Won Ho Jo

Preparation of chitosan self-aggregates as a gene delivery system

Hydrophobically modified chitosan containing 5.1 deoxycholic acid groups per 100 anhydroglucose units was synthesized by an EDC-mediated coupling reaction. Formation and characteristics of self-aggregates of hydrophobically modified chitosan were studied by fluorescence spectroscopy and dynamic light scattering method. The critical aggregation concentration (cac) of the self-aggregate was determined by measuring the fluorescence intensity of pyrene as a fluorescent probe. The cac value in PBS solution (pH 7.2) was 1.7x10(-2) mg/ml. Mean diameter of self-aggregates in PBS solution (pH 7.2) was 162 +/- 18 nm with an unimodal size distribution. Charge complex formation between self-aggregates and plasmid DNA was confirmed by electrophoresis on an agarose gel. Migration of DNA on an agarose gel was completely retarded above a charge ratio ( +/-) of 4/1 at pH 7.2. The free DNA dissociated from the complexes was observed by electrophoresis above pH 8.0 at a fixed charge ratio of 4/1. An efficient of COS-1 cells was achieved by self-aggregates/DNA complexes.

Fluoro‐Substituted n‐Type Conjugated Polymers for Additive‐Free All‐Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%

Fluorinated n-type conjugated polymers are used as efficient electron acceptor to demonstrate high-performance all-polymer solar cells. The exciton generation, dissociation, and charge-transporting properties of blend films are improved by using these fluorinated n-type polymers to result in enhanced photocurrent and suppressed charge recombination.

A Fluorinated Phenylene Unit as a Building Block for High‐Performance n‐Type Semiconducting Polymer

Copolymers composed of diketopyrrolopyrrole and phenylene units with different numbers of fluorine subsitution are synthesized. When the effect of the number of fluorine substitution on the n-channel transporting property is investigated, the polymer with four fluorine substitutions exhibits the best n-type charge-transporting behavior with an electron mobility of 2.36 cm(2) V(-1) s(1).

Fabrication of Highly Conductive and Transparent Thin Films from Single-Walled Carbon Nanotubes Using a New Non-ionic Surfactant via Spin Coating

Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for fabricating high-quality single-walled carbon nanotube (SWCNT) films by a simple spin coating method. The absence of charge repulsion between SWCNT/surfactant complexes successfully leads to formation of a dense network of SWCNTs on the substrate through a single deposition of spin coating. When the SWCNT film was treated with nitric acid and thionyl chloride after washed with dichloromethane and water, a high-performance SWCNT film with the sheet resistance of 59 ohm/sq and the transparency of 71% at 550 nm was successfully obtained. Since the SWCNT film exhibits a high value of σ(dc)/σ(ac) (∼17) and excellent dimensional stability after releasing from the substrate, the film can be used as a transparent electrode in flexible optoelectronic devices.

Effects of comonomers and shear on the melt intercalation of styrenics/clay nanocomposites

Abstract The effect of polar comonomers introduced into polystyrene (PS) chains on the melt intercalation into organosilicate is investigated in terms of shear and annealing time. In the case, where the interaction between polymers and silicate layers is relatively weak, e.g. PS/organosilicate hybrids, it is difficult for the polymer chains to diffuse into interlayers of organosilicate and the hybrids are unstable under shear. For hybrids in which the interaction is enhanced by incorporating polar comonomers, e.g. acrylonitrile (AN) and methylvinyl oxzoline (OZ), their structures are stable even after a long mixing time. However, they do not show an exfoliated structure under the mixing conditions used in this study. The interaction between the comonomer and the silicate surface is investigated indirectly via both X-ray diffractional and FTIR spectral analyses. It is concluded that the structural stability of polymer/organosilicate hybrids is very dependent on the relative magnitude of the interactions between polymers, organic modifiers, and silicate layers.

Semi-crystalline random conjugated copolymers with panchromatic absorption for highly efficient polymer solar cells

An effective approach to extend the light absorption range of conjugated polymers for high performance photovoltaics is synthesis of copolymers composed of at least two different chromophores with a complementary absorption range. For this purpose, we synthesized random conjugated copolymers consisting of DPP and isoindigo as co-electron accepting units in donor–acceptor type conjugated copolymers. The random copolymers exhibited both broad light absorption and low-lying HOMO level, which contribute to enhancement of JSC and VOC, respectively. Furthermore, the predominant face-on orientation of the random copolymers on the substrate is beneficial for charge transport in the device. The polymer solar cell (PSC) based on the random copolymer has shown a promising efficiency of 6.04% which is the highest value among PSCs based on random copolymers.

