Dong-Kyu Park

Synthesis and characterization of color tunable electro-luminescent polymer by blending oxadiazole containing polymer

Abstract The electro-optical properties of novel polymers such as poly(phenylenevinylene-alt-N-hexylcarbazolevinylene) ([poly(PPV-N-hexylCvz)]) poly[1′,4′-phenylene-1′′,4′′-(2′′-(2′′′′-ethylhexyloxy))phenylene-2,5-(1′′′,4′′′-phenylene)-1,3,4-oxadiazolyl] (PPEPPO) and their blends were characterized. The photoluminescence and electroluminescence (EL) spectra of the blend films composed of poly(PPV-N-hexylCvz) and PPEPPO were mainly contributed from the luminance of poly(PPV-N-hexylCvz) even at low poly(PPV-N-hexylCVz) ratios. Remarkably enhanced EL efficiency was observed as increase the ratio of PPEPPO. This probably results from the energy transfer in the excited state from PPEPPO, with a larger band gap, to poly(PPV-N-hexylCvz) with a smaller band gap. When the ratio of PPEPPO was increased, we observed color emission from green to blue and slightly increased the turn-on voltage compared to poly(PPV-N-hexylCvz). The forward bias turn-on voltage for the light emitting diodes are 5 V for poly(PPV-N-hexylCvz) and 40 V for PPEPPO, respectively.

Covalently attached multilayer self-assemblies of single-walled carbon nanotubols and diazoresins

Layer-by-layer (LBL) multilayer films have been constructed from multiple hydroxyl group directly modified single-walled carbon nanotubes (single-walled carbon nanotubols, SWNTols) and light-sensitive diazoresin (DR). Ultraviolet–visible (UV–vis) absorbance spectra confirm that the alternate assemblies of SWNTols and DR result in uniform film growth. Fourier transformed infrared (FT-IR) spectra of the films show that the main driving force for DR/SWNTol assembly is attributed to hydrogen bond attractions rather than electrostatic interactions. Upon UV irradiation, the hydrogen bonds of the multilayer films transform into covalent bonds, which significantly improves the stability of the DR/SWNTol multilayer films towards etching by polar solvent. Atomic force and scanning electron microscopies (AFM and SEM) indicate high structural homogeneity of the assembled composite films.

Synthesis and cycling behavior of LiMn2O4 cathode materials prepared by glycine-assisted sol-gel method for lithium secondary batteries

Spinel-type LiMn2O4 powders having submicron, narrow particle-size distribution and excellent phasepure particles have been synthesized at low temperatures from metal acetate aqueous solution containing glycine as a chelating agent by a sol-gel method. The dependence of the physicochemical properties and cycling characteristics of the spinel LiMn2O4 powders on the various calcination temperatures has been extensively studied. It was found that the physicochemical properties of the LiMn2O4 powders could be controlled by simply varying the calcination temperature. Glycine-assisted LiMn2O4 powders have shown excellent rechargeability and delivered discharge capacity of 119 mAh/g for more than 150th cycles in Li/polymer electrolyte/liMn2O4 cells.

Balanced charge injection in multilayer polymer light-emitting diode with water soluble nonconjugated polymer dispersed by ionic compounds

The authors have fabricated highly efficient polymeric light-emitting diode (PLED) from ionic compound dispersed water soluble nonconjugated polymer, polyurethane (PU), which was used as an ultrathin hole blocking and electron injection layer (HB-EIL) on the top of commercially available blue-emitting polymer, polyfluorene. The device with HB-EIL showed a maximum quantum efficiency of 1.7%, while the one without HB-EIL showed an efficiency of 0.6%. They propose that the better performance in PLED with PU layer was due to a well balanced charge injection in emitting layer after the enhanced electron injection due to ionic compound in the insulating PU layer.

Synthesis and Photophysical Properties of a Blue Light-Emitting Hyperbranched Poly(fluorene)

A novel hyperbranched conjugated poly(fluorene) (Hyper-PDHF) was obtained through the Suzuki coupling polymerization. Hyper-PDHF has good solubility in common organic solvents and showed good thermal stability up to 428°C with less than 5 wt% weight loss. The photophysical properties of Hyper-PDHF in solution and net solid film are inspected and compared with poly(di-n-hexylfluorene) (PDHF) having linear structure. An absorption maximum of Hyper-PDHF film was determined at 334 nm which was far blue shifted than that of linear PDHF film (381 nm). Hyper-PDHF showed absolute blue photoluminescence (PL) peak at 418 nm with 394 and 444 nm as shoulder peaks. Hyper-PDHF showed also high fluorescence quantum yield as about Φsol = 0.81. In addition, Hyper-PDHF film exhibited almost no longer wavelength emission peaks even the film was annealed at 80°C for 1 hr with illuminating UV (365 nm) in air condition. The high PL efficiency and no aggregate or excimer emission of Hyper-PDHF results from the inhibition of intramolecular or intermolecular interaction by the introduction of the hyperbranched network into the poly(fluorene) backbone.

