Recent advances of organic emitters in deep-red light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) are kind of easily fabricated and low-cost light-emitting devices that can efficiently convert electric power to light energy. Compared with blue and green LECs, the performance of deep-red LECs is limited by the high non-radiative rate of emitters in long-wavelength region. While various organic emitters with deep-red emission have been developed to construct high-performance LECs, including polymers, metal complexes, and organic luminous molecules (OLMs), but this is seldom summarized. Therefore, we overview the recent advances of organic emitters with emission at the deep-red region for LECs, and specifically highlight the molecular design approach and electrochemiluminescence performance. We hope that this review can act as a reference for further research in designing high-performance deep-red LECs.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

Tuning the UV/Vis Absorption Spectra of Electrochromic Small Molecular Radicals Through Bridge Modulation

Studies on the optical properties of donor-bridge-acceptor (DBA) materials in their radical anion state are rare but important. Such investigations can help to extend the application of DBA materials to opto-electrochemical devices and no longer limit them to optical physics research. In this work, a series of new DBA materials, TACzs, for overcoming this limitation are reported. All TACzs show strong intramolecular charge transfer (ICT) in their photoexcited states, leading to noticeable solvatochromism. Besides, the electronic structures of their radical anions show great variability, displaying different absorption spectra and diverse colors. Moreover, the potential application of TACzs as electrochromic and electro-fluorochromic materials are discussed. This work demonstrates that manipulating the π bridge between the donor and acceptor in the DBA system is an effective pathway not only to tailor the ICT properties of materials in their neutral state, but also to tune the absorption characteristics of their radical anion state, which makes them very promising for applications in electroluminescent and electrochemical devices.

Simple luminescent phenanthroimidazole emitters for solution-processed non-doped organic light-emitting electrochemical cells

Organic luminescent materials with leveraging properties have attracted urgent demand for their commercial application in lighting devices.

Strong Doping and Electroluminescence Realized by Fast Ion Transport through a Planar Polymer/Polymer Interface in Bilayer Light-Emitting Electrochemical Cells

Bilayer light-emitting electrochemical cells are demonstrated with a top conjugated polymer (CP) emitting layer and a solid polymer electrolyte (SPE) underlayer. Fast, long-range ion transport through the planar CP/SPE interface leads to doping and junction electroluminescence in the CP layer. All bilayer cells have pairs of aluminum electrodes separated by 2 or 11 mm at their inner edges, creating the largest planar (lateral) cells that can be imaged with excellent temporal and spatial resolutions. To understand how in situ electrochemical doping occurs in the CP layer without any ionic species mixed in, the planar bilayer cells are investigated for different CPs, CP layer thickness, operating voltage, and operating temperature. The bilayer cells are much faster to turn on than control cells made from a single mixed CP/SPE layer. The cell current and the doping propagation speed exhibit a linear dependence on the operating voltage and an Arrhenius-type temperature dependence. Unexpectedly, long-range ion transport in the CP layer and across the CP/SPE interface does not impede the doping reactions. Instead, the doping reactions are limited by the bulk resistance of the extra-wide SPE underlayer. In bilayer cells with a thin red-emitting CP layer, ion transport and doping reactions can penetrate the entire CP layer in the vertical direction. In thicker MEH-PPV or the blue-emitting cells, the doping did not reach the top of the CP layer. This led to broadened emitting junctions and/or unexpected junction locations. The bilayer LECs offer unique opportunities to investigate the ion transport in pristine CPs, the CP/SPE interface, and the SPE using highly sensitive and reliable imaging techniques. Removing the inert electrolyte polymer from the semiconducting CP can potentially lead to high-performance electrochemical light-emitting/photovoltaic cells or transistors.

Revealing the Impact of Heat Generation Using Nanographene-Based Light-Emitting Electrochemical Cells

Self-heating in light-emitting electrochemical cells (LECs) has been long overlooked, while it has a significant impact on (i) device chromaticity by changing the electroluminescent band shape, (ii) device efficiency because of thermal quenching and exciton dissociation reducing the external quantum efficiency (EQE), and (iii) device stability because of thermal degradation of excitons and eliminate doped species, phase separation, and collapse of the intrinsic emitting zone. Herein, we reveal, for the first time, a direct relationship between self-heating and the early changes in the device chromaticity as well as the magnitude of the error comparing theoretical/experimental EQEs-that is, an overestimation error of ca. 35% at usual pixel working temperatures of around 50 °C. This has been realized in LECs using a benchmark nanographene-that is, a substituted hexa-peri-hexabenzocoronene-as an emerging class of emitters with outstanding device performance compared to the prior art of small-molecule LECs-for example, luminances of 345 cd/m² and EQEs of 0.35%. As such, this work is a fundamental contribution highlighting how self-heating is a critical limitation toward the optimization and wide use of LECs.

