Seong Taek Lee

Carrier trapping and efficient recombination of electrophosphorescent device with stepwise doping profile

We have presented a physical concept for enhancing efficiency and lifetime of doped electrophosphorescent organic light-emitting devices. In order to provide a control parameter for higher device performance, a stepwise doping concentration profile at the emission layer was prepared. A more than 30% improvement of power efficiency was obtained for green electrophosphorescent device with a higher doping ratio at the emission layer-hole transport layer interface. We explained the carrier trapping and transport mechanism with direct recombination of an exciton in an iridium-based dopant system. When compared to green device, phosphorescent red devices showed a more significant charge trapping effect at low doping concentration, which is responsible for shifting the recombination zone far from the emission layer-hole transport layer interface. Therefore, charge trapping by doping control in an emission layer could be utilized for a charge-balancing technique for the confinement of a triplet exciton.

Improved blue light-emitting polymeric device by the tuning of drift mobility and charge balance

We have prepared blue polymer-small molecule hybrid electroluminescence devices with improved efficiency and lower driving voltage by the statistical design method. Analysis of time-of-flight measurement shows that amorphous small molecule hole-transporter blended with a blue light-emitting polymer increases the field-dependent hole mobility, with transition from nondispersive to dispersive transport induced by the charge-trapping effect. Moreover, at the electroluminescent devices with different electron injection/transport layer (LiF/Al, LiF/Ca/Al, and Alq3/LiF/Al), efficiency was further increased. We have analyzed that carrier mobility of a multilayered device can also be controlled by the change of electron injection and transport layers. We find that structural design and matching overall charge balance is an essential factor to improve both the operating voltage and efficiency of existing blue polymer devices.

Effects of cathode thickness and thermal treatment on the design of balanced blue light-emitting polymer device

The interface between layered conjugated polymer and electrode is a most important factor to improve the performance and lifetime of polymeric light-emitting devices (PLEDs). In this work, a blue PLED with improved stability was achieved by the combination of optimized cathode structure as well as thermal treatment of light-emitting polymer (LEP). Experimental evidence of the initial luminance “settling in” stage was found to be dependent upon the cathode structure, while the long-term slope of luminance as a function of elapsed time is governed by the annealing conditions. Our study revealed the importance of extrinsic design of device for the improvement of PLED stability. Experimental data shows that a blue PLED annealed at 170°C and 6nm LiF at LiF∕Ca∕Al cathode retained the best lifetime, which can be explained by the improved polymer–metal interface and LEP’s charge mobility.

53.1: Invited Paper: LITI (Laser Induced Thermal Imaging) Technology for High-Resolution and Large-Sized AMOLED

In this paper, we describe a novel color patterning method for the fabrication of high resolution and large format full-color AMOLEDs. Laser Induced Thermal Imaging (LITI) is a laser addressed thermal patterning technology with unique advantages such as excellent transfer film thickness uniformity, multi-layer stack transfer ability, high resolution and scalability to large-Size mother glass. We developed and optimized transfer films, structure of OLED layers, and scanning conditions for the patterning of the evaporated small molecules. As a result, we achieved excellent LITI device stability, which gave the device life time is more than 20,000 hrs with 2.0″ QVGA device architecture for 150 cd/m2 white brightness. As a first step toward the mass production, we set up Gen 4 LITI pilot system.

High efficiency AMOLED using hybrid of small molecule and polymer materials patterned by laser transfer

Abstract Laser‐Induced Thermal Imaging (LITI) is a laser addressed patterning process and has unique advantages such as high‐resolution patterning with over all position accuracy of the imaged stripes of within 2.5 micrometer and scalability to large‐size mother glass. This accuracy is accomplished by real‐time error correction and a high‐resolution stage control system that includes laser interferometers. Here the new concept of hybrid system that complement the merits of small molecule and polymer to be used as an OLED; our system can realize easy processing of light emitting polymers and high luminance efficiency of small molecules. LITI process enables the stripes to be patlerned with excellent thickness uniformity and multi‐stacking of various functional layers without having to use any type of fine metal shadow mask. In this study, we report a full‐color hybrid OLED using the multi‐layered structure consisting of small molecules and polymers.

11.3: Control of Emission Zone in a Full Color AMOLED with a Blue Common Layer

We report a novel structure for a full-color AMOLED (Active Matrix Organic Light Emitting Diode) eliminating the patterning process of a blue emitting layer. The patterning of the three primary colors, RGB, is a key technology in the OLED fabrication process. Conventional full color AMOLED containing RGB layers includes the three opportunities of the defects to make an accurate position and fine resolution using various technologies such as fine metal mask, ink-jet printing and laser-induced transfer system. We can skip the blue patterning step by simply stacking the blue layer as a common layer to the whole active area after pixelizing two primary colors, RG, in the conventional small molecular OLED structure. The red and green pixel showed equivalent performances without any contribution of the blue emission.

Alternative approach to large‐sized AMOLED HDTV

Abstract— In this work, alternative approaches to existing technologies for the fabrication of large-sized AMOLEDs, such as non-laser crystallization methods for poly-Si TFT fabrication and color patterning using laser-induced thermal imaging (LITI), is proposed. In particular, it was found that the super grain crystallization (SGS) method resulted in high-performance TFTs in terms of mobility and off-current. The feasibility of these techniques for large-sized AMOLEDs is demonstrated by 17-in. UXGA AMOLED displays which show good brightness uniformity.