Electroluminescent Polymer Research Articles

BINOL induces chirality in polyfluorene-based electroluminescent polymers in solid state

Nowadays circularly polarized luminescence (CPL) gains much attraction because it can be applied to a variety of fields. An efficient method to achieve CPL is blending achiral emissive polymers with chiral small-molecule additives as a chiral inducer. In this work, we demonstrate that 1,1′-Bi-2-naphthol (BINOL) acts as a chiral-inducer for poly (9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) in a thin film. In the thin films of F8BT/BINOL before annealing, only circular dichroism (CD) signals of BINOL were observed. After the annealing at 120 °C, clear CD signals of F8BT were observed indicating that the chiroptical property was induced on the F8BT thin films. The CD signal intensity was enhanced by increasing the annealing temperature up to 200 °C. By optimizing the concentration of BINOL and the annealing temperature, we obtained a dissymmetry CD value of 0.012. Photoluminescence quantum yield of the F8BT/BINOL thin films was also increased after the annealing.

Electroluminescent Polymers with Cationic Iridium-Containing Fragments

Electroluminescent Polymers with Cationic Iridium-Containing Fragments

High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence.

Stretchable light-emitting materials are the key components for realizing skin-like displays and optical biostimulation. All the stretchable emitters reported to date, to the best of our knowledge, have been based on electroluminescent polymers that only harness singlet excitons, limiting their theoretical quantum yield to 25%. Here we present a design concept for imparting stretchability onto electroluminescent polymers that can harness all the excitons through thermally activated delayed fluorescence, thereby reaching a near-unity theoretical quantum yield. We show that our design strategy of inserting flexible, linear units into a polymer backbone can substantially increase the mechanical stretchability without affecting the underlying electroluminescent processes. As a result, our synthesized polymer achieves a stretchability of 125%, with an external quantum efficiency of 10%. Furthermore, we demonstrate a fully stretchable organic light-emitting diode, confirming that the proposed stretchable thermally activated delayed fluorescence polymers provide a path towards simultaneously achieving desirable electroluminescent and mechanical characteristics, including high efficiency, brightness, switching speed and stretchability as well as low driving voltage.

Mayer Rod‐Coated Organic Light‐Emitting Devices: Binary Solvent Inks, Film Topography Optimization, and Large‐Area Fabrication

Homogeneous pinhole‐free functional films are essential in the manufacturing of large‐area high‐performance flexible electronics. Mayer rod coating is used to create high‐quality organic electroluminescent (EL) films herein. Various factors that affect the film quality of Mayer rod coating, including Mayer rod coating processing, solvent engineering, emitting material concentrations, and the ionic conductor proportions, are systematically investigated and optimized. Accordingly, uniform large‐area EL polymer films are achieved using Mayer rod coating. Meanwhile, large‐area polymer‐based light‐emitting electrochemical cells (PLECs) manufactured by printing methodology are successfully constructed. The resulting printed large‐area PLECs manifest enhanced performance by a factor of 2.37 compared with the corresponding spin‐cast counterparts.

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (gabs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (gPL) of the order of 10−3 were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of gabs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (gEL) of 1.09 × 10−4 and −1.02 × 10−4 at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Dependence of the performance of light-emitting diodes on the molecular weight of the electroluminescent polymer PFO-MEH-PPV

Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m2, and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.

Monoradically luminescent polymers by a super acid-catalyzed polymerization and deep-red electroluminescence

Luminescent radicals have received great attention recently due to their idiographic luminescent properties and potential 100% utilization efficiency of doublets under electrical excitation. However, luminescent radical polymers are rarely explored owing to their challenging molecular design and synthesis. Herein, we report an efficient approach to construct luminescent radical polymers by a super acid-catalyzed polymerization reaction proceeded at room temperature, which intrinsically avoid the heavy metal catalyst. The obtained polymers exhibit the unique paramagnetic signals, good thermal properties, and excellent photo-stability. Moreover, the quantifiable electroluminescence of such radical polymers was demonstrated for the first time.

Carbazole Substituted BODIPYs.

Difluoroboron-dipyrromethenes (BODIPYs) are highly popular fluorescent dyes with applications as NIR probes for bioimaging, fluorescent tags/sensors and as photosensitizers in cancer therapy and organic photovoltaics. This review concentrates on the synthesis and spectral properties of BODIPY dyes conjugated with carbazole heterocycle. The carbazole is an electron rich tricyclic compound and due to its excellent electronic properties, it is extensively used in the production of electroluminescent materials and polymers. This review highlights the recent progress made on the series of BODIPY derivatives containing carbazole ring at alpha, beta, and meso-positions of the BODIPY skeleton. Carbazole based hybrid BODIPYs, carbazole linked aza-BODIPYs and carbazole-fused BODIPYs are also discussed.