Bright, stable, and efficient red light-emitting electrochemical cells using contorted nanographenes

This work rationalizes, for the first time, the electroluminescent behavior of a representative red-emitting contorted nanographene -i.e., hexabenzoovalene derivative - in small molecule light-emitting electrochemical cells (SM-LECs). This new emitter provides devices with irradiances of ca. 220 μW cm^-2 (242 cd m^-2), external quantum efficiencies (EQE) of 0.78% (<25% loss of the maximum theoretical EQE), and stabilities over 200 h. Upon optimizing the device architecture, the stability increased up to 3600 h (measured) and 13 000 h (extrapolated) at a high brightness of ca. 30 μW cm^-2 (34 cd m^-2). This represents a record stability at a high brightness level compared to the state-of-the-art SM-LECs (1000 h at 0.3 μW cm^-2). In addition, we rationalized one of the very rare LEC examples in which the changes of the electroluminescence band shape relates to the dependence of the relative intensity of the vibrational peaks with electric field, as corroborated by dynamic electrochemical impedance spectroscopy assays. Nevertheless, this exclusive electroluminescence behavior does not affect the device color, realizing one of the most stable, bright, and efficient red-emitting SM-LECs up to date.

Oligomer Electrolytes for Light-Emitting Electrochemical Cells: Influence of the End Groups on Ion Coordination, Ion Binding, and Turn-on Kinetics

The electrolyte is an essential constituent of the light-emitting electrochemical cell (LEC), since its operating mechanism is dependent on the redistribution of mobile ions in the active layer. Recent developments of new ion transporters have yielded high-performance devices, but knowledge about the interactions between the ionic species and the ion transporters and the influence of these interactions on the LEC performance is lacking. We therefore present a combined computational and experimental effort that demonstrates that the selection of the end group in a star-branched oligomeric ion transporter based on trimethylolpropane ethoxylate has a paramount influence on the ionic interactions in the electrolyte and thereby also on the performance of the corresponding LECs. With hydroxyl end groups, the cation from the salt is strongly coordinated to the ion transporter, which leads to suppression of ion pairing, but the penalty is a hindered ion release and a slow turn-on for the LEC devices. With methoxy end groups, an intermediate coordination strength is seen together with the formation of contact ion pairs, but the LEC performance is very good with fast turn-on. Using a series of ion transporters with alkyl carbonate end groups, the ion transporter:cation coordination strength is lowered further, but the turn-on kinetics are slower than what is seen for devices comprising the methoxy end-capped ion transporter.

Dynamic Bipolar Electrode Array for Visualized Screening of Electrode Materials in Light-Emitting Electrochemical Cells

Charge injection at a metal/semiconductor interface is of paramount importance for many chemical and physical processes. The dual injection of electrons and holes, for example, is necessary for electroluminescence in organic light-emitting devices. In an electrochemical cell, charge transfer across the electrode interface is responsible for redox reactions and Faradic current flow. In this work, we use polymer light-emitting electrochemical cells (PLECs) to visually assess the ability of metals to inject electronic charges into a luminescent polymer. Silver, aluminum, and gold microdisks are deposited between the two driving electrodes of the PLEC in the form of a horizontal array. When the PLEC is polarized, the individual disks functioned as bipolar electrodes (BPEs) to induce redox p- and n-doping reactions at their extremities, which are visualized as strongly photoluminescence-quenched growth in the luminescent polymer. The three metals initially generate highly distinct doping patterns that are consistent with differences in their work function. Over time, the doped regions continue to grow in size. Quantitative analysis of the n/p area ratio reveals an amazing convergence to a single value for all 39 BPEs, regardless of their metal type and large variation in the size of individual doped areas. We introduce the concept of a dynamic BPE, which transforms from an initial metal disk of a fixed size to one that is a composite of p- and n-doped polymer joined by the initial metallic BPE. The internal structure of the dynamic BPE, as measured by the n/p area ratio, reflects the properties of only the mixed conductor of the PLEC active layer itself when the area ratio converges.

