Organic Light-emitting Electrochemical Cells Research Articles

In Situ Surface-Enhanced Raman Spectroscopy on Organic Mixed Ionic-Electronic Conductors: Tracking Dynamic Doping in Light-Emitting Electrochemical Cells.

In the domain of organic mixed ionic-electronic conductors (OMIECs), simultaneous transport and coupling of ionic and electronic charges are crucial for the function of electrochemical devices in organic electronics. Understanding conduction mechanisms and chemical reactions in operational devices is pivotal for performance enhancement and is necessary for the informed and systematic development of more promising materials. Surface-enhanced Raman spectroscopy (SERS) is a potent tool for monitoring electrochemical evolution and dynamic doping in operational devices, offering enhanced sensitivity to subtle spectral changes. We demonstrate the utility of SERS for in situ tracking of doping in OMIECs in an organic light-emitting electrochemical cell (LEC) containing a conjugated polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]; MEH-PPV), a molecular anion (lithium triflate), and an electrolyte network (poly(ethylene oxide); PEO). SERS enhancement is achieved via an interleaved layer of gold particles formed by spontaneous breakup of a deposited thin gold film. The results successfully highlight the ability of SERS to unveil time-resolved MEH-PPV doping and polaron formation, elucidating the effects of triflate ion transfer in the operating device and validating the electrochemical doping model in LECs.

Synthesis and characterization of UV organic light-emitting electrochemical cells (OLECs) using phenanthrene fluorene derivatives for flexible applications

This paper details how two new small molecules, based on phenanthrene, were developed, and tailored for light-emitting device applications. An account is provided of both the compound synthesis and the methodologies employed in device fabrication. The ink formulation was improved by the use of triflate counterions. Standard bottom emitting devices were constructed on ITO glass along with top emitting devices on a sputter coated silver on glass substrate. Both structures exhibit UV emissions from the synthesized molecules. Successful EL emission within the UV spectrum range has been achieved by spray coating these active molecules onto glass slides. The optimized solution-processed devices produce UV emission using a semi-transparent silver nanowire top electrode. This results in electroluminescence (EL) peaking at 398 nm, with a maximum EL emission intensity of 20.5 μW/cm2.

Progress on Phenanthroimidazole Derivatives for Light-Emitting Electrochemical Cells: An Overview

Phenanthroimidazole derivatives are currently frequently used chemical compounds in the active layers of various devices, such as organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs). Their advantages include simple and cheap synthesis, good solubility in organic solvents, easy processing, high thermal stability, excellent electron transport ability, and very good optical and electroluminescent properties. The construction of the LEC is based on a very simple construction in which the active layer plays a significant role. Thus, the compounds used in it are extremely important because they play the role of more than just emitters. This review summarizes the current state of the art regarding phenanthroimidazole derivatives used in the active layer in LECs. The synthesis of the considered compounds is presented in detail in the first part of the review. Next, research on the physicochemical properties of phenanthroimidazole derivatives is presented. Finally, the latest reports on LEC test devices are presented.

Exploring the Electroluminescent Applications of Imidazole Derivatives.

Recently, the fields of organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) have improved tremendously with regard to tunable emission, efficiency, brightness, and thermal stability. Imidazole derivatives are excellent deep blue-green light-emitting layers in the OLED or LEC devices. This Review summarizes the major breakthroughs of various electroluminescence (EL) layers with imidazole-containing organic or organometallic derivatives, the molecular design principles, and their light-emitting performances as effective EL materials. The highly tunable chemical structures and flexible molecular design strategies of imidazole-based compounds are advantages that provide great opportunities for researchers. They can provide a good basis for the design and development of new EL materials with narrower emission and higher efficiency. Moreover, imidazole compounds have demonstrated breakthrough performances in thermally activated delayed fluorescence (TADF) properties where triplet excitons are utilized to inhibit anti-intersystem quenching, showing great promise in breaking the theoretical external quantum efficiencies (EQE) limits in traditional fluorescent devices.

Printable biflourene based ultra-violet (UV) organic light-emitting electrochemical cells (OLECs) with improved device performance

A series of printable UV emitting ionic bifluorene derivatives have been prepared incorporating pendent alkylimidazolium groups. Herein, we detail the synthesis of compounds and the methods used in device fabrication. We show how ink formulation is improved by increasing the solubility of the active bifluorene through extension of the alkyl chain length and switching the counter ion from PF 6 − to CF 3 SO 3 − . We also show how organic light emitting electrochemical cells (OLECs) can be fabricated by spray coating to achieve an active layer with a thickness of ∼150–200 nm, leading to working devices with a turn on voltage of around 6.5 V. This gives electroluminescent (EL) that peaks between 385 nm and 390 nm with a maximum EL emission intensity of 1.29 μW/cm 2 . Thus, EL emission within the UV range has been demonstrated successfully with the synthesised molecules via spray coating onto glass slides. UV light emitting ionic bifluorene small molecules have been synthesised and used to fabricate the OLECs on ITO glass slides. Spray coating technique has been utilized to deposit the functional layers in the OLECs. While the OLEC light emission layer is optimized to the thickness of ∼250 nm, the encapsulated devices demonstrated the turn on voltage of 6.5 V. This fabrication process can be adopted to achieve the light emitting textiles.