Cyclometalated Iridium(III) Complexes with a Norbornene-Substituted Picolinate Ligand and Electroluminescent Polymers Based on them

Cyclometalated Iridium(III) Complexes with a Norbornene-Substituted Picolinate Ligand and Electroluminescent Polymers Based on them

Recent development of crown-substituted polyfluorenes for blue light-emitting devices in organic electronics

The efficient photoluminescent polymers containing crown-substituted fluorene and diphenyl anthracene units were synthesized by aromatic nucleophilic substitution reaction. The light-emitting polymers contain benzo-15-crown-5 groups at the C-9 position of fluorene and diphenyl anthracene-emitting segments in the main chain. Introduction of a crown ether group in polymer backbone leads to its high thermal stability at varying temperature ranges and good solubility in common organic solvents such as CH2Cl2, CHCl3, THF, and toluene. The novel combination of pendant crown ether and anthracene moiety enhanced the electroluminescence of polymers. The synthesized polymers were effectively characterized by elemental analyses, GPC, TGA, FTIR, 1H, 13C NMR spectroscopy and UV–Visible spectroscopy. The electrochemical measurements and optical properties of the polymers were also studied. The obtained polymers showed good photoluminescence and electroluminescence properties with blue fluorescence.

(Invited) Alternating Current Electroluminescence for Stimuli-Interactive Sensing Display

Development of stimuli-interactive display capable of spontaneously visualizing various external human sensible inputs has been of great interest and tremendous efforts are devoted to the visualization of nonvisible human senses such as touch, smell, taste, and sound. Field induced electroluminescence of either organic or inorganic fluorescent materials under alternating current (AC) has been extensively studied and its unique device architecture in which an emitting layer is separated with an insulator from electrode offers a new platform for designing and developing emerging stimuli-interactive displays. In this presentation, high-performance field-induced AC polymer electroluminescence (AC-PEL) devices are demonstrated with high brightness, high efficiency and color and intensity-tunability. We also present a pressure interactive AC display sensor that allows for both sensing and visualisation of pressure. Light emission upon exposure to an AC field between two electrodes is controlled by the capacitance change of the insulator arising from the pressure applied on top. Besides capacitive pressure sensing, our EL sensor allows for direct visualisation of the static and dynamic information of position, shape, and size of a pressurising object on a non-pixelated single device platform. Finally, the presentation shows that simultaneous sensing and visualization of the conductive substance is achieved when the conductive object is coupled with the light emissive material layer on our novel parallel-type AC-PEL device. A variety of conductive materials can be detected regardless of their work functions, and thus information written by a conductive pen is clearly visualized, as is a human fingerprint with natural conductivity.

Exciton–Polaron Interactions in Polyfluorene Films with β-Phase

Fluorescence quenching by electric charges is an important loss mechanism in high-brightness organic light-emitting diodes (OLEDs), but its effect is difficult to quantify in working devices. Here, we combine an electrochemical technique to control the charge density with time-resolved photoluminescence to distinguish between different quenching mechanisms. The material studied was the blue electroluminescent polymer poly(9,9-dioctylfluorene) with β-phase. Our results show that quenching occurs by Forster resonance energy transfer and is mediated by exciton diffusion. We determine the quenching parameters over a wide range of charge concentrations and estimate their impact on the OLED efficiency roll-off at high current density. We find that fluorescence quenching by charges and singlet–triplet exciton annihilation are the two main mechanisms leading to the efficiency roll-off. Our results suggest that hole polarons are not very effective quenchers of singlet excitons, which is important for understanding...

26.2: Invited Paper: Electroluminescent Polymers for Solution-processed PLEDs

26.2: <i>Invited Paper:</i> Electroluminescent Polymers for Solution-processed PLEDs

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy.

Solution-processed organic light-emitting diodes (s-OLED) consisting of TAPC/TmPyPB interfacial exciplex host and polymer PAPTC TADF emitter are prepared, simultaneously displaying ultralow voltages (2.50/2.91/3.51/4.91 V at luminance of 1/100/1000/1000 cd m-2), high efficiencies (14.9%, 50.1 lm W-1), and extremely low roll-off rates (J50 of 63.16 mA cm-2, L50 of ca. 15000 cd m-2). Such performance is distinctly higher than that of pure-PAPTC s-OLED. Compared to pure-PAPTC, the advanced emissive layer structure of TAPC:PAPTC/TmPyPB is unique in much higher PL quantum yield (79.5 vs 36.3%) and nearly 4-fold enhancement in kRISC of the PAPTC emitter to 1.48 × 107 s-1.

Doping-free circularly polarized electroluminescence of AIE-active chiral binaphthyl-based polymers.

AIE-active chiral polymer enantiomers (S-/R-P) can emit green circularly polarized electroluminescence (CP-EL) with gEL up to 0.024 without alignment layers and chiral dopants, which represents the first example of CP-OLEDs based on AIE-active main-chain chiral polymers.

Simple light-emitting electrochemical cell using reduced graphene oxide and a ruthenium (II) complex.

