Polymer Electroluminescent Devices Research Articles

Achieving deep red to near-infrared emission in dinuclear platinum (II) complexes with the donor-π-acceptor-type cyclometalated ligand

A novel deep red-emitting dinuclear platinum (II) complex, (TPA2niq)2Pt2(C8OXT)2, featuring a donor-π-acceptor (D-π-A) type ligand, was synthesized and characterized. Here, TPA2niq represents the 4-(tert-butyl)-N-(4-(tert-butyl)phenyl)-N-(4-(6-(isoquinolin-1-yl)naphthalen-2-yl)phenyl)aniline unit, while C8OXT was the abbreviation for the bridging ligand 5-(4-octyloxyphenyl)-1,3,4-oxadiazole-2-thiol. Its photophysical, electrochemical, and electroluminescent characteristics were primarily studied. It was found that (TPA2niq)2Pt2(C8OXT)2 exhibited a saturated deep red emission with a peak at 686 nm in dichloromethane. Furthermore, the (TPA2niq)2Pt2(C8OXT)2-doped polymer electroluminescent devices (PLEDs), fabricated through a solution process, exhibited outstanding electroluminescence (EL) properties, with an emission peak at 684 nm, a maximum external quantum efficiency (EQE) of 3.78%, and a maximum radiant emittance of 4478 mW/Sr/m2. By incorporating the D-π-A type ligand into the dinuclear platinum (II) complex, the PLEDs achieved efficient deep red emission.

Improved Characteristics of Fluorene‐Type Polymer Light‐Emitting Devices Based on Multilayer Formation from Polymers and Solution‐Processable Wide‐Bandgap Inorganic Copper(I) Thiocyanate with p‐Type Conduction and High Refractive Index

The functional solution‐processable multilayer formation is a significant factor in the improved characteristics of polymer light‐emitting devices. The improved characteristics of direct current and alternating current (AC)‐driven fluorene‐type polymer light‐emitting devices based on multilayer structures utilizing polymers and solution‐processable inorganic wide‐bandgap semiconductor, copper(I) thiocyanate, CuSCN, which exhibits p‐type conduction and high refractive index, are investigated. For the polymer light‐emitting diodes based on poly(9,9‐dioctylfluorene‐co‐benzothiadiazole), F8BT with CuSCN, the plateau of current efficiency indicates the carrier injection and transport to be balanced at lower current density and the balance to be maintained through the wide current range. CuSCN acts as the hole transport and electron‐blocking layer, although CuSCN acts as a fluorescence quencher for F8BT. The introduction of inorganic/organic hybrid dielectric mirrors, which consists of the alternating layers of inorganic CuSCN and insulating poly(vinylidene fluoride‐trifluoroethylene), P(VDF‐TrFE), into the AC voltage‐driven polymer electroluminescent device (ACEL) structure, is an effective way to achieve both the spectral narrowing and color tunability. Using high‐refractive‐index CuSCN, the color‐tunable emission for ACEL with distributed Bragg reflector mirrors and the electroluminescent (EL) emission from a weak microcavity effect are achieved.

Microcavity polymer electroluminescent devices with solution-processed dielectric distributed Bragg reflectors utilizing inorganic copper (I) thiocyanate and insulating polymers

Abstract The concept of using printed inorganic/organic hybrid distributed Bragg reflectors (DBRs) utilizing inorganic semiconductor and insulating polymers in microcavity polymer electroluminescent devices is introduced to provide an approach to achieve the spectral narrowing and the strong forward directionality. The large refractive index contrast of approximately 0.5 (0.44) between inorganic copper(I) thiocyanate, CuSCN, and insulating polymer of poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE) (cellulose acetate, CA) results in the fabrication of solution-processed inorganic-organic hybrid dielectric DBRs with high reflectivity (>90%) from nanostructures consisting of only four (five) bilayers. For DBRs composed of CuSCN/CA alternative dielectric layers, all-solution processed microcavity polymer light-emitting diode based on highly conductive poly(ethylenedioxythiophene):poly(styrenesulfonate) anode except for Ag cathode exhibits the narrowing of EL spectrum with a full width at half maximum of approximately 25 nm and the maximum luminance of above 10,000 cd/m2. From the viewpoint of dielectric DBRs based on ferroelectric polymer P(VDF-TrFE) with both low refractivity and high permittivity, we demonstrate a microcavity AC voltage-driven polymer electroluminescent device (μcACEL) which exhibits the spectral narrowing and the strong forward directionality. This work is anticipated to be useful for the development of solution-processed μcACEL with unique device architecture.

