Ja-Ryong Koo

Electrical properties of porphyrin-based switching devices

Abstract Molecular switching devices were reported utilizing Langmuir–Blodgett (LB) monolayer films containing the 5,10,15,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) as a redox-active component. From current–voltage (I–V) characteristics, it was found that the devices exhibit outstanding switching diode and tunneling diode behavior at room temperature, which seems to be due to the asymmetric device structure. These electrical properties of the devices may be applicable to active components for the memory and/or logic circuits in the future.

Flexible bottom-emitting white organic light-emitting diodes with semitransparent Ni/Ag/Ni anode.

We fabricated a flexible bottom-emitting white organic light-emitting diode (BEWOLED) with a structure of PET/Ni/Ag/Ni (3/6/3 nm)/ NPB (50 nm)/mCP (10 nm)/7% FIrpic:mCP (10 nm)/3% Ir(pq)(2) acac:TPBi (5 nm)/7% FIrpic:TPBi (5 nm)/TPBi (10 nm)/Liq (2 nm)/ Al (100 nm). To improve the performance of the BEWOLED, a multilayered metal stack anode of Ni/Ag/Ni treated with oxygen plasma for 60 sec was introduced into the OLED devices. The Ni/Ag/Ni anode effectively enhanced the probability of hole-electron recombination due to an efficient hole injection into and charge balance in an emitting layer. By comparing with a reference WOLED using ITO on glass, it is verified that the flexible BEWOLED showed a similar or better electroluminescence (EL) performance.

Amorphous Indium Gallium Zinc Oxide Thin-Film Transistors with a Low-Temperature Polymeric Gate Dielectric on a Flexible Substrate

Amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a solution-processed polymeric gate dielectric of cross-linked poly(4-vinylphenol) (c-PVP) film were fabricated on a poly(ethylene terephthalate) (PET) substrate on which an a-IGZO film, as the active channel layer, was deposited by radio frequency (RF) sputtering. The entire TFT fabrication process was carried out at a temperature below 110 °C. The device exhibited an on/off ratio of 1.5×106 and a high field-effect mobility of 10.2 cm2 V-1 s-1, which is, to our knowledge, the best result ever achieved among a-IGZO TFTs with polymeric gate dielectrics on a plastic substrate.

Improved Performance of Pentacene Thin-Film Transistors with Al2O3 Gate Dielectric: Annealing Effect on the Surface Properties

We have fabricated pentacene thin-film transistors (TFTs) with modified Al2O3 layers as gate dielectrics. The effects of thermal treatments on the E-beam-processed Al2O3 layers in the TFTs were investigated by using various analytical tools such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscope (AFM), and a contact angle method. From the results of the analyses, it was confirmed that the performance of thermally treated Al2O3 gate dielectrics is much better than that of non-treated ones. Finally, pentacene TFTs were fabricated with optimum conditions and improved electrical properties were obtained for low-voltage (below -3 V) TFTs as follows: a mobility of 0.9 cm2 V-1 s-1, a threshold voltage of -1.3 V, a sub-threshold slope of 0.12 V/decade, and an on/off current ratio of 2.2 ×105.

Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light-emitting diodes

We investigate multiple quantum well [MQW] structures with charge control layers [CCLs] to produce highly efficient blue phosphorescent organic light-emitting diodes [PHOLEDs]. Four types of devices from one to four quantum wells are fabricated following the number of CCLs which are mixed p- and n-type materials, maintaining the thickness of the emitting layer [EML]. Remarkably, such PHOLED with an optimized triplet MQW structure achieves maximum luminous and external quantum efficiency values of 19.95 cd/A and 10.05%, respectively. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs within each EML. It also shows a reduction in the turn-on voltage from 3.5 V (reference device) to 2.5 V by the bipolar property of the CCLs.

