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Cho, Hyunsu,Yoo, Seunghyup Optical Society of America 2012 Optics express Vol.20 No.2
<P>Circular polarizer (CP)-free, high-contrast inverted top-emitting organic light-emitting diodes (ITOLEDs) are demonstrated by using a semitransparent Ag electrode capped with a single dielectric layer as a top anode and Cs(2)CO(3) (1.5 nm)/Al (1.5 nm)/Cr (100nm) as a partially absorbing bottom cathode. Low luminous reflectance is achieved by combining the broadband absorption of Cr, the weak but inherent cavity structure, and the optimal thickness of the capping layer yielding a high transmittance of a top electrode. With the optimized organic capping layer, contrast-enhanced ITOLEDs exhibit a luminous reflectance as low as 3.6% with a large thickness margin. Their luminous efficiency is shown to be comparable to or even higher than that of CP-based conventional OLEDs.</P>
Gupta, D,Seunghyup Yoo,Changhee Lee,Yongtaek Hong IEEE 2011 IEEE transactions on electron devices Vol.58 No.7
<P>In this paper, we present the experimental and simulation results of the stress-recovery characteristics of solution-processed ZnO thin-film transistors under gate bias and current stress conditions. Under both stress conditions, we invariably observed a positive threshold voltage shift (Δ<I>VT</I>) that is initially associated with changes in the values of subthreshold slope and off-current, which later becomes constant on prolonging the stress time. However, Δ<I>VT</I> was less for current stress, compared with gate bias stress. This stress-induced Δ<I>VT</I> is speculated to be caused by defect creation in the active layer and charge trapping at the semiconductor-dielectric interface. Following a stretched exponential model, at room temperature, a characteristics time of 1.6 × 10<SUP>3</SUP>-3.6 × 10<SUP>3</SUP>s during stress and 7.7 × 10<SUP>3</SUP>-15.7 × 10<SUP>3</SUP>s during recovery was obtained under all gate bias and current stress conditions. The Δ<I>VT</I>-time measurements performed under various temperatures yield an activation energy of ~ 0.5 and ~ 0.7 eV for the stress and recovery periods, respectively. The device simulation indicates that Δ<I>VT</I> is mainly caused by the increase in accept or like defects of the density of states in the ZnO channel layer. Furthermore, it was found that the deep lying states are responsible for the change in the value of inverse subthreshold slope.</P>
Controlling the Threshold Voltage of Organic Thin-Film Transistors by Transition Metal Oxides
Hanul Moon,Dongmo Im,Seunghyup Yoo IEEE 2013 IEEE electron device letters Vol.34 No.8
<P>We demonstrate an effective, noble metal-free method to control the threshold voltages (VT) of organic thin-film transistors (OTFTs). Through covering an Al gate electrode with a high work function (WF) transition metal oxide (TMO) layer of WO<SUB>3</SUB> or MoO<SUB>3</SUB>, VT is shifted in a positive direction from -2.15 V to -1.40 V or -0.89 V, respectively, with respect to that of OTFTs with a bare Al gate electrode. Together with a thin dielectric layer of cross-linked Cytop, the reduced magnitude of V<SUB>T</SUB> allows for a low-voltage switching operation with a gate voltage as small as 2 V. The amount of V<SUB>T</SUB> shift is shown to correlate well with the change in the WF of the gate electrode upon TMO deposition.</P>
Hanul Moon,Mincheol Kim,Seunghyup Yoo IEEE 2011 IEEE electron device letters Vol.32 No.8
<P>We propose a systematic fabrication method for stable high-performance organic thin-film transistors (TFTs) that are potentially compatible with high-density integrated circuits. Ag/PEDOT:PSS bilayers provide source/drain (S/D) electrodes with low sheet resistance and efficient hole injection capabilities, leading to high-performance bottom-contact pentacene TFTs with a saturation mobility of 0.19 cm<SUP>2</SUP>/ V·s. Patterned dielectric layers based on a fluoropolymer Cytop function as a hydrophobic bank structure to define S/D electrodes in a self-aligned manner from Ag ink and a PEDOT:PSS solution while simultaneously improving the electrical stability of pentacene TFTs.</P>
Cho, Hyunsu,Yun, Changhun,Yoo, Seunghyup The Optical Society 2010 Optics express Vol.18 No.4
<P>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.</P>
Cho, Hyunsu,Choi, Jung-Min,Yoo, Seunghyup Optical Society of America 2011 Optics express Vol.19 No.2
<P>Highly transparent organic light-emitting diodes (TrOLEDs) are demonstrated using damage-free top cathodes of Cs<sub>2</sub>CO<sub>3</sub>/ Ag capped with ZnS layers. The presence of ultrathin Cs<sub>2</sub>CO<sub>3</sub> layers not only improves the electron injection properties but also makes Ag thin films more continuous and uniform, resulting in ideal top electrodes with low sheet resistance and high transmittance. The combination of the uniform Ag morphology enabled by Cs<sub>2</sub>CO<sub>3</sub> and the optimized thickness of ZnS capping layers results in TrOLEDs that have a peak transmittance as large as 80% with a luminous transmittance of 76.4%. These TrOLEDs exhibit a low turn-on voltage of 2.6V due to injection improvement by the Cs<sub>2</sub>CO<sub>3</sub> layers.</P>
Resistive Switching Characteristics of Sol–Gel Zinc Oxide Films for Flexible Memory Applications
Sungho Kim,Hanul Moon,Gupta, D.,Seunghyup Yoo,Yang-Kyu Choi IEEE 2009 IEEE transactions on electron devices Vol.56 No.4
<P>Unipolar resistive switching devices are investigated for nonvolatile memory applications in a metal-insulator-metal structure in which the insulator layer is based on sol-gel-derived zinc oxide (ZnO) films prepared by a simple spin-coating process followed by thermal annealing. Fast programming ( les 50 ns) and a high off-to-on resistance ratio ( ges 10<SUP>4</SUP>) is demonstrated. The influences on the switching behaviors according to the crystallinity of the ZnO films are studied as a function of the annealing temperature. In addition, the devices are fabricated on a flexible plastic substrate and exhibit excellent durability upon repeated bending tests, demonstrating their potential for flexible low-cost memory devices.</P>
Jae-Woo Park,Dongyun Lee,Hakyoung Kwon,Seunghyup Yoo,Jongmoo Huh IEEE 2009 IEEE electron device letters Vol.30 No.7
<P>We report on performance improvement of n-type oxide-semiconductor thin-film transistors (TFTs) based on TiO<SUB>x</SUB> active channels grown at 250degC by plasma-enhanced atomic layer deposition. TFTs with as-grown TiO<SUB>x</SUB> films exhibited the saturation mobility (mu<SUB>sat</SUB>) as high as 3.2 cm<SUP>2</SUP>/V ldr s but suffered from the low on-off ratio (I<SUB>ON</SUB>/I<SUB>OFF</SUB>) of 2.0 times 10<SUP>2</SUP> ldr N<SUB>2</SUB>O plasma treatment was then attempted to improve I<SUB>ON</SUB>/I<SUB>OFF</SUB>. Upon treatment, the TiO<SUB>x</SUB> TFTs exhibited I<SUB>ON</SUB>/I<SUB>OFF</SUB> of 4.7 times 10<SUP>5</SUP> and mu<SUB>sat</SUB> of 1.64 cm<SUP>2</SUP>/V ldr s, showing a much improved performance balance and, thus, demonstrating their potentials for a wide variety of applications such as backplane technology in active-matrix displays and radio-frequency identification tags.</P>