Structures, photoresponse properties and DNA binding abilities of 4-(4-pyridinyl)-2-pyridone salts

Three salts [perchlorate (2), chloride (3) and tetrafluoroborate (4)] were synthesized from a 1-(2-aminoethyl)-6-hydroxy-2-oxo-1,2-dihydro-[4,4′-bipyridine]-3,5-dicarbonitrile compound (1) and characterized by spectroscopic and single crystal X-ray diffraction methods. Various noncovalent interactions (e.g., anion⋯π⁺, π⋯π, lp⋯π) are explored in the solid state crystal structure of the salts. Optical band gaps of all the four compounds were determined from their solid-state UV-vis spectrum. Electrical properties like electrical conductivity, photosensitivity, etc. were calculated and the results revealed that they have potential to act as optoelectronic devices. The values of the electrical parameters increase several times when they are exposed to visible light rather than in dark conditions. The light sensing properties of the salts (2–4) are enhanced compared to that of the mother organic compound 1 but the magnitude of this enhancement is not same for the three salts. This observation has been rationalized by theoretical considerations. Moreover, the DNA binding ability of one of the representative salts (compound 2) was examined to check the biological importance of the synthesized salts.

The optical band gap energies in several 2-pyridone derivatives have been measured using solid state UV to explore their semiconductor behavior. The electric current measurements for the four compounds exhibit enhanced photoconduction properties under irradiation of light.

Polypyridyl ligands as a versatile platform for solid-state light-emitting devices

A comprehensive review of tuneable polypyridine complexes as the emissive components of OLED and LEC devices is presented, with a view to bridging the gap between molecular design and commercialization.

Luminescent copper(i) complexes with bisphosphane and halogen-substituted 2,2'-bipyridine ligands

Heteroleptic [Cu(P^P)(N^N)][PF6] complexes, where N^N is a halo-substituted 2,2'-bipyridine (bpy) and P^P is either bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) have been synthesized and investigated. To stabilize the tetrahedral geometry of the copper(i) complexes, the steric demands of the bpy ligands have been increased by introducing 6- or 6,6'-halo-substituents in 6,6'-dichloro-2,2'-bipyridine (6,6'-Cl2bpy), 6-bromo-2,2'-bipyridine (6-Brbpy) and 6,6'-dibromo-2,2'-bipyridine (6,6'-Br2bpy). The solid-state structures of [Cu(POP)(6,6'-Cl2bpy)][PF6], [Cu(xantphos)(6,6'-Cl2bpy)][PF6]·CH2Cl2, [Cu(POP)(6-Brbpy)][PF6] and [Cu(xantphos)(6-Brbpy)][PF6]·0.7Et2O obtained from single crystal X-ray diffraction are described including the pressure dependence of the structure of [Cu(POP)(6-Brbpy)][PF6]. The copper(i) complexes with either POP or xantphos and 6,6'-Cl2bpy, 6-Brbpy and 6,6'-Br2bpy are orange-to-red emitters in solution and yellow-to-orange emitters in the solid state, and their electrochemical and photophysical properties have been evaluated with the help of density functional theory (DFT) calculations. The emission properties are strongly influenced by the substitution pattern that largely affects the geometry of the emitting triplet state. [Cu(POP)(6,6'-Cl2bpy)][PF6] and [Cu(xantphos)(6,6'-Cl2bpy)][PF6] show photoluminescence quantum yields of 15 and 17%, respectively, in the solid state, and these compounds were tested as luminophores in light-emitting electrochemical cells (LECs). The devices exhibit orange electroluminescence and very short turn-on times (<5 to 12 s). Maximum luminance values of 121 and 259 cd m-2 for [Cu(POP)(6,6'-Cl2bpy)][PF6] and [Cu(xantphos)(6,6'-Cl2bpy)][PF6], respectively, were achieved at an average current density of 100 A m-2. External quantum efficiencies of 1.2% were recorded for both complexes.

