Hong-Ku Shim

High-photosensitivity p-channel organic phototransistors based on a biphenyl end-capped fused bithiophene oligomer

We report highly photosensitive organic phototransistors (OPTs) based on a 2,5-bis-biphenyl-4-yl-thieno[3,2-b]thiophene (BPTT). The measured maximum sensitivity and the ratio of photocurrent to dark current (Iph∕Idark) in BPTT OPTs were 82A∕W and 2.0×105 under 380nm UV light with 1.55mW∕cm2, respectively. The prepared OPTs show a photocurrent response similar to the absorption spectrum of BPTT. The major mechanisms for photocurrent amplification in this device were verified from experimental results as photovoltaic (turn-on) and photocurrent effect (turn-off) by a fitting to theoretic equations.

Polymer light-emitting devices using ionomers as an electron injecting and hole blocking layer

The effect of ion concentration, neutralization level and counterions in ionomers was systematically studied to obtain the optimal electroluminescent (EL) characteristics in polymer light-emitting diodes using poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) for the emissive layer and sulfonated polystyrene (SPS) ionomers for the electron-injecting layer. The optimum ion concentration of NaSPS was determined to be at 6.7 mol %. Ionomers with a higher neutralization level make the EL device more efficient, with the highest efficiency being at 200% overneutralization. The ionomer with a smaller metal counter ion greatly enhances the efficiency of EL devices with the indium–tin–oxide/MEH-PPV/LiSPS/Al device having the highest EL quantum efficiency, 1.18% photons/electron. The dominant factor in enhancing the luminance is the number of ionic dipoles near the cathode irrespective of the type of metal counterions, while the hole blocking mostly depends on the restriction of chain segmental ...

Light-Emitting Characteristics of Conjugated Polymers

This article reviews mainly the results of our recent research on the relationship between the structure and the luminescence properties of PPV derivatives. PPV derivatives are particularly useful in an effort toward the establishment of such relationship because their chemical structures can be manipulated very systematically. Attachment of a wide variety of substituents, inclusion of kinky structural units, modification of main chain structures by inclusion of hole- and/or electron-transferring structures, and blending of polymers having different optical and electronic properties are representative approaches. The device characteristics of the light-emitting diodes (LEDs) fabricated from these polymers are discussed in relation to their structures. In certain cases, their photoluminescence (PL) properties are compared with their electroluminescence (EL) properties.

High mobility organic single crystal transistors based on soluble triisopropylsilylethynyl anthracene derivatives

A series of new anthracene based semiconductors are designed and synthesized. By substituting appropriate acenes at the 2,6-positions of triisopropylsilylethynyl anthracene (NMR, IR, DSC and TGA spectra and crystallographic information of TIPSAntBT and TIPSAntNa), two different derivatives were prepared. Especially, TIPSAntNa (naphthalene as a side group) showed superior performance when it was used as channel material. A hole mobility as high as 3.7 cm2 V−1 s−1 was obtained from single crystal OFETs. To elucidate the origin of this high performance, we carried out comparative studies to investigate the direct relationship between the molecular-packing parameters and the field-effect mobility in single-crystal OFETs because the performance of such single-crystal OFETs is not affected by defects and grain boundaries. Comparing TIPSAN single crystal OFETs having four different acene derivatives, and applying the concept of molecular overlap ratio along the long/short axis, we could show that the effective π-stacking area dominantly determines the field-effect mobility of π-stacked materials. In the case of TIPSAntNa, a large π-stacking area and a small π-stacking distance enabled the highest field-effect mobility.

A new family of bis-DCM based dopants for red OLEDs

We have synthesized a series of novel bis-DCM derivatives as candidate red dopants for use in organic light-emitting devices (OLEDs), by introducing various donor-substituted aryl rings. Compared to DCJTB (621 nm), the novel dopants (637–677 nm) showed more red-shifted emission in 1,2-dichloroethane. Using bis-DCMNEtOBu (7) as a dopant, we fabricated OLEDs with the configuration of ITO/4,4′,4″-tris(3-methylphenylamino)triphenylamine (m-MTDATA) (20 nm)/N,N′-bis(1-naphthyl)-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) (40 nm)/tris(8-quinolinolato)aluminium (Alq3) : red dopant (35 nm, x wt%)/Alq3 (35 nm) LiF/Al. The device with a doping concentration of 1.25 wt% showed pure red emission at λmax = 654 nm (chromaticity coordinate: x = 0.67, y = 0.33) and a maximum brightness of 2500 cd m−2. The chromaticity coordinates were almost independent of current density. Moreover, highly efficient red emission (x = 0.63, y = 0.36) was obtained in the 0.74 wt% doped device. The maximum external quantum efficiency was 4.46% at 7 V, the current efficiency was 3.43 cd A−1, and the power efficiency was 1.64 lm W−1. The highest brightness of 8300 cd m−2 was obtained at 19.6 V.