Wide-Bite-Angle Diphosphine Ligands in Thermally Activated Delayed Fluorescent Copper(I) Complexes: Impact on the Performance of Electroluminescence Applications.

We report a series of seven cationic heteroleptic copper(I) complexes of the form [Cu(P^P)(dmphen)]BF4, where dmphen is 2,9-dimethyl-1,10-phenanthroline and P^P is a diphosphine chelate, in which the effect of the bite angle of the diphosphine ligand on the photophysical properties of the complexes was studied. Several of the complexes exhibit moderately high photoluminescence quantum yields in the solid state, with ΦPL of up to 35%, and in solution, with ΦPL of up to 98%. We were able to correlate the powder photoluminescence quantum yields with the % Vbur of the P^P ligand. The most emissive complexes were used to fabricate both organic light-emitting diodes and light-emitting electrochemical cells (LECs), both of which showed moderate performance. Compared to the benchmark copper(I)-based LECs, [Cu(dnbp)(DPEPhos)]+ (maximum external quantum efficiency, EQEmax = 16%), complex 3 (EQEmax = 1.85%) showed a much longer device lifetime (t1/2 = 1.25 h and >16.5 h for [Cu(dnbp)(DPEPhos)]+ and complex 3, respectively). The electrochemiluminescence (ECL) properties of several complexes were also studied, which, to the best of our knowledge, constitutes the first ECL study for heteroleptic copper(I) complexes. Notably, complexes exhibiting more reversible electrochemistry were associated with higher annihilation ECL as well as better performance in a LEC.

Spray-Coated Organic Light-Emitting Electrochemical Cells Realized on a Standard Woven Polyester Cotton Textile

In this article, organic light-emitting electrochemical cells (OLECs) have been fabricated on a substrate consisting of a standard woven polyester cotton textile. The textile substrate is presmoothed by first screen printing an ultraviolet (UV) curable polyurethane layer, termed the interface layer. Solution-processed spray coating is then utilized to deposit all the functional layers on the textile to achieve fully spray-coated flexible OLECs. OLEC devices were initially fabricated on ITO-coated glass substrates for process optimization. Then, the OLECs were fabricated on flexible textile using the optimized spray coating process. Finally, both OLECs devices on the glass and textile substrates were encapsulated by stencil printing epoxy before testing in an ambient environment. The OLECs on ITO-coated glass exhibit a turn-on voltage of 3 V, a brightness level of 200 cd/m2 and a lifetime of at least 60 min. The OLECs on textile exhibit a turn-on voltage of 4 V, a brightness level of 80 cd/m2 level and a lifetime of at least 30 min.

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.

Fully stretchable active-matrix organic light-emitting electrochemical cell array

Intrinsically and fully stretchable active-matrix-driven displays are an important element to skin electronics that can be applied to many emerging fields, such as wearable electronics, consumer electronics and biomedical devices. Here, we show for the first time a fully stretchable active-matrix-driven organic light-emitting electrochemical cell array. Briefly, it is comprised of a stretchable light-emitting electrochemical cell array driven by a solution-processed, vertically integrated stretchable organic thin-film transistor active-matrix, which is enabled by the development of chemically-orthogonal and intrinsically stretchable dielectric materials. Our resulting active-matrix-driven organic light-emitting electrochemical cell array can be readily bent, twisted and stretched without affecting its device performance. When mounted on skin, the array can tolerate to repeated cycles at 30% strain. This work demonstrates the feasibility of skin-applicable displays and lays the foundation for further materials development.

Supramolecular Approach in Energy Conversion Devices

This review summarizes investigations carried out at the Laboratory of Photochemistry and Energy Conversion (LFCE) in the University of Sao Paulo dealing with design and characterization of ruthenium(II), rhenium(I) and iridium(III) polypyridine complexes with desired photochemical and photophysical properties in light of the development of optoelectronics and photoinduced energy conversion systems. First, the breakthroughs on molecular engineering of emissive ReI, RuII and IrIII complexes for the development of highly efficient light-emitting devices, such as organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs), are presented. Then, the photochemical and photophysical properties of fac-[Re(CO)3(NN)(trans-L)]+ complexes (NN = bidentate polypyridyl ligands and trans-L = stilbene-like ligand), which find use in molecular machines and photosensors, are discussed. Finally, dye-sensitized energy conversion devices based on RuII complexes and natural dyes, such as dye-sensitized solar cells (DSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs), are reviewed, highlighting some strategies for photoanode engineering aiming at improved device efficiencies.