A Versatile and Simple Approach to Generate Light Emission in Semiconductors Mediated by Electric Double Layers

The light-emitting device is the primary device for current light sources. In principle, conventional light-emitting devices need heterostructures and/or intentional carrier doping to form a p-n junction. This junction formation is, however, very difficult to achieve for most emerging semiconductors, and the fabrication of light-emitting devices is invariably a significant challenge. This study proposes a versatile and simple approach to realize light-emitting devices. This proposed device requires only a semiconducting film with two electrodes that are covered with an electrolyte. This unique structure achieves light emission at a voltage slightly larger than the bandgap energy of materials. This study applies this concept to emerging direct bandgap semiconductors, such as transition metal dichalcogenide monolayers and zinc oxide single crystals. These devices generate obvious light emission and provide sufficient evidence of the formation of a dynamic p-i-n junction or tunneling junction, presenting a versatile technique to develop optoelectronic devices.

Time-Resolved Chemical Mapping in Light-Emitting Electrochemical Cells

An understanding of the doping and ion distributions in light-emitting electrochemical cells (LECs) is required to approach a realistic conduction model which can precisely explain the electrochemical reactions, p-n junction formation, and ion dynamics in the active layer and to provide relevant information about LECs for systematic improvement of function and manufacture. Here, Fourier-transform infrared (FTIR) microscopy is used to monitor anion density profile and polymer structure in situ and for time-resolved mapping of electrochemical doping in an LEC under bias. The results are in very good agreement with the electrochemical doping model with respect to ion redistribution and formation of a dynamic p-n junction in the active layer. We also physically slow ions by decreasing the working temperature and study frozen-junction formation and immobilization of ions in a fixed-junction LEC device by FTIR imaging. The obtained results show irreversibility of the ion redistribution and polymer doping in a fixed-junction device. In addition, we demonstrate that infrared microscopy is a useful tool for in situ characterization of electroactive organic materials.

σ-Hammett parameter: a strategy to enhance both photo- and electro-luminescence features of heteroleptic copper(i) complexes

Hammett was also right for devices. This work directly links the enhancement of both the photoluminescence properties in solid-state and the electroluminescence features in light-emitting electrochemical cells (LECs) with a rational ligand design using the σ-Hammett parameter.

Phenothiazine based blue emitter for light-emitting electrochemical cells

Electroluminescence of the sulfone form of phenothiazine derivative was tuned to blue emission from green emission in the sulfide form.

Light-Emitting Electrochemical Cells: A Review on Recent Progress

The light-emitting electrochemical cell (LEC) is an area-emitting device, which features a complex turn-on process that ends with the formation of a p-n junction doping structure within the active material. This in-situ doping transformation is attractive in that it promises to pave the way for an unprecedented low-cost fabrication of thin and light-weight devices that present efficient light emission at low applied voltage. In this review, we present recent insights regarding the operational mechanism, breakthroughs in the development of scalable and adaptable solution-based methods for cost-efficient fabrication, and successful efforts toward the realization of LEC devices with improved efficiency and stability.

Large Size Color-tunable Electroluminescence from Cationic Iridium Complexes-based Light-emitting Electrochemical Cells

Solution-processable light-emitting electrochemical cells (LECs) with simple device architecture have become an attractive candidate for application in next generation lighting and flat-panel displays. Herein, single layer LECs employing two cationic Ir(III) complexes showing highly efficient blue-green and yellow electroluminescence with peak current efficiency of 31.6 cd A(-1) and 40.6 cd A(-1), respectively, have been reported. By using both complexes in the device, color-tunable LECs with a single spectral peak in the wavelength range from 499 to 570 nm were obtained by varying their rations. In addition, the fabrication of efficient LECs was demonstrated based on low cost doctor-blade coating technique, which was compatible with the roll to roll fabrication process for the large size production. In this work, for the first time, 4 inch LEC devices by doctor-blade coating were fabricated, which exhibit the efficiencies of 23.4 cd A(-1) and 25.4 cd A(-1) for the blue-green and yellow emission, respectively. The exciting results indicated that highly efficient LECs with controllable color could be realized and find practical application in large size lighting and displays.

