Myeong-Suk Kim

High-efficiency stacked white organic light-emitting diodes

We report efficient tandem white organic light-emitting diodes (WOLEDs) by using bathocuproine:Cs2CO3∕MoO3 as an effective interconnecting layer. We utilized two primary colors of sky blue and orange fluorescent emitters to obtain efficient white electroluminescence. Although single WOLEDs using two adjacent emitting layers showed a maximum current efficiency of 7.96cd∕A with Commission Internationale d’Eclairage (CIE) coordinates of (0.28, 0.34), the tandem WOLED device made by stacking two single color OLEDs in series demonstrated doubled maximum current efficiency of 17.14cd∕A with CIE coordinates of (0.28, 0.41). The stacking of different single color OLEDs in series instead of double stacking of WOLEDs can be useful to achieve highly efficient WOLEDs because it can reduce the number of layers of the devices.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

A stable blue host material for organic light-emitting diodes

We have developed a high performance fluorescent host material, 6-anthracene-9-yl-2,3-di-p-tolyl benzo[b]thiophene (ATB), for blue organic light-emitting diodes. ATB formed a stable amorphous solid state film with a high glass transition temperature (Tg=116°C). The multilayer devices fabricated using ATB as a blue host material showed higher power efficiency (6.4lm∕W at 1000cd∕m2) than the conventional device using 2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN) (4.3lm∕W at 1000cd∕m2). The half lifetime of the ATB device (6480h at 1000cd∕m2) was also enhanced compared to the TBADN device (5341h at 1000cd∕m2).

Local modification of the thin YBa2Cu3O7−y microstrips by the voltage-biased atomic force microscope tip

The atomic force microscope (AFM) tip biased at around −15 V is found to be capable of locally modifying the entire thickness of 40-nm-thick semiconducting or superconducting YBa2Cu3O7−y microstrips in air. We show, using combined electrical and AFM measurements, that the local regions underneath the surface of the semiconducting or superconducting YBa2Cu3O7−y microstrips are transformed into either nonconducting or nonsuperconducting regions, respectively, upon applying the negatively biased AFM tip. The conductance of the nonsuperconducting regions is also found to be comparable to that of the superconducting regions before modification at 298 K.

Spontaneous assembly of ordered atomic wires with a long interwire distance on a stepped atomic template

Indium atomic wires with a long interwire distance of 5.73 nm were ordered spontaneously at room temperature on a stepped atomic template using a Si(557) surface. The long interwire distance is very interesting because, in general, interwire interactions are needed to order atomic wires in such a way that ordered atomic wires have a short interwire distance of just a few A. The Si(557) surface is composed of four steps, i.e., one (111) step and three (112) steps, with a very similar local structure to each other. However, mobile indium atoms at room temperature were adsorbed specifically onto the second Si(112) step while maintaining the overall structure of the stepped atomic template, as observed by scanning tunneling microscopy, which results in the ordered atomic wires with the long interwire distance. This was supported by first-principles calculations.

A novel Dual String NOR (DuSNOR) memory cell technology scalable to the 256 Mbit and 1 Mbit flash memories

We have developed a novel NOR-type flash memory technology named Dual String NOR (DuSNOR). In the DuSNOR cell array, two adjacent cell strings share a source line and up to 128 cell transistors can be attached to a string. This makes the cell size of DuSNOR smaller than NAND, without sacrificing the advantages of the NOR-type cell. Both the program and erase operations utilize the Fowler-Nordheim tunneling. DuSNOR cells with a cell size of 1.60 /spl mu/m/sup 2/ was fabricated using 0.5 /spl mu/m design rules. It was found that the DuSNOR cell has a fast operating speed, reduced overprogram and disturb problems, and excellent endurance characteristics. DuSNOR is a promising technology for high density, high speed, and random-access flash memories with a small cell size, excellent device characteristics and simplicity in the fabrication process.