidation of the coreactant at the anode (C2O42- – 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). Note that only the anode of the device is shown. (b) Operation of a DC-driven ECLD based on 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). Note that only the cathode of the device is shown.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Schematic description of an AC-driven ECLD operation based on 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+ – e- → Ru(bpy)33+); 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).


Page 28 of 35

Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 4. Schematic description of the LEC operation based on ED model (a) and ECD model (b). Conduction of electrons is depicted in blue arrows and that of holes is depicted in 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) A photograph of the model LEC under operation (6 V).


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) Schematic description of a flexible ECLD containing a liquid emissive layer. Images were redrawn from Ref. 72. (b) Schematic description of a microfluidic ECLD. Demonstration of multicolor ECLD using blue, yellow, and red ECL luminophores. A series of photographs showing the degradation of the microfluidic ECLD with time (upper). A series of photographs showing the recovery of the microfluidic ECLD after injecting a fresh ECL solution into the microfluidic device. (lower). Reprinted with permission from Ref. 42. Copyright 2014 Elsevier B.V. All rights reserved.


Page 30 of 35

Page 31 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

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 correactant (TPA). By applying different electrode potentials, the mixture yields different colors based on different contributions of the two luminophores in luminescence. Reprinted with permission from Ref. 75. Copyright 2012 American Chemical Society. (b) A 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, Ir(df-ppy)2(ptp)). Reprinted with permission from Ref. 76. Copyright 2014 American Chemical Society. (c) Color tuning in an AC-driven ECLD containing a mixture of two ECL luminophores (rubrene (RUB) and DPA (diphenyl antracene). Because different ECL luminophores have different optimal frequencies resulting in the maximal light emission, the ECLD emitted different colors at different operating frequencies. Reproduced with permission of Ref. 16. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Dual-functional ECLDs. (a) A reflective/emissive dual-mode display containing both the electrochromic and ECL materials in a single cell. Copyright 2013 The Society of Photopolymer Science and Technology (SPST). The device operates in its reflective mode under a DC bias and in its emissive mode under an AC bias. (b) An energy storage/light-emitting dual-mode device containing an ECL active layer sandwiched between pseudo-capacitive 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 of Ref. 18. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Page 32 of 35

Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 8. Solid-state ECLDs with a gel-type active layer. (a) Gelation of ECL active layer achieved from oxide nanoparticles as filler. Reprinted with permission from Ref 15. Copyright 2009, The Electrochemical Society. (b) Gelation of ECL active layer achieved by exploiting a physical-crosslinked self-assembled network of triblockcopolymer. Demonstration of a patterned, flexible ECLD. Reprinted with permission from Ref. 17. Copyright 2014 American Chemical Society. (c) Gelation of ECL active layer achieved by exploiting a physicalcrosslinked self-assembled network of block-copolymer in ECL solution. Demonstration of a sticker-type ECLD. Reprinted with permission of Ref. 56. Copyright 2016 Nature Publishing Group.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

.

Figure 9. (a) A metal mask-printed ECLD. Schemes are redrawn from Ref. 15. The photograph is reprinted with permission from Ref. 15. Copyright 2009, The Electrochemical Society. (b) An ECLD in an unconventional parallel-electrode structure. As a proof-of-concept, an ECL solution (a mixture of [Ru(bpy)3][(PF6)2] and [EMIM[TFSI]) was dropped between two parallel ITO electrodes separated by 1.5 mm. Under an AC bias (60 Hz, ±5 V), light emission at the edge of the two electrodes could be clearly observed.


Page 34 of 35

Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

TOC


Light-Emitting Devices Based on Electrochemiluminescence

Recommend Documents