Emissive Polyelectrolytes As Interlayer for Color Tuning and Electron Injection in Solution-Processed Light-Emitting Devices

Herein we present a solution-processed hybrid device architecture combining organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) in a bilayer architecture. The LEC interlayer promotes the charge injection from an air-stable Ag cathode as well as permits the color tuning of the device emission. To this end, we used an alcohol-soluble anionic polyfluorene derivative, the properties of which were investigated by absorption and photoluminescence spectroscopy as well as by cyclic voltammetry. The bilayer device exhibited operating voltages ∼6 V and a color tuning of the emission spectrum dependent on the LEC interlayer thickness. The hybrid devices presented a color emission ranging from the yellow (x = 0.39, y = 0.47) toward the green region (x = 0.29, y = 0.4) of the Commission Internationale de I'Eclairage (CIE) 1931 chromaticity diagram.

Doping Evolution and Junction Formation in Stacked Cyanine Dye Light-Emitting Electrochemical Cells

Cyanine dyes are fluorescent organic salts with intrinsic conductivity for ionic and electronic charges. Recently ( J. Am. Chem. Soc. 2013 , 135 , 18008 - 18011 ), these features have been exploited in cyanine light-emitting electrochemical cells (LECs). Here, we demonstrate that stacked, constant-voltage driven trimethine cyanine LECs with various counteranions develop a p-i-n junction that is composed of p- and n-doped zones and an intrinsic region where light-emission occurs. We introduce a method that combines spectral photocurrent response measurements with optical modeling and find that at maximum current the intrinsic region is centered at ∼37% away from the anode. Transient capacitance, photoluminescence and attenuance experiments indicate a device situation with a narrow p-doped region, an undoped region that occupies ∼72% of the dye layer thickness and an n-doped region with a maximum doping concentration of 0.08 dopant/cyanine molecule. Finally, we observe that during device relaxation the parent cyanines are not reformed. We ascribe this to irreversible reactions between doped cyanine radicals. For sterically conservative cyanine dyes, this suggests that undesired radical decomposition pathways limit the LEC long-term stability in general.

Peripheral halo-functionalization in [Cu(N^N)(P^P)]+ emitters: influence on the performances of light-emitting electrochemical cells

Trends in the performance data of [Cu(N^N)(P^P)]⁺-based LECs in which N^N ligands bear peripheral F, Cl, Br or I substituents reveal that fluoro-groups are beneficial, but heavier halo-substituents lead to poor devices.

Benefits of using BODIPY–porphyrin dyads for developing deep-red lighting sources

The syntheses, as well as the photophysical and electrochemical characterization, of two novel BODIPY–porphyrin dyads and their first application in lighting schemes are provided.

Non-doped deep blue light-emitting electrochemical cells from charged organic small molecules

Pure deep-blue light-emitting LEC devices were fabricated utilizing charged organic small molecules. with CIE coordinates of (0.15, 0.09) and (0.16, 0.10) for compound 1 and compound 2, respectively.

Cunning metal core: efficiency/stability dilemma in metallated porphyrin based light-emitting electrochemical cells

The syntheses, the photophysical/electrochemical characterization, and the first application in LECs of different metallated porphyrins are provided.

Delayed fluorescence in a solution-processable pure red molecular organic emitter based on dithienylbenzothiadiazole: a joint optical, electroluminescence, and magnetoelectroluminescence study

The discovery of triplet excitons participating in the photoluminescent processes in a growing number of pure organic emitters represents an exciting impetus for a diversity of promising opto, bio, and optoelectronic applications. In this contribution, we have studied a small-molecule dithienylbenzothiadiazole-based red-emitting dye red-1b, which shows clearly delayed fluorescence under optical and electrical excitation. The OLED device that contained red-1b as a nondoped solution-processable emitter exhibited a moderately high utilization of exciton amounting to ≈31% and slow efficiency roll-off. Magnetoelectroluminescence measurements revealed the coexistence of reverse intersystem crossing from the lowest triplet state to singlet state (RISC, E-type triplet to singlet up-conversion) and triplet-triplet annihilation (TTA, P-type triplet to singlet up-conversion). Specifically, in low current-density regime, the moderately high exciton utilization is attributed to RISC (i.e., thermally activated delayed fluorescence, TADF), whereas in high current-density regime, TTA may contribute to suppressing efficiency roll-off. Furthermore, the results showed that red-1b may represent a new kind of organic red emitters that display delayed fluorescence in a way differing from the few red emitters investigated so far.

Dynamically doped white light emitting tandem devices

Solution-processed, salt-containing, blue and orange light-emitting layers lead to efficient white light-emitting devices when arranged in a tandem configuration separated by a thin metal layer.