(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.

High-performance alternating current field-induced chromatic-stable white polymer electroluminescent devices employing a down-conversion layer

In this work, a high-performance alternating current (AC) filed-induced chromatic-stable white polymer electroluminescence (WFIPEL) device was fabricated by combining a fluorophor Poly(9,9-dioctylfluorene) (PFO)-based blue device with a yellow down-conversion layer (YAG:Ce). A maximum luminance of this down-conversion FIPEL device achieves 3230cdm−2, which is 1.41 times higher than the device without the down-conversion layer. A maximum current efficiency and power efficiency of the down-conversion WFIPEL device reach 19.7cdA−1 at 3050cdm−2 and 5.37lmW−1 at 2310cdm−2 respectively. To the best of our knowledge, the power efficiency is one of the highest reports for the WFIPEL up to now. Moreover, Commison Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.30) is obtained by adjusting the thickness of the down-conversion layer to 30μm and it is kept stable over the entire AC-driven voltage range. We believe that this AC-driven, down-conversion, WFIPEL device may offer an easy way towards future flat and flexible lighting sources.

Electroluminescent Devices: Frequency‐Dependent, Alternating Current‐Driven, Field‐Induced Polymer Electroluminescent Devices with High Power Efficiency (Adv. Mater. 48/2014)

Frequency-dependent, high-performance organic electroluminescent devices driven by alternating current (AC) are demonstrated through impedance matching of the device to the driving source by D. L. Carroll and co-workers on page 8133. The device may allow for specific advantages, including elimination of rectifiers while integrating into 110/220 V and 50/60 Hz AC power sources, reduced Ohmic-heating-based failures at color component interfaces, and greater manufacturing tolerances over large areas.

Alternating current-driven, white field-induced polymer electroluminescent devices with high power efficiency

A solution-processed, all-phosphor, three-color (i.e., blue, green, and red), alternating current-driven white field-induced polymer electroluminescent device (WFIPEL), with low operational voltage, high luminance, high efficiency, high color-rendering index (CRI), and excellent color-stability, was demonstrated. The devices employed poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) [P(VDF–TrFE–CFE)] dielectric modified by single-walled carbon nanotubes (SWNTs) to further improve the dielectric characteristics, as the insulating layer. This significantly lowers the driving voltage of the device. Moreover, hole-generation layer and electron-transporting layer with high conductivity were used to more efficiently form and confine excitons in the emissive layer. The resulting WFIPEL devices show significant improvements in performance as compared to previous reports. Specifically, the devices exhibit a low turn-on voltage of 10V, a maximum luminance of 7210cdm−2, a maximum current efficiency and power efficiency of 33.8cdA−1 and 10.5lmW−1, and a CRI of 82. The power efficiency is even 10 times higher than the highest previous report (1lmW−1).

Frequency-dependent, alternating current-driven, field-induced polymer electroluminescent devices with high power efficiency.

A significant enhancement in power efficiency for alternating current-driven field-induced polymer electroluminescent devices is demonstrated by employing a high-k ferroelectric polymer dielectric through impedance matching of the device with the driving source. A peak power efficiency of 34.1 lm W(-1) at a frequency of 65 kHz is achieved, which is 2 to 12 times higher than the previous highest reports.