We report the use of conducting substrates from reduced graphene oxide (rGO) applied in light-emitting electrochemical cells (LECs) in which we used an electroluminescent polymer as a light-emitting layer. The emitting layer was composed of an organic metal complex of tris(2, 2'-bipyridine) ruthenium (II) linked to the sodium tetrafluoroborate anion ([Ru(bpy)3]+2)(BF4)-1. The luminance generated from the electroluminescent device was 6.89 Cd/m2 when applying a voltage of 13.48 V and a current of 10.19 mA. The luminance of this device was kept on almost constant for 36.84 min. The prolonged lifetime of the electrochemical device was achieved by depositing an rGO thin layer with a 161.3 nm thickness inside the LEC structure.

Cyclometallated iridium(III) complex with 1-phenylisoquinoline and norbornene-substituted pyrazolonate ligands and related electroluminescent polymers

A new cyclometallated iridium(III) complex NBEpzIr(Piq)2 (I) (NBEpzH is 1-phenyl-3-methyl- 4-(5-bicyclo[2.2.1]hept-5-en-2-yl)-5-pyrazolone, PiqH is 1-phenylisoquinoline) is synthesized. The structure of the compound is determined by X-ray diffraction analysis (СIF file CCDC no. 1521037). Copolymers with the carbazole and iridium-containing fragments in the side chains (P1–P3) are prepared from monomer I by the ring-opening metathesis polymerization method. Their photoluminescence and electroluminescence properties are studied. Copolymers P1–P3 exhibit an intense photoluminescence and electroluminesce of red color. The maximum luminance (3010 cd/m2) and current efficiency (15.1 cd/A) are achieved for emitter P2.

Synthesis and Characterisation of Novel Thiophene Based Azomethine Polymers and Study of Their Liquid Crystalline, Electrochemical and Optoelectronic Properties

This work reports the synthesis, structural characterisation, liquid crystallinity, luminescence and electroluminescence of novel thiophene azomethine polymers. The polymers under study were prepared via oxidative polymerisation of four novel monomers at room temperature using iron (III) chloride. The chemical structures of the prepared monomers and polymers were confirmed by infrared and 1H and 13CNMR spectroscopy. Molecular masses were determined for monomers and polymers by gas/liquid chromatography-mass spectrometry (GC/LC-MS) and by gel-permeation (size exclusion) chromatography (SEC), respectively. Thermal stability studies of the prepared materials were achieved by thermogravimetric analysis (TGA), and the onset of weight loss To and the endset Tmax were calculated from the thermograms. Liquid crystalline mesophases and phase changes of the monomers and polymers were studied by differential scanning calorimetry (DSC) and polarised optical microscopy (POM), and the glass transition temperatures Tg of the polymers were determined from the DSC curves. The electrochemical band gaps, HOMO and LUMO energy levels were measured by cyclic voltammetry. UV-visible absorption-emission spectra (liquid and solid films) of the polymers were obtained at room temperature with different solvents. Optical band gaps were calculated from the absorption edges, and were in good agreement with those estimated from cyclic voltammetry. Mixing the polymers with lanthanide salts such as EuCl3 and YbCl3 gave modified fluorescence, and the light emitted was much more intense than that from the pure polymers. Polymer based light-emitting diodes (PLEDs) were fabricated by spin coating, and their current-voltage characteristics were measured. In preliminary work, the polymer devices were found to produce electroluminescent spectra similar to the PL spectra of the corresponding samples. Molecular modelling studies were performed both on polymer segments and monomer molecules; the absorption spectra of the prepared polymers, HOMO and LUMO energy levels were calculated with ZINDO using AMI geometry optimisation.

Electroluminescent copper-containing polymers based on copper(I) norbornene-substituted complexes

New copper(I) complexes with norbornene-substituted benzimidazole ligands were synthesized. On the base of the obtained compounds carbon-chain copper-containing copolymers possessing photo- and electroluminescent properties were obtained by the metathesis polymerization method. Polymeric emitters show electroluminescence of yellow and orange colors with a maximum brightness as high as 629 cd/m2.

Study of White Electroluminescence from a Single-Component Polymer Using an Electrolyte-Gated Diode

White electroluminescence (EL) of a single-component polymer (SCP) in a polymer light-emitting diode (PLED) was measured to show blue-green and red emission peaked at 494 and 627 nm spectrally, the CIE coordinates of (0.36, 0.34), and a color temperature of 6247 K. The electric excited red emission is significantly different from the obtained photoluminescence (PL) spectrum, attributing to the formation of electromers under an electric field. The white EL of SCP was also investigated in an electrolyte-gated organic light-emitting diode in which the luminance of EL was tuned from 0.3 cd/m2 at VG = −4 V to 6000 cd/m2 at VG = 7 V under the diode bias of 8 V. Although the current efficiency of SCP diodes increased, the relative intensities of red emission with reference to blue-green emission decreased when switching gate potential from 0 to 8 V at a constant driving voltage. These results indicate the generation of electromers is favored with imbalanced charge carriers, besides the conformation changes of mo...