Shun-Chi Chang

MULTICOLOR ORGANIC LIGHT-EMITTING DIODES PROCESSED BY HYBRID INKJET PRINTING

734 O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0906-0734

DUAL-COLOR POLYMER LIGHT-EMITTING PIXELS PROCESSED BY HYBRID INKJET PRINTING

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Degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes

A hybrid inkjet printing (HIJP) technology, which combines a pin-hole free polymer buffer layer and an inkjet printed polymer layer, allows the patterning of high quality polymer light-emitting devices. In this letter, we present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using this HIJP technique. In this demonstration, the polymer buffer layer is a wide band gap, blue emitting semiconducting polymer prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconducting polymer which was printed onto the buffer layer. When a proper solvent was selected, the printed polymer diffused into the buffer layer and efficient energy transfer took place generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for...

Organic/polymeric electroluminescent devices processed by hybrid ink-jet printing

The degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes (PLEDs) is investigated. The active medium of our PLED is a polymer blend comprising poly(vinylcarbazole) (PVK), [2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] (t-PBD), and platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The cyclic voltammetry result shows that the reductive reversibility of PtOX is poor. This result suggests that PLED doped with PtOX is not stable if PtOXs trap electrons and turn into anionic PtOX species. This was indeed verified by fabricating single-layer PLEDs with various amounts of electron-transporting material, t-PBD. A slower degradation rate was observed from the devices with higher concentration of t-PBD, because of the reduction of the electron accumulation at the PtOX sites. The half decay lifetime of our phosphorescent polymer LED has been improved by a factor of ∼40, from 1.2 to 45 h.

High Performance Polymer Light-Emitting Diodes Fabricated by a Low Temperature Lamination Process**

Ink-jet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting (or conjugated) organic molecules and polymers are solution processable, IJP becomes an ideal method for printing polymer/organic light-emitting diodes with high resolution. In this review article, we present the hybrid ink-jet printing technology (HIJP), which consists of an ink-jet printed layer in conjunction with another uniform spin-coated polymer layer, which serves as a buffer layer to seal the pin holes between the ink droplets. This HIJP technology has been successfully applied to the fabrication of polymer light-emitting logos, multicolor polymer/organic light-emitting diodes, and the built-in shadow mask for the cathode patterning for pixelated polymer LEDs.

Highly efficient polymer light-emitting devices using a phosphorescent sensitizer

We report on the successful demonstration of high performance polymer light-emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. Atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.

Polymer solution light-emitting devices

Highly efficient single-layer polymer light-emitting diodes that employ Nile Red as a fluorescent dye, with a green phosphorescent-sensitizer, bis(2-phenyl pyridinato-N,C2′) iridium (acetylacetonate) doped in a PVK/PBD host are demonstrated. The function of the phosphorescent sensitizer is to convert the triplet exciton into a singlet exciton during the energy transfer process. Therefore, ideally, all the excitons can be utilized. For comparison, devices with the same structure, but using a fluorescent sensitizer instead of a phosphorescent sensitizer, were fabricated. The efficiency of the phosphor-sensitized device is 6.4 cd/A, almost triple that of lumophor-sensitized or nonsensitized devices. This result indicates that not only singlet excitons but also triplet excitons are efficiently transferred from the host to the fluorescent dye when a phosphorescent material is used as a sensitizer.

Pyramid-shaped pixels for full-color organic emissive displays

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.

High-performance flexible polymer light-emitting diodes fabricated via a low-temperature plastic laminated process

Organic electroluminescent emissive displays are composed by pixels consisting red-green-blue (RGB) light-emitting diodes(LEDs) in a planar arrangement. When operated, these RGB LEDs are biased independently to produce the required color. In this manuscript, we describe a promising pixel structure, the pyramid-shaped pixel (PSP) for the integration of organic light-emitting diodes(OLEDs) in full-color organic emissive displays. The RGB light-emitting diodes are constructed on the walls of the pyramid structure. When operated, the RGB LEDs emit photons through the base of the pyramid structure, hence these RGB LEDs share the same emissive area to produce the required color. The PSP structure offers the advantage of being a full color emissive pixel comprising of individual RGB OLEDs with very high resolution. In addition, pyramid pixel does not require shadow mask to pattern the organic materials during the vacuum deposition process.

Dual-color polymer light-emitting pixels processed by hybrid inkjet printing

In this manuscript, we report on the successful fabrication of high performance polymer light emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. The atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.