Self Assembled Monolayer for Molecular Electronics

Molecular electronic devices were fabricated with thiol derivatives as redox-active component. This molecules have a thiol radical to allow monolayer formation by the self assembly method, and then this self assembled monolayer (SAM) were inserted between patterned Pt bottom electrode (30 nm) and Ti (5 nm)/Al (30 nm) top electrodes in crossbar array architectures. Before self assembly process, the substrate was treated by a piranha solution or a UV-ozone treatment. From contact angle measurements, before forming SAM, both the substrates treated by the piranha solution and the UV-ozone were hydrophilic condition. After forming SAM, the substrates were maintained with hydrophobic condition in case of the octadecanethiol (C18SH) and hydrophilic condition in case of the 11-mercapto-1-undecanol (HSC11OH). Such results were shown that the SAM was well packed. Also, the current-voltage (I–V) characteristics of the SAM devices along the direction vertical to substrate were measured.

Efficient Hole Injection for Top-Emitting Organic Light-Emitting Diodes Using Nickel Oxide by Oxygen Plasma Treatment

To improve the performance of top-emitting organic light-emitting diodes (TEOLEDs), the effect of oxygen plasma treatment duration on the electrical properties of multi metal Ni/Ag/Ni thin film anodes was investigated. The results revealed that a Ni/Ag/Ni thin-film layer formed upon oxygen plasma treatment for 180 s effectively increased electron-hole recombination probability by improving their charge balance.

NO2 Gas Detection Characteristics of Octa-dodecyloxy Copper-phthalocyanine Langmuir-Blodgett (LB) Films

Metallo-phthalocyanines(MPcs) are very sensitive to toxic molecules such as electron affinitive NO 2 gas and also chemically and thermally stable since lots of MPcs have been studied for the potential chemical gas sensors for NO2 using their electrical conductivity. In this study, thin films of octa-dodecyloxy copper -phthalocyanine were prepared by Langmuir-Blodgett(LB) method and characterized by using UV/Vis absorption spectroscopy, and ellipsometry. It was found that the proper transfer surface pressure for the film deposition was 25mN/m and the limiting area per molecule was 112 A 2 /molecule. The film thickness of one layer was 64 A. Current-voltage(I-V) characteristics of these films were investigated as a function of film thickness.

Enhanced Hybrid Blue Organic Light Emitting Diodes with a Multi-Emitting Layer Using Fluorescent and Phosphorescent Emitters

We investigated efficient blue organic light emitting diodes (OLEDs) containing fluorescent and phosphorescent emitters in the emitting layer (EML). We fabricated several hybrid blue OLEDs with different thicknesses or switching positions for the fluorescent and the phosphorescent EMLs using the fluorescent deep blue emitter 1,6-Bis(N-phenyl-p-CN-phenylamino)-pyrenes and the phosphorescent blue emitter iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate. The fabricated device E stacked a 20 nm thickness of phosphorescent EML and a 10 nm thickness of fluorescent EML. Hybrid blue OLEDs have many advantages in terms of their electrical characteristics, as they have a lower efficiency roll-off than phosphorescent OLEDs and a higher efficiency than fluorescent OLEDs.

Analysis of Charge Transfer Mechanism in Molecular Memory Device using Temperature-dependent Electrical Measurement

A molecular memory device which has a structure of Al//ASA-15 LB monolayer/Ti/Al device, was fabricated. To study a charge transfer mechanism of molecular memory devices, current density-voltage (J-V) characteristics were measured at an increasing temperature range from 10 K to 300 K with an interval of 30 K. Strong temperature-dependent electrical property and tunneling through organic monolayer at low bias (below 0.5 V) were appeared. These experimental data were fitted by using a theoretical formula such as the Simmons model. In comparison between the theoretical and the experimental results, it was verified that the fitting results using the Simmons model about direct tunneling was fairly fitted below 0.5 V at both 300 K and 10 K. Hopping conduction was also dominant at all voltage range above 200 K due to charges trapped by defects located within the dielectric stack, including the , organic monolayer and Ti interfaces.