Polymer Electroluminescent Devices Research Articles

Electroabsorption Measurement of Polymer Electroluminescent Devices with Insulating Layer

We use the electroabsorption technique to measure the electric field in the emitting layer of ITO/MEHPPV/ionomer/Al light-emitting. Sodium sulfonated polystyrene (SSPS) was used as the ionomer. It is observed that the built-in potential in the emitting layer is lowered when the ionomer layer is inserted. On the contrary, the current density increases with the ionomer layers. These results indicate that the ionomer enhances the electron injection from the cathode to improve the balanced injection of electrons and holes into the light-emitting devices. Therefore, the quantum efficiency increases significantly in the EL device with the ionomer layer.

Optical Patterning in Organic EL Devices

Fabrication of micropatterned multicolor organic electroluminescent (EL) devices by the “photobleaching method” is described. In this approach, we take advantage of the instability of organic fluorescent molecules upon photooxidation; that is, organic molecules can be oxidized or bleached by exposure to light in the presence of oxygen. Several types of multicolor devices having a dye-dispersed polymer emitter layer were fabricated. In order to tune the emission color and pattern the device, the polymer emitter layer is exposed to ultraviolet light or visible light after formed by spin coating so that specific dyes are photooxidized and become non-emissive. Due to this photobleaching process, the light-exposed area have different emission colors from the non-exposed area, and, using a fine-patterned photomask, micropatterned multicolor EL devices were successfully fabricated. This photobleaching method is the simplest method for the fabrication of multicolor polymer EL devices and can be applicable to the fabrication of full color displays having arrays of red-green-blue pixels.

Polymer gel light-emitting devices

Polymers can be processed into several forms such as solid films, solutions, and gels. Traditional conjugated polymer electroluminescent devices are thin film, solid-state devices consisting of a thin polymer film sandwiched between two electrodes. Recently, a light-emitting device based on polymer solutions has been demonstrated. In this letter, we report the generation of luminescence from a polymer gel in a compact cell configuration. This unique polymer gel light-emitting device (GLED) consists of a thin layer of a polymer gel sandwiched between two electrode/glass substrates. The emission spectrum is consistent with the photoluminescence spectrum obtained from the polymer gel. The device shows an operating mechanism similar to the electrogenerated chemiluminescence.

Electroluminescent Properties of Dye-Dispersed Polyurethane with Stilbene Dye Pendant

The dye-dispersed polymer electroluminescent device using soluble polyurethane derivative (PU) with stilbene side chain was prepared. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) was used as a lumophore. The DCM-dispersed polymer exhibit strong photoluminescence (PL) in the red spectral region peaking at 640 nm. Single-layer electroluminescent devices with indium-tin-oxide (ITO) and magnesium-silver (Mg:Ag) as electrodes were fabricated via spin-coating from DCM-dispersed PU solution. Red emission was observed from the devices when a DC voltage was exerted. The EL colors turned from green to red by DCM.

Efficient, low operating voltage polymer light-emitting diodes with aluminum as the cathode material

Efficient polymer electroluminescent (EL) devices have been fabricated using stable metals such as aluminum (Al) as the cathode material. These light-emitting diodes are fabricated with ITO as the anode, a soluble dialkoxy-PPV containing an appropriate surfactant additive as the EL polymer and Al as the cathode. The external quantum efficiency (QE) is above 2%, comparable with control devices (without the additive) with calcium as the cathode material. These Al cathode devices show promising stress life. Enhancements in the QE are also found for other EL polymers which emit blue, green, yellow and red light.

Polymer solution light-emitting devices

Traditional conjugated polymer electroluminescent devices are thin-film solid-state devices consisting of a thin polymer film sandwiched between two electrodes. In this letter, we demonstrate the generation of luminescence from polymer solutions in a compact polymer solution configuration. This unique polymer solution light-emitting device (SLED) consists of a thin layer of a polymer solution sandwiched between two transparent indium–tin–oxide/glass substrates. When biased, the device turns on at slightly above the band-gap energy and emits bright luminescence. The emission spectrum is consistent with the photoluminescence spectrum obtained from the polymer solution. We suggest that the mechanism of the SLED is due to the electrogenerated chemiluminescence effect. The SLED combines the advantages of low operating voltage, and easy and low-cost fabrication. The SLED is also a highly transparent emissive device when transparent materials are used for the electrodes and the substrates.

Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo

Inkjet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting (or conjugated) polymers are solution processable, IJP technology becomes an ideal method for printing polymer light-emitting diodes with high resolution. Unfortunately, the polymer film printed from an inkjet printer usually consists of pin-holes, and this intrinsic character makes it unsuitable for fabricating high quality polymer electronic devices, particularly for devices in the sandwich structure. In this letter, we submit a hybrid structure, which consists of an inkjet printed layer in conjunction with another uniform spin coated polymer layer, as an alternative to the regular inkjet printed structure. The uniform layer serves as a buffer layer to seal the pin-holes and the IJP layer is the layer consisting of the desired pattern, for example the red–green–blue dots for a multicolor display. To demonstrate, we applied this hybrid technology to fabricate efficient and large area polymer light-emitting logos. The use of this concept represents a whole new technology of fabricating polymer electronic devices with lateral patterning capability.

4.4L: Late‐News Paper: Ink‐Jet Printing Technology and Its Application in Polymer Multicolor EL Displays

AbstractInkjet printing (IJP) technology is a popular technology for desktop publishing. In this abstract, we present the first successful demonstration of patterning the polymer electroluminescent devices using the IJP technology. The use of this concept represents a whole new technology of fabricating polyemr multicolor displays with high precision lateral patterning capability.

Degradation process in polymeric electroluminescent devices

To investigate the initial degradation process in polymeric electroluminescent (EL) devices, in situ measurement of photoluminescence image change was carried out. The Al electrode was sunk or protruded during the device operation. Formation of the dented dark spot is observed by using an atomic force microscope and a scanning electron microscope, which is the critical factor of the degradation.

Red electroluminescent devices utilizing dye-dispersed soluble polyimide thin film as an emission layer

The dye-dispersed polymer electroluminescent device using the soluble poly(ether imide) (PEI) of relatively high glass transition temperature was successfully prepared. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4 H-pyran(DCM) was used as a lumophore. The device was turned on at around 12 V. and the brightness at 20 V was very strong. The color of the emitted light was red, which is almost the color of the dispersed dye alone. The stability of the device was also found to be good, even after packaging the cathode side with a polymeric glue at 110 °C.

In situ investigation of degradation in polymeric electroluminescent devices using time-resolved confocal laser scanning microscope

Degradation progress in polymeric electroluminescent devices during device operation was clarified by in situ measurement of photoluminescence image change using time resolved confocal laser scanning microscope. It is found that the degradation stems from two kinds of dark spots, which have the dented circle shape and the dome shape. Dented circle shapes grow during the operation. Dome shapes, where the polymer layers are detached from the indium-tin-oxide electrode, appear suddenly and do not change in size. Comparison of the Auger analysis depth profiles of the dented and the normal sites indicates that the polymer layer at the dented sites disappears. Time-resolved confocal laser scanning microscopy is found to be a powerful technique to analyze the degradation process.

Improvement of quantum efficiency in polymer electroluminescence devices by inserting pmma langmuir-blodgett films

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir-Blodgett films were used as the thin tunneling barrier. The enhancement may result from the improved balance between the electron and hole injection into the light emitting devices.

Organic Electroluminescence Device Based on an Electrodeposited Poly(3‐substituted thiophen) Film

An electrochemically deposited composite film of poly(3‐substituted thiophen) and insulating nitrile butadiene rubber (NBR) was used as the emission layer of a polymer electroluminescence (EL) device. The composite film on an ITO substrate was uniform, and by using this film current leakage was prevented. The device with the composite film as the EL emission layer shows rectification properties, the emitted color depending on the substituent of thiophen units. By coating with NBR after the electrodeposition of a poly(3‐substituted thiophen) film, EL characteristics were improved compared with the device with NBR coating performed before electropolymerization. Moreover, by using electropolymerized poly(3‐n‐octylthiophen) film as a hole transporting layer, the luminance of an organic EL device with a poly(N‐vinylcarbazole) dip‐coating layer was remarkably enhanced. The addition of the hole transporting layer reduced the turn‐on bias voltage and increased the emission intensity to 700 cd m−2.