Magneto-Electroluminescence in ITO/MEH-PPV:PEO:LiCF3SO3/Al Polymer Light-Emitting Electrochemical Cells.

Magnetic field effects (MFE) have been extensively studied in organic light emitting diodes because of their potential application in organic spintronics devices. However, only a few studies on MFE in organic light-emitting electrochemical cells (LEC) have been reported. In this paper, magnetic field effects on the electroluminescence of an LEC device with the structure of ITO/MEH-PPV:PEO:LiCF3SO3/Al were studied at various temperatures. The luminance–current–voltage curves of the device shows the typical bi-polar characteristics of LECs; positive magnetic electroluminescence (MEL) was observed with a value of about 2.5% (B = 42 mT, 250 K), showing a Lorentzian line shape. With a decrease in temperature, the MEL value and the threshold voltage increased accordingly, below the possible mechanism is discussed.

Solution-Processed Organic Optical Upconversion Device.

Imaging in the near-infrared (NIR) is getting increasingly important for applications such as machine vision or medical imaging. NIR-to-visible optical upconverters consist of a monolithic stack of a NIR photodetector and a visible light-emitting unit. Such devices convert NIR light directly to visible light and allow capturing a NIR image with an ordinary camera. Here, five-layer organic solution-processed upconverters (OUCs) are reported which consist of a squaraine dye NIR photodetector and a fluorescent poly( para-phenylene vinylene) copolymer (super yellow)-based organic light-emitting diode (OLED) or light-emitting electrochemical cell (LEC), respectively. Both OLED-OUCs and LEC-OUCs convert NIR light at 980 nm to yellow light at around 575 nm with comparable device metrics of performance, such as a turn-on voltage of 2.7-2.9 V and a NIR-to-visible photon conversion efficiency of around 1.6%. Because of the presence of a salt in the emitting layer, the LEC-OUC is a temporally dynamic device. The LEC-OUC turn-on and relaxation behavior is characterized in detail. It is demonstrated that a particular ionic distribution and thereby the LEC-OUC status can be frozen by storing the device in the presence of a small voltage applied. This provides a test chart for quantitative measurements.

Using Distributed Energy States of Graphene Quantum Dots for an Efficient Hole-Injection Media in an Organic Electroluminescent Device

In this letter, graphene quantum dots (GQDs) were used for an efficient hole-injection layer in organic electroluminescent devices. The spin-coated microplasmasynthesized GQDs had small diameters and high areal density (3.2 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The microstructure was studied using X-ray photoelectron spectroscopy, which indicated that the GQDs contained 31.5% C = O and 10.5% C-O functional groups. This indicated the highest occupied molecular orbit (HOMO) of the GQDs was approximately 5.3 eV. The photoluminescence excitation spectra suggested that the GQDs had a broad distribution of energy states (>1 eV). The material Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which has strong red-light emission at approximately 625 nm, was used as the luminescent material. The HOMO level of Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> was approximately 5.7 eV. Because the HOMO level for the GQDs was 5.3 eV with an energy level distribution of 1 eV, the GQDs could adequately bridge the work function of indium-tin oxide (ITO) to the HOMO of the luminescent material (Ru(bpy) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) for hole injection. After applying the GQDs, the emission intensity, luminous efficiency, and external quantum efficiency of the organic light-emitting electrochemical cell were enhanced approximately 45.6%, 44.2%, and 33%, respectively. The results indicated that surface-functionalizedGQDs were suitable for hole injection in organic electroluminescent devices.

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes.

Phosphorescent organometallic compounds based on heavy transition metal complexes (TMCs) are an appealing research topic of enormous current interest. Amongst all different fields in which they found valuable application, development of emitting materials based on TMCs have become crucial for electroluminescent devices such as phosphorescent organic light-emitting diodes (PhOLEDs) and light-emitting electrochemical cells (LEECs). This interest is driven by the fact that luminescent TMCs with long-lived excited state lifetimes are able to efficiently harvest both singlet and triplet electro-generated excitons, thus opening the possibility to achieve theoretically 100% internal quantum efficiency in such devices. In the recent past, various classes of compounds have been reported, possessing a beautiful structural variety that allowed to nicely obtain efficient photo- and electroluminescence with high colour purity in the red, green and blue (RGB) portions of the visible spectrum. In addition, achievement of efficient emission beyond such range towards ultraviolet (UV) and near infrared (NIR) regions was also challenged. By employing TMCs as triplet emitters in OLEDs, remarkably high device performances were demonstrated, with square planar platinum(II) complexes bearing π-conjugated chromophoric ligands playing a key role in such respect. In this contribution, the most recent and promising trends in the field of phosphorescent platinum complexes will be reviewed and discussed. In particular, the importance of proper molecular design that underpins the successful achievement of improved photophysical features and enhanced device performances will be highlighted. Special emphasis will be devoted to those recent systems that have been employed as triplet emitters in efficient PhOLEDs.