High-color-quality white emission in AC-driven field-induced polymer electroluminescent devices

The high-color-quality white emission in an AC-driven field-induced electroluminescence (FIPEL) device consisting of a white emitting ter-polymer: poly(fluorene–benzothiadiazole–quinoline) PF–BT–QL combined with a red emitting dye: Bis(2-methyl-dibenzof,hquinoxaline)(acetylacetonate)iridium (III) Ir(MDQ)2(acac) was achieved. The wide EL emission effectively covered the visible spectral region at the concentration of 5% Ir(MDQ)2(acac) in PF–BT–QL and largely enriched the color rendering capability with a CIE (0.36, 0.38) close to the ideal equal-energy white (0.33, 0.33) and a CRI as high as 97.4, close to the blackbody curve characteristic and CCT between 3034K and 5334K which are required for high-quality white-light illumination. When further increasing the concentration of Ir(MDQ)2(acac) to 10%, leading to a more pure white with CIE (0.36, 0.37) and a CRI as high as 97.1. Surprisingly, the FIPEL devices containing 20% and 30% Ir(MDQ)2(acac) in PF–BT–QL still exhibit high-quality white emission with CIE (0.42, 0.37) and (0.32, 0.38) and CRI 93.9 and 88.9 at high electric field, respectively. To the best of our knowledge, there are no reports of two-component FIPELs with a CRI>90, especially with such a high concentration of the phosphor dopant. We attribute this to the unique carrier injection characteristics of the AC-driven field induced device. This further suggests its great potential application in display and solid state lighting.

Effects of electrode modification using calcium on the performance of alternating current field-induced polymer electroluminescent devices

In this work, the effects of electrode modification by calcium (Ca) on the performance of AC field induced polymer electroluminescence (FIPEL) devices are studied. The FIPEL device with Ca/Al electrode exhibits 550 cd m−2, which is 27.5 times higher than that of the device with only an Al electrode (20 cd m−2). Both holes and electrons are injected from one electrode in our FIPEL device. We found that the electron injection can be significantly enhanced by a Ca modification on the Al electrode without greatly affecting the hole injection. Therefore, the electrons and holes can be effectively recombined in the emissive layer to form more excitons under the AC voltage, leading to effective light emission. The device emitted much brighter light than other AC-based organic EL devices. This result provides an easy and effective way to improve FIPEL performance.

Emission characteristics in solution-processed asymmetric white alternating current field-induced polymer electroluminescent devices

In this work, the emission characteristics of a blue fluorophor poly(9, 9-dioctylfluorene) (PFO) combined with a red emitting dye: Bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) [Ir(MDQ)2(acac)], are examined in two different asymmetric white alternating current field-induced polymer electroluminescent (FIPEL) device structures. The first is a top-contact device in which the triplet transfer is observed resulting in the concentration-dependence of the emission similar to the standard organic light-emitting diode (OLED) structure. The second is a bottom-contact device which, however, exhibits concentration-independence of emission. Specifically, both dye emission and polymer emission are found for the concentrations as high as 10% by weight of the dye in the emitter. We attribute this to the significant different carrier injection characteristics of the two FIPEL devices. Our results suggest a simple and easy way to realize high-quality white emission.

Effect of multi-walled carbon nanotubes on electron injection and charge generation in AC field-induced polymer electroluminescence

We investigate the use of multi-walled carbon nanotubes (MWNTs) dispersed in an emissive layer of poly (N-vinylcarbazole) (PVK):fac-tris(2-phenylpyri-dine)iridium(III) [Ir(ppy)3] in alternating current (AC) field-induced polymer electroluminescence (FIPEL) devices. A symmetric device structure, with the polymer/MWNT composite between two dielectric layers, was used to study the effect of MWNTs on charge generation within the active layer. An asymmetric device structure, using one dielectric layer, was used to study band alignment effects of carbon nanotubes in charge injection from a contact. The presence of MWNTs within the emissive layer facilitates effective internal charge generation in the symmetric devices, as would be expected if they acted as a charge source. However, electron injection under AC-driven fields also increases in the asymmetric devices, suggesting a modification to band alignment. Increase in light emission of five times is achieved in composite devices compared to devices with the pure polymer. From the trends in behavior with nanotube loading, we suggest that the nanotubes effectively doped the polymer, modifying energy level alignment in the device and increasing field-induced polarization currents. The combined effects of electron injection and charge generation may pave the way for widespread use of MWNTs in high-performance FIPEL devices.

