Changhun Yun

Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics

The optical properties of dielectric-metal-dielectric (DMD) transparent electrodes are investigated from the perspectives of organic light-emitting diodes (OLEDs). A joint experimental and theoretical study showed that the optical characteristics of OLEDs based on DMD electrodes can be widely tuned to fulfill the requirements of a target application through careful control of the microcavity effect, transmittance of DMD electrodes, and a correlation of these two factors with the emission spectra of the emitted materials. Upon variation of the DMD structure, near-Lambertian emission and a 100% improvement in the luminous efficiency are demonstrated, respectively. Optimization strategies are also discussed that are relevant to forward luminous efficiency, total optical power, and angular/ spectral characteristics.

Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes

We report on the fabrication of efficient inverted organic light-emitting diodes (IOLEDs) using pentacene films as an electron injection/transport layer between Al and Alq3 layers. These IOLED devices turn on at 4.7 V and exhibit a luminous efficiency of 9.5 cd/A without any dopants or reactive metals. Analysis using space-charge-limited characteristics of electron-only devices and ultraviolet photoelectron spectroscopy measurement of metal/organic interfaces indicates that the efficient IOLED characteristics can be attributed partly to the electron mobility of pentacene that is 102–104 times larger than that of Alq3 and to the effective reduction in injection barrier at contacts.

Fullerene-Derivative-Embedded Nanogap Field-Effect-Transistor and Its Nonvolatile Memory Application†

Adeviceplatformwithnanochannels,fabricatedbyeithertopdown or bottom-up nanofabrication technologies, has been attracting notable attention in various fields and applications. Asarepresentativecaseofsuchnanochanneldeviceplatform,a nanogap- embedded field-effect-transistor (FET) has shown a great potential as a fast and label-free detection tool for a specific target material by monitoring the changes in electrical characteristicssuchasthethresholdvoltageorconductivity. [1,2] In a recent publication, for example, we reported that the nanogap-embedded FET can detect biomolecules in a very sensitive way by confining them within the nanogap and by monitoring the induced change in the effective dielectric constant of gate dielectrics, which is directly tied to the threshold voltage of the FET under study. [1] Alternatively, a conductivity change that was induced upon bridging the gap with a conductive nanomedium was used as an unlabeled biosensor. [3,4] Inthosedevices,itisoftendifficulttoconfirmina

Doping-Free Inverted Top-Emitting Organic Light-Emitting Diodes With High Power Efficiency and Near-Ideal Emission Characteristics

Inverted top-emitting organic light-emitting diodes (ITOLEDs) with high power efficiency and near-ideal emission characteristics are demonstrated by using the combination of the following: 1) an electron-injection layer composed of Cs2CO3, which lowers the turn-on voltage; 2) an electron-transporting layer with optimal electron mobility, which enhances the electron current and thus improves the carrier balance; and 3) a dielectric/metal/dielectric multilayer electrode that works as a damage-free top transparent anode optimized to achieve high efficiency and ideal emission characteristics. By this approach, ITOLEDs with power efficiency values of 3.8 and 30 lm W-1 are demonstrated in fluorescent and phosphorescent types, respectively, at a luminance value of 1000 cd m-2 with little distortion in spectral/angular characteristics.

Digital-Mode Organic Vapor-Jet Printing (D-OVJP): Advanced Jet-on-Demand Control of Organic Thin-Film Deposition

Digital-mode organic vapor-jet printing (D-OVJP) is demonstrated by producing a series of organic vapor jets. D-OVJP not only inherits all the benefits of a conventional OVJP but also provides an advanced, straightforward control over organic deposition with a pixel-to-pixel precision. Digitally-controlled film thickness and high-performance thin-film transistors are demonstrated with D-OVJP, proving its potential applicability to organic electronics and related areas.

Influence of phosphorescent dopants in organic light-emitting diodes with an organic homojunction

We report on phosphorescent homojunction organic light-emitting diodes (HJOLEDs) using p-i-n structures based on a single ambipolar organic semiconductor, 4,4′-Bis(carbazol-9-yl)-biphenyl, as matrix organic materials. In HJOLEDs, the phosphorescent dopant molecules play an important role in controlling the charge balance inside the emissive layer. We observe a four-fold enhancement in the luminous efficacy at 1000 cd/m2 from 3.7% to 12.9% by varying the emitter molecule. The influence of the energy level of the emitter molecule on charge balance is investigated by analyzing current density vs. voltage curves with the trap-limited current theory and by analyzing the electroluminescence spectra.

High-Performance Pentacene Thin-Film Transistors Fabricated by Organic Vapor-Jet Printing

Organic vapor-jet printing, a maskless direct printing method, is used to fabricate high-performance pentacene thin-film transistors. By combining the optimal carrier gas temperature and the surface treatment of gate dielectrics, a mobility of 0.46 (±0.03) cm2 V-1 s-1 and an on-off ratio greater than 107 are achieved. Morphological analyses indicate that the relatively high carrier gas temperature and low surface energy of the dielectric surface are the keys in achieving the level of performance comparable to that of devices based on conventional technologies.

High-Performance Pentacene Thin-Film Transistors Fabricated by Printing Technology

A high-performance bottom-contact pentacene thin-film transistor (TFT) is realized with its channel and electrodes fabricated by a simple printing process. By applying reverse offset printing of a nanosilver paste ink to the source/drain electrodes and organic vapor-jet printing to the thin pentacene layer, TFTs with a channel length of 20 μm are realized in a precise yet relatively simple fashion. The oxide formed during the processing of the silver ink is shown to help reduce the injection barrier between the source and pentacene, making it possible to realize high-performance bottom-contact TFTs without special treatment for the electrodes.

Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes

We report on the characteristics of enhanced and balanced white-light emission from bi-directional organic light-emitting diodes (BiOLEDs) enabled by the introduction of micro-cavity effects. The insertion of an additional metal layer between the indium tin oxide anode and the hole transporting layer results in similar light output of our BiOLEDs in both top and bottom direction and in reduced distortion of the electroluminescence spectrum. Furthermore, we find that by utilizing MC effects, the overall current efficiency can be improved by 26.2% compared to that of a conventional device.

Improved lifetime of highly flexible OLEDs based on multilayered transparent electrodes with enhanced barrier performance

— Flexible organic light-emitting diodes (FOLEDs) showing enhanced barrier properties under repeated mechanical stress are reported. By combining metal-based multilayer transparent electrodes (MTEs) as highly flexible anodes replacing ITO electrodes and sol-gel organic-inorganic hybrimers which function as both planarizing films and barrier layers, the proposed FOLEDs not only exhibit a level of performance comparable to that of ITO-based reference devices but also show a superior mechanical flexibility with “after-bending” lifetime close to that of ITO-based devices.