Electroluminescence of polymer blend composed of carbazole group contained PMA and MEH-PPV

Abstract Alkylstyryl carbazole group containing poly(methacrylate) (Cz-PMA) was prepared and blended with poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). The blends were characterized by UV-Vis, photoluminescence (PL), and electroluminescence (EL) studies. The polymer blends showed two isolated PL emission peaks at 440 and 560 nm corresponding to their component polymers, but the only one EL emission peak of the MEH-PPV/Cz-PMA blended light-emitting diode (LED) appeared near 580 nm which corresponds to that of MEH-PPV. This result indicates that the energy of excited Cz-PMA was transferred into MEH-PPV, resulting in the enhancement of EL intensity.

New selenophene-based semiconducting copolymers for high performance organic thin-film transistors

A series of new selenophene-based organic semiconducting copolymers, poly(5,5′-bis(3-dodecylthiophen-2-yl)-2,2′-biselenophene) (PDT2Se2) and poly(5,5′-bis(4,4′-didodecyl-2,2′-bithiophen-5-yl)-2,2′-biselenophene) (PDT4Se2), were successfully synthesized by Stille and oxidative coupling reactions. Our aim was to investigate the effects of the selenophene units and inserted dodecylthiophenes on the optical and electrochemical properties of these copolymers, their intermolecular ordering in the film state, and hence their thin-film transistor (TFT) performance. X-ray-diffraction (GIXRD and XRD) and theoretical calculations for models of these polymers were used to show that PDT2Se2 films have well organized interlayer packing and π–π stacking, whereas the films of PDT4Se2, which contain regioregularly inserted additional dodecylthiophenes next to the repeat units of PDT2Se2, have a long-range amorphous structure. The TFT characteristics of these polymers are strongly dependent on the intermolecular ordering of the polymer chains. PDT2Se2 exhibited a high hole transporting mobility of 0.02 cm2V−1 s−1 due to its excellent intermolecular ordering, whereas PDT4Se2 exhibited a very poor mobility of 1.4 × 10−5 cm2V−1 s−1 due to its amorphous characteristics, which result from the repulsion between the additional dodecylthiophenes. These results confirm that it is important to consider intermolecular ordering in the design of semiconducting materials for high performance OTFTs.

In situ investigation of degradation in polymeric electroluminescent devices using time-resolved confocal laser scanning microscope

Degradation progress in polymeric electroluminescent devices during device operation was clarified by in situ measurement of photoluminescence image change using time resolved confocal laser scanning microscope. It is found that the degradation stems from two kinds of dark spots, which have the dented circle shape and the dome shape. Dented circle shapes grow during the operation. Dome shapes, where the polymer layers are detached from the indium-tin-oxide electrode, appear suddenly and do not change in size. Comparison of the Auger analysis depth profiles of the dented and the normal sites indicates that the polymer layer at the dented sites disappears. Time-resolved confocal laser scanning microscopy is found to be a powerful technique to analyze the degradation process.

Syntheses, electrical and nonlinear optical properties of FPV derivatives containing alkoxynitrostilbene group

Abstract Poly[2-(3-{4-[2-(4-nitrophenyl)vinyl]phenoxy}propoxy)phenylenevinylene], poly(NO2STPV), containing nonlinear optical chromophore and its copolymers with PPV were prepared through the Wesslings precursor method. These polymers were characterized by using UV-visible and FT-IR spectroscopy. The electrical conductivities of FeCl 3 -doped poly (NO2STPV) and its copolymer films were in range of 10 −3 ∼ 10 −4 S/cm. The copolymer films were readily drawn upto draw ratio of 4.8∼6.4, and their conductivities were ∼10 −2 S/cm. The third order nonlinear optical susceptibility, χ (3) , was measured by using degenerate four-wave mixing (DFWM) technique at 602 nm. The measured χ (3) value of poly(NO2STPV) was 1.7 × 10 −10 esu.

Synthesis and Luminescent Properties of Blue Light Emitting Polymers Containing both Hole and Electron Transporting Units

Poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene-1,4-phenylene], POFB-MEH-PPV, poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-9,9-dihexyl-2,7-fluorenediylvinylene-1,4-phenylene], POFB-PF, and poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-N-ethylhexyl-3,6-carbazolevinylene-1,4-phenylene], POFB-PK, were synthesized by the well-known Wittig condensation polymerization. We incorporated the high electron affinity (octafluorobiphenyl) and hole-transporting (carbazole, fluorene, and dialkoxyphenyl) units into the conjugated main chain. The conjugation lengths are limited to the blue-emission region by ether linkage. The resulting polymers were completely soluble in common organic solvents such as chloroform, 1,2-dichloroethane, and cyclohexanone, and exhibited good thermal stability up to 300°C. The synthesized polymers showed UV-visible absorbance and photoluminescence (PL) in the ranges of 350–385 nm and