AC Field-Induced Polymer Electroluminescence with Single Wall Carbon Nanotubes

We developed a high-performance field-induced polymer electroluminescence (FPEL) device consisting of four stacked layers: a top metal electrode/thin solution-processed nanocomposite film of single wall carbon nanotubes (SWNTs) and a fluorescent polymer/insulator/transparent bottom electrode working under an alternating current (AC) electric field. A small amount of SWNTs that were highly dispersed in the fluorescent polymer matrix by a conjugate block copolymer dispersant significantly enhanced EL, and we were able to realize an SWNT-FPEL device with a light emission of approximately 350 cd/m(2) at an applied voltage of ±25 V and an AC frequency of 300 kHz. The brightness of the SWNT-FPEL device is much greater than those of other AC-based organic or even inorganic ELs that generally require at least a few hundred volts. Light is emitted from our SWNT-FPEL device because of the sequential injection of field-induced holes and then electron carriers through ambipolar carbon nanotubes under an AC field, followed by exciton formation in the conjugated organic layer. Field-induced bipolar charge injection provides great material design freedom for our devices; the energy level does not have to be aligned between the electrode and the emission layer, and the balance of the carrier injected and transported can be altered in contrast to that in conventional organic light-emitting diodes, leading to an extremely cost-effective and unified device architecture that is applicable to all red-green-blue fluorescent polymers.

Multicolor single‐layer electroluminescence device using poly(L‐glutamate) for hole transport

AbstractHelical poly(L‐glutamate) with carbazole (Cz) side chains (PCELG) was synthesized as a hole‐transport host material for a dye‐doped polymer electroluminescence (EL) device. The main‐ and side‐chain conformations were investigated by a combination of polarized infrared spectroscopy and semiempirical quantum chemical calculation. In an electrically poled PCELG crystalline film, the main chain was found to assume a right‐handed α‐helical conformation with an order parameter of ∼0.8. The Cz plane at the terminal side chain is inclined by about 44° toward the helical axis, creating a regular stacked structure conforming to the rigid α‐helical backbone. The principal EL characteristics of these devices were determined and compared with those of devices made with poly(N‐vinyl carbazole) (PVCz). For green and blue light, EL devices using PCELG exhibited luminance efficiencies comparable to those using PVCz. To the best of our knowledge, this was the first demonstration of primary colors emitted by an organic EL device with a polypeptide hole‐transport host material. The experimental results suggested that the total concentration and spatial arrangement of the Cz groups play an important role in determining the EL characteristics of dye‐doped polymer EL devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 496–502, 2010

Dual functions of a new n-type conjugated dendrimer: light-emitting material and additive for polymer electroluminescent devices

We demonstrate a novel light-emitting diode (LED) of a graded bilayer structure that comprises poly(N-vinylcarbazole) (PVK) with good hole transport ability as the energy donor and a new distyrylanthracene-triazine-based dendrimer with enhanced electron transport ability as the light-emitting molecule. The device contains a graded bilayer structure of the PVK film covered with the dendrimer film prepared by sequential spin-casting of the dendrimer layer from a solvent that only swells the PVK layer. The bilayer device demonstrated a significantly enhanced electoluminescence quantum efficiency compared with the dendrimer single layer device or the PVK : dendrimer blend device with optimized composition. We also prepared composite LEDs with an MEH-PPV : emissive dendrimer blend. By doping the electron-deficient MEH-PPV layer with a small amount of the distyrylanthracene-triazine-based dendrimer, we could not only enhance the device performance but also depress the long-wavelength emission of MEH-PPV.