Multilayer polymer electroluminescent devices: Color-variable emission through charge carrier confinement

We have succeeded in obtaining polarity-dependent color-variable emission from polymer electroluminescent (EL) devices with a three-layer structure. The emissive polymers used are poly(3-dodecylthiophene), poly( p-phenylenevinylene) and its derivative, and poly(9,9-dihexylfluorene). The emissive layers are separated by a layer of wide-band-gap hole-transporter, poly(N-vinylcarbazole) or an aromatic diamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine. When the EL devices are operated under forward or reverse bias, they exhibit the emission from the negatively-biased polymer layer. The energy band diagram of the devices implies that the separating layer confines excitons in the negatively-biased polymer layer

Molecular Conformation and Application of Stereoregular PMMA Langmuir-Blodgett Films

Molecular conformations of stereoregular poly(methyl methacrylate) (PMMA) monolayers have been investigated by scanning probe microscopes. Isotactic and syndiotactic PMMAs were found to have right and left hand helical structures, respectively. On the contrary atactic PMMA showed rather random arrangement of the chains. It has been demonstrated that the PMMA Langmuir-Blodgett(LB) films can be utilized to form nanoscale patterns down to 50 nm and to form a geodesic lens. It has also been manifested that the quantum efficiency of a polymer electroluminescent device can be significantly enhanced by inserting the PMMA LB films between the emitting layer and the cathode. All the applications utilize the unique characteristics of the LB films to form thin and uniform films in the molecular level.

Color-Variable Emission in Multilayer Polymer Electroluminescent Devices Containing Electron-Blocking Layer

Color-variable emission in multilayer polymer electroluminescent (EL) devices has been obtained by the use of poly(N-vinylcarbazole) (PVCz) as an electron-blocking layer. The emissive polymers used were poly ( p-phenylenevinylene), poly(3-dodecylthiophene), and poly(2,5-dinonyloxy- p-phenylenevinylene), whose emission colors were green, red, and orange, respectively. When EL devices consisting of two emissive polymer layers separated by a PVCz layer were driven under either forward or reverse bias mode, emission from the negatively biased polymer was observed. PVCz is assumed to act as an electron-blocking layer due to its small electron affinity (2.3 eV) compared with those of the emissive polymers (2.7–3.5 eV), and to facilitate exciton confinement in the negatively biased polymer layer, where radiative recombination of the charge carriers occurs.

Enhanced quantum efficiency in polymer electroluminescence devices by inserting a tunneling barrier formed by Langmuir–Blodgett films

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir–Blodgett films were used as the thin tunneling barrier. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing the effective barrier for hole injection. The effects improve the balanced injection of electrons and holes into the light-emitting devices.

Color-variable electroluminescence from multilayer polymer films

Color-variable emission has been observed from a multilayer polymer electroluminescent device composed of three polymer layers: perfluoropropylated poly(p-phenylene) (F-PPP) sandwiched between two emissive polymers, poly (3-dodecylthiophene) (PDT) and poly (p-phenylenevinylene) (PPV). Green and red emissions were observed when a negative bias was applied to the PPV and PDT side electrodes, respectively. F-PPP was supposed to act as an electron-blocking layer due to its small electron affinity (2.2 eV) compared with those of PPV (2.7) and PDT (3.5), and to facilitate the confinement of the excitons in the negatively biased polymer layer, where the radiative recombination of the charge carriers takes place.

Alternating-current light-emitting devices based on conjugated polymers

Most polymer electroluminescent devices to date are represented as tunnel diodes and operate under direct-current (dc) driving field. Here we report the fabrication of symmetrically configured alternating-current (ac) light-emitting (SCALE) devices based on conjugated polymers. The new devices consist of an emissive polymer layer sandwiched between two redox polymer layers. This configuration enables the SCALE devices to work under both forward and reverse dc bias as well as in ac modes. The nearly ohmic electrode/redox polymer contacts improve the charge injection efficiency significantly and make the SCALE device operation insensitive to electrode work functions. Symmetric operation supports the key role of redox polymer/emissive polymer interface states.

Application of Electropolymerized Poly(thiophen derivative)/NBR Composite Film to an Electroluminescence Emission Layer

Abstract A semiconducting composite film of poly(3-substituted thiophen) and insulating nitrile butadiene rubber (NBR) was applied to an emission layer of polymer electroluminescence device. The uniform composite was deposited by electropolymerization of phenyl-, hexyl- and octylthiophen onto an indium-tin oxide glass pre-coated with NBR thin film. By utilizing indium as rectifying contact, visible light was observed from the composites under forward bias. The emission color depended on the substituents in thiophen units.