Hyo-Sang Lee

A Highly Planar Fluorinated Benzothiadiazole‐Based Conjugated Polymer for High‐Performance Organic Thin‐Film Transistors

High-mobility and low-voltage-operated organic field-effect transistors (OFETs) are demonstrated by the design of a new fluorinated benzothiadiazole-based conjugated polymer with fluorinated high-k polymer dielectrics. A record-breaking high hole mobility of 9.0 cm(2) V(-1) s(-1) for benzothiadiazole-based semiconducting polymers is achieved by the excellent planarity of the semiconducting polymer.

Transient characteristics of a GdBCO racetrack pancake coil without turn-to-turn insulation

This paper reports the transient characteristics of two types of magnetically coupled GdBCO racetrack pancake coils: (1)?a pair of insulated coils (INS?INS coils) and (2)?a pair of non-insulated coils (NI?NI coils). In order to investigate the electromagnetic behavior of the INS?INS and NI?NI coils, charging/sudden discharging tests were performed with respect to the distance between primary and secondary coils of the coil pairs. The charging/sudden discharging test results show that the amount of induced current and measured axial magnetic field decreased with increasing distance between the primary and secondary coils. Additionally, charge and discharge delays were clearly observed in the NI?NI coils. The simulated results show good agreement with the experimental results, which validates the proposed approach involving an equivalent circuit model.

Quench and recovery characteristics of a racetrack double pancake coil wound with YBCO-coated conductor

The reliability of high temperature superconducting (HTS) racetrack coils is one of the most important factors for the development of large-scale rotating machines. However, it is necessary to investigate the stability and normal zone propagation characteristics of racetrack coils for large-scale applications such as ship propulsion motors and power generators. In this study, the quench/recovery characteristics of a racetrack-type, double pancake (DP) coil, which could be applied to HTS rotating machines, were investigated using the voltage and temperature profiles in a cryogenic conduction cooling system. The minimum quench heating flux and quench propagation velocity of the racetrack DP coil are also discussed.

Synergistic effects of solvent and polymer additives on solar cell performance and stability of small molecule bulk heterojunction solar cells

We developed p-DTS(FBTTh2)2:PC71BM-based small molecule bulk heterojunction solar cells using 1,8-diiodooctane (DIO) and small amounts of PCDTBT polymer. In the film, PCDTBT effectively suppresses the over aggregation of the p-DTS(FBTTh2)2 donor phase and promotes formation of percolating networks between the donor and acceptor phases. Moreover, the portion of p-DTS(FBTTh2)2 crystallites with the face-on orientation in the blend film is significantly increased and phase separation is decreased, which enables efficient charge generation and transport. Consequently, these solar cells consistently exhibit high fill factors and photocurrent densities and high efficiencies in the range 6.68–8.13% regardless of the DIO content (0.4–3 v/v%). In contrast, a large variation was found in the performance of the solar cells with blend films processed with DIO alone, with efficiencies of 2.75–6.68% depending on the DIO content. More importantly, the PCDTBT-processed solar cells exhibit remarkably improved stability under heating and 65 °C/85% RH. Thus, processing photoactive layers utilizing a combination of DIO and PCDTBT is an effective way of preparing promising small molecule solar cells: DIO promotes the crystallinity of the donor phase, and the intermolecular interactions between the polymer and the push–pull moiety in p-DTS(FBTTh2)2 induce distribution of donor crystallites to form percolating networks by suppressing donor over-segregation.

Investigation of multifilament MgB2 superconducting joint technique for development of MRI magnets

This study presents the investigation of superconducting joints fabricated using multifilament magnesium diboride (MgB2) wires for the development of persistent-current mode magnetic resonance imaging (MRI) magnets. The critical current of the jointed samples decreased with increasing cutting angle because the smaller cutting angle allowed greater exposure of the MgB2 filament, thereby increasing the contact area for the wire-bulk-wire connection. In addition, an appropriate pressing pressure (300 MPa) was necessary to establish the multifilament MgB2 joint without significant degradation of superconducting properties. The resistance of the optimal MgB2 joint, measured using the field-decay technique, was <1.5 × 10-14 Ω. Therefore, the proposed joint technique can be employed for developing multifilament MgB2 MRI magnets operating in the persistent-current mode.