Synthesis and photophysical property of well-defined donor–acceptor diblock copolymer based on regioregular poly(3-hexylthiophene) and fullerene

A new, well-defined diblock copolymer (P3HT-b-C60) based on regioregular poly(3-hexylthiophene) (P3HT) and fullerene was synthesized. First, regioregular P3HT was synthesized through Grignard metathesis polymerization, and then methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) were copolymerized by using an end-functionalized P3HT as a macroinitiator for the atom transfer radical polymerization to yield a diblock copolymer (P3HT-b-P(MMA-r-HEMA)). A fullerene derivative functionalized with carboxylic acid, [6,6]-phenyl-C61-butyric acid (PCBA), was then chemically linked to the HEMA unit in the second block (P(MMA-r-HEMA)) to produce a diblock copolymer with the second block containing fullerenes. Annealing thin films of the copolymer revealed nanometer-scale phase separation, a more suitable morphology for enabling excitons generated in the P3HT domain to more efficiently reach the donor–acceptor interface, relative to simple blends of P3HT and C60. As a result, photoluminescence of the P3HT-b-C60diblock copolymer in the films showed a complete quenching of photoluminescence of P3HT, which is indicative of charge transfer between P3HT and fullerene.

Fluorination on both D and A units in D–A type conjugated copolymers based on difluorobithiophene and benzothiadiazole for highly efficient polymer solar cells

Fluorination of conjugated polymers is one of the effective strategies to tune the frontier energy levels for achieving high efficiency polymer solar cells. In this study, three fluorinated D–A polymers, consisting of 3,3′-difluoro-2,2′-bithiophene and 2,1,3-benzothiadiazole (BT) with different numbers of fluorine substitution, were synthesized in order to investigate the effect of fluorination on their photovoltaic properties. The polymers with fluorinated BT show lower frontier energy levels, improved polymer ordering, and a narrower fibril size in the blend with PC71BM. The polymer with mono-fluorinated BT exhibits a superior PCE of 9.14% due to a high SCLC hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination. This result demonstrates that the fluorine content in conjugated polymers should be controlled for optimizing optoelectrical and photovoltaic properties of fluorinated conjugated polymers.

Enhanced Performance and Air Stability of Polymer Solar Cells by Formation of a Self‐Assembled Buffer Layer from Fullerene‐End‐Capped Poly(ethylene glycol)

morphology control, [ 3 ] and device optimization. [ 4 ] To date, a power conversion effi ciency (PCE) over 5% has been obtained by using poly(3-hexylthiophene) (P3HT) [ 5 ] or other low-bandgap polymers [ 6 ] as a donor and [6,6]-phenyl-C 60 -butyric acid methyl ester (PCBM) as an acceptor. One of the most important issues of PSCs is that the effi ciency is lower than conventional Si-based solar cells or dyesensitized solar cells. [ 7 ] Various approaches for morphology control such as thermal annealing, [ 8 ] solvent annealing, [ 9 ] solvent mixture, [ 10 a–e] and microwave annealing [ 10 f ] have been proposed to increase the effi ciency. These methods have been very effective to give nanoscale phase-separated morphology in the horizontal direction (parallel to the fi lm surface). However, the methods have limited control over the vertical distribution of the components in active layer, although the vertical distribution is very critical for effective transport of charge carriers. [ 11 ]

Synthesis of C60-end capped P3HT and its application for high performance of P3HT/PCBM bulk heterojunction solar cells

A new C60-end capped poly(3-hexylthiophene) (P3HT-C60) was synthesized via a simple three-step process, and used as a compatibilizer for P3HT/PCBM composite for the purpose of controlling the morphology of P3HT/PCBM composite film, and thus improving the long-term thermal stability of solar cell performance. When a small amount of P3HT-C60 was added to P3HT/PCBM, the bicontinuous and nanometre-scale film morphology was developed and preserved for 2 h of annealing at 150 °C. Furthermore, the addition of P3HT-C60 as a compatibilizer suppressed large-scale phase separation of P3HT/PCBM composite even after prolonged annealing time (8 days), and as a result, the P3HT/PCBM/P3HT-C60 bulk heterojunction solar cells exhibited the excellent long-term thermal stability of device performance.