Jae Min Hong

An alternating copolymer consisting of light emitting and electron transporting units

An alternating copolymer composed of fluorenedivinylene as the light emitting unit and pyridine as the electron transporting one was synthesized by employing the Wittig reaction. The copolymer which has conjugation throughout the molecular chain is soluble in both polar and nonpolar solvents. The copolymer has a band gap energy of 2.85 eV deduced from an ultraviolet-visible absorption spectrum, and ionization potential and electron affinity of –5.67 and –2.82 eV, respectively, deduced from a cyclic voltammogram. The photoluminescence (PL) emission maximum was observed at 440 or 540 nm depending on the solvent used in making the solution for spin-casting. The copolymer was also capable of transporting electrons and could be used as an electron transporting layer. A light emitting diode (LED) fabricated with a blend of polyvinylcarbazole (PVK) with a fluorene-based light emitting material, and this copolymer as an electron transporting layer, exhibited an electroluminescence (EL) emission maximum at 475 nm with a full width at the half maximum (FWHM) of 50 nm and a quantum efficiency of 0.1%, where indium tin oxide (ITO) and Al were used as the anode and cathode, respectively.

J-aggregation induced low bandgap anthracene-based conjugated molecule for solution-processed solar cells

A new anthracene-based X-shaped conjugated molecule, HBTATHT, was synthesized. Thin film transistors based on unannealed HBTATHT showed a carrier mobility of 0.15 cm(2) V(-1) s(-1) (I(on/off) = 7.9 × 10(6)). Further, a solution processed solar cell made of HBTATHT exhibited promising power conversion efficiencies of 4.84% and 4.70% with PC(61)BM (1:0.8 wt ratio) and PC(71)BM (1:0.6 wt ratio), respectively.

Enhanced performance of organic photovoltaic devices by photo-crosslinkable buffer layer

AbstractThe performance of organic photovoltaic devices was enhanced by insertion of the photo-crosslinkable buffer layer. This buffer layer was formed by a photo curable precursor with bithiophene and pentadienyl moieties. The fill factor of the device with this buffer layer exceeded 0.7 in the organic photovoltaic cell. The characteristic of the photo-crosslinkable property enabled this buffer layer to be inserted between the hole extraction layer and the active layer, which formed an ohmic contact with both layers. The insertion of the buffer layer induced a 20% enhancement in conversion efficiency by small increases in the short-circuit current and the open-circuit voltage, and a huge increase in the fill factor. This photo-crosslinkable buffer layer acted as a leakage current reducing layer as measured by the reduced dark current. The high crystallinity and smooth surface of this buffer layer resulted in improved surface morphology and internal packing, thus enhanced the fill factor.

Selective photonic sintering of Ag flakes embedded in silicone elastomers to fabricate stretchable conductors

Stretchable conductors have recently attracted much attention because of rapid developments in wearable and deformable electronics. Conductors are one of the essential components of electronic devices. A photonic sintering process with intense pulsed light from a xenon lamp is used for sintering printed ink to fabricate conductors, without damaging the substrate. However, it is difficult to achieve the most desired properties such as high conductivity and mechanical stretchability in stretchable conductors fabricated by photonic sintering. Here, highly stretchable conductors are developed by the selective sintering of conductive materials embedded in silicone elastomers, overcoming this drawback. The conductors are composed of conductive Ag flakes and elastomeric Ecoflex. Photonic sintering generates conductive paths without damaging the transparent elastomer. As a result, a stretchability of 500% is achieved without the need for any structural designs. Furthermore, a wireless power transfer circuit incorporating the conductors was successfully applied to operate a light emitting diode wirelessly. This approach opens up the possibility of developing new types of stretchable and deformable electronics for future applications.

A Study on the Fabrication of Organic Thin Film Transistor Sensors using Gravure Printing

Conducting polymer TFT was fabricated with gravure printing technique using a vapor polymerization method. Polyvinyl alcohol (PVA) with excellent film forming characteristics was used as a matrix polymer. After the printing of the oxidant dispersed PVA dissolved in DI water, the vapor polymerization of the pyrrole monomer formed a thin conducting polymer film. The conductivity of the film was dependent on the concentration of the oxidant and the polymerization time. In order to be used for TFT application, the conductivity had to be reduced by controlling the various conditions. The effect of exposure to humidity on TFT signal was studied.