Synthesis and Photophysics of New Donor-Acceptor Copolymers Based on Fluorene and Phenylquinolines

The synthesis and photophysics of two new donor-acceptor conjugated A-B type copolymers based on conjugated 2,6-disubstitued-4-phenyl-quinoline and 2-substitued-4-phenyl-quinoline, and 9,9-dialkyl-fluorene linked through the 1,4 and 1,3,5 phenyl rings via the Friedländer condensation and Suzuki coupling reaction are presented. The band gap energy of the polymers 1 and 2 measured from the thin films were 430 nm and 380 nm corresponding to 2.88 eV and 3.26 eV, respectively. The photoluminescence (PL) spectra of the thin films of the polymers 1 and 2 showed maximum peaks at 427 and 397 nm corresponding to the deep blue and UV ranges respectively. The red shift of the polymer 1 relative to the polymer 2 can be accounted for by the increased conjugation length because the quinoline unit is oriented to include the phenyl ring in the backbone of polymer 1 . Both polymers have similar trends of irreversible oxidation. The ionization energy of polymers 1 and 2 were measured by cyclic voltammetry (CV) and were found to be 5.60 and 5.65 eV, respectively.

Highly efficient organic light-emitting diodes based on donor-acceptor small molecules

The authors have fabricated highly efficient organic light-emitting diodes from donor-acceptor small molecules, 2-(4-methyl-quinolin-2-yl)-10-phenyl-phenothiazine (MQPTZ) and 2-(4-phenyl-quinolin-2-yl)-10-phenyl-phenothiazine (PQPTZ). Their devices turned on at 4.0V and emitted strong green light near 510nm. The maximum luminance of 60 000 and 55000cd∕m2 at 10.4V and the maximum power efficiencies of 5.75 and 2.5lm∕W at 5V were obtained from the MQPTZ and the PQPTZ, respectively. The overall performance of their devices in terms of current-voltage, luminance, and external quantum efficiency were discussed and compared with those from the device with tris-(8-hydroxyquinolineolato) aluminum.

Surface alignment of ferroelectric liquid crystals using side chain ferroelectric liquid crystal polymer

New surface alignment material for surface stablized ferroelectric liquid crystal displays (SSFLCDs) is investigated. Side chain ferroelectric liquid crystal polymer (SCFLCP) such as Poly{methyl 4-[[[S-(-)-2-methyl-1-butoxy]carbonyl] phenyl ]4′-hexyloxybenzoyl}Siloxane was used as the alignment layer which resulted in good uniform alignment of FLC molecules without zig-zag defects. However, RN-715 polyimide alignment layer showed random alignments with some zig-zag defects in the SSFLC cells. After the field stablization of±20 V AC, the typical stripe-shaped patterns appeared. Good electro-optical performance was observed by using SCFLCP.

Increasing the efficiency of a multilayer bulk heterojunction solar cell using water-soluble conjugated polymer as an electron injection layer

A bulk heterojunction solar cell (BHJ-SC) was fabricated from the conventional active polymer blend P3HT:PCBM by constructing a polymer multilayer structure with novel electron-transporting water-soluble conjugated polymers (WSCPs), poly[9,9′-bis[2-(2-(2-(6′′-(N,N,N-trimethylammonium) ethoxy) ethoxy) ethoxy) ethyl]fluorine, and its copolymer with a phenyl on top of the polymer active layer. In the multilayer structure BHJ-SC, each WSCP coordinated by Na+ ions to the oxygen site on the side chain of the polymer acts as a good electron injection layer for the higher electron collection at the cathode metal. This causes up to 85% increase in the BHJ-SCs power conversion efficiency (PCE) compared to that of the BHJ-SC without WSCPs. This report will discuss the effect of the electron injection layer in the BHJ-SC in terms of the current/voltage characteristics, PCE, electron affinity, and charge mobility of water-soluble polymers.

Poly(p-phenylenevinylene)-Based Alternating Copolymers With Hole Transport Moiety

Phenylenevinylene and aromatic amine segments-based alternating copolymers were synthesized by Horner-Wittig-Emmons polymerisation reaction. The molecular weights of the copolymers were in the range of 39,500 - 26,100 and higher than those of conventional Wittig polymerization method. The maximum absorption and photoluminescence (PL) spectrum of the copolymers are shown at 420, 442 nm and 529, 545 nm with alkyl and alkyloxy substitutents into the phenylenevinylene backbone, respectively. The phenylenevinylene part acts as the emission role and the aromatic amine portions impart the hole transporting mobility and increase the thermal stability. Polymer light emitting diodes (PLEDs) were fabricated by sandwitching the emissive copolymers between ITO and calcium electrodes. The emission colors could be tuned from 492 nm to 530 nm by introducing the alkyl and alkoxy substitutents into the phenylenevinylene ring with applied electric field.