Improvement of Polymeric Electroluminescent Device Structures Through Simulation of UV-Vis Absorption Spectra of PANI/PVS Films

Artificial neural networks (ANN) were used to simulate the absorption spectra of PANI/PVS films deposited by the layer-bylayer (LBL) technique with different number of bilayers and doping levels. The PAni/PVS films are used to enhance electrical and optical performance of polymeric electroluminescent devices. In this work one hidden-layer Multilayer Perceptron (MLP) architecture was used. It is shown a comparison between experimental and simulated UV-Vis absorption spectra. This approach is proposed for the determination of thin-film design so as not to overlap the absorption spectra of Pani/PVS films with the device active layer emission spectra in order to enhance the device performance.

Self‐Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Abstract A new approach to forming a gradient hole‐injection layer in polymer light‐emitting diodes (PLEDs) is demonstrated. Single spin‐coating of hole‐injecting conducting polymer compositions with a perfluorinated ionomer results in a work function gradient through the layer formed by self‐organization, which leads to remarkably efficient single‐layer PLEDs (ca. 21 cd A–1). The device lifetime is significantly improved (ca. 50 times) compared with the conventional hole‐injection layer, poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonate). These results are a good example for demonstrating that the shorter lifetime of PLEDs compared with small‐molecule‐based organic LEDs (SM‐OLEDs) is not mainly due to the inherent degradation of the polymeric emitter itself. Hence, the results open the way to further improvements of PLEDs for real applications to large‐area, high‐resolution, and full‐color flexible displays.

Effect of Thermal Annealing on the Charge Carrier Mobility in a Polymer Electroluminescent Device

Effect of thermal annealing on the time-of-flight charge carrier mobility of a polymer electroluminescent device was investigated. The average hole mobility of the device before annealing was ∼1×10−6 cm2/V/s, while the thermally annealed device demonstrated both dispersive carrier transport characteristics and enhanced carrier mobility (∼3×10−6 cm2/V/s). This is attributed to the increased interchain interactions achieved by thermal annealing. It was clarified that with the higher degree of interchain interactions the faster becomes the charge carrier mobility. We propose the thermal annealing method as a convenient and effective mean to improve the carrier mobility of the devices based on organic/polymeric semiconducting materials.

Surface treatments of indium‐tin oxide substrates for polymer electroluminescent devices

AbstractIn this work, three different sets of processing techniques (wet, dry, and combined treatments) were utilized to modify the surfaces of indium‐tin oxide (ITO) substrates for polymer electroluminescent devices (PELDs), and the influence of surface treatments on the surface properties of ITO substrates were investigated by X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle, and four‐point probe. The surface energies of ITO substrates were also calculated from the measured contact angles. Experimental results show that the surface properties of the ITO substrates strongly depend on the surface treatments. Oxygen plasma treatment effectively improves the ITO surface properties since plasma decreases the surface roughness and sheet resistance, improves the surface stoichiometry and wetting. Furthermore, the PELDs with the differently treated ITO substrates as hole‐injecting electrodes were fabricated and characterized. We observe that the optical and electrical characteristics of devices are greatly influenced by the surface treatments on ITO substrates. Oxygen plasma treatment decreases turn‐on voltage, increases brightness and efficiency, and thereby improves the device performance of PELDs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-performance low-temperature transparent conducting aluminum-doped ZnO thin films and applications

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by radio frequency magnetron sputtering using a hydrogen–argon gas mixture at room temperature. The effect of hydrogen partial pressure on the optical and electrical properties of AZO films is investigated. The hydrogen partial pressure was varied from 0 to 1.25×10 −3 Pa during film growth. Polycrystalline Al-doped ZnO films having a preferred orientation with the c-axis perpendicular to the substrate were obtained with a root mean square roughness of ∼2 nm. At an optimal condition, the AZO films with a resistivity of 5.1×10 −4 Ω cm and an optical transparency of over 83% in the visible wavelength region are achieved. The AZO films with desired properties were used as an anode contact in polymeric electroluminescent devices and the electroluminescence performance of these OLEDs was studied.