Dynamics of Charge Distribution in Sandwich-Type Light-Emitting Electrochemical Cells Probed by the Stark Effect

A sandwich-type structure composed of thin films of functional materials is the dominant architecture of organic optoelectronic devices. During device operation, injected or inside-generated electrical charges arrange in a specific spatial distribution. This distribution affects the electric field profile throughout the layers and has a fundamental impact on charge transport and recombination/separation processes. However, probing of the charge distribution is challenging because the hidden active layers are experimentally not directly accessible. Here, we study the temporal evolution of organic salt-based light-emitting electrochemical cells and demonstrate that electroabsorption spectroscopy in combination with electrical capacitance measurements enables the determination of the distribution of the injected, uncompensated electronic charge inside these devices. For constant-voltage operating conditions and over a period of hours, the Stark effect signal intensity and the capacitance increase steadily, but to a different extent. We demonstrate that this difference sensitively depends on the position and distribution width of injected mobile electrons. Estimates show a substantial spreading over the active layer of the injected electron density with time, screening the electric field behind the charge peak.

ITO-Free, Fully Solution Processed Transparent Organic Light-Emitting Electrochemical Cells on Thin Glass

Organic light emitting diodes (OLEDs) have the advantages of self-emitting property, high luminous efficiency, full-colour capability, wide viewing angle, high contrast, low power consumption, low weight and flexibility. All these capacitate OLEDs to be an attractive optical source for display and lighting applications. However, the current manufacturing cost is still relative high, which obstacle its success in the display and lighting market. The main problems are indispensable low work function material for cathode and/or electron injection layer such as Ba, LiF, which require vacuum deposition and/or inert fabrication atmosphere, and widely used anode ITO which are not solution processable. Therefore, the fully solution processing manufacturing techniques are under intense investigation in order to reduce the cost. Compared to OLEDs, its derivation, organic light-emitting electrochemical cells (OLECs), show an intrinsic advantage in fully solution processing. By mixing metal ions and a solid electrolyte with light-emitting polymers as active materials, an in-situ doping and in-situ PN-junction can be generated within a three layer sandwich device. Because of this doping effect, work function adaption is not necessary and air-stable flexible electrodes like PEDOT:PSS can be used. Hereby, a manufacturing process for fully solution-processed transparent OLECs is presented, which consist of inkjet printed PEDOT:PSS anode and cathode, spin-coated ZnO interlayer, and spin-coated yellow light-emitting polymer plus dopant. Inkjet printing the electrode give the advantages of mask-free and spin-coating provides the homogeneous active layers. Furthermore, by manufacturing on 70 µm thin glass, a relative flexible transparent light source can be realized and enable the potential emerging applications like signal display on textiles.

Water-free synthesis of ZnO quantum dots for application as an electron injection layer in light-emitting electrochemical cells

Large-area light emitters like organic (OLEDs) or quantum dot light-emitting devices (QLEDs) and light-emitting electrochemical cells (LECs) have gained increasing interest due to their cost-effective fabrication on various even flexible substrates.

Fully Solution Processed Organic Light-emitting Electrochemical Cells (OLEC) with ZnO Interlayer for Lab-on-Chip Applications

Microfluidic lab-on-a-chip devices can be used for chemical and biological analyses such as DNA tests or environmental monitoring. In order to make a monolithic and cost-efficient/disposable sensing device, direct integration of excitation light source for fluorescent sensing is often required. A manufacturing process for fully solution-processed blue organic light-emitting electrochemical cells (OLECs) is presented, which consist of pre-patterned ITO, spin-coated ZnO buffer layer and blue light-emitting polymer plus dopants and an inkjet-printed PEDOT:PSS transparent top anode. Furthermore, the fully transparent blue OLEC is able to emit &gt; 2000 cd/m2 light under pulsed driving mode, which fulfils requirements for simple fluorescent on-chip sensing applications. Furthermore, ITO electrodes can be replace by PEDOT:PSS transparent electrodes when a ZnO interlayer is solution processed on top, which enable the mask-free and fully solution processing integration on chips.

Fully Solution Processed Organic Light-emitting Electrochemical Cells (OLEC) with ZnO Interlayer for Lab-on-Chip Applications

Fully Solution Processed Organic Light-emitting Electrochemical Cells (OLEC) with ZnO Interlayer for Lab-on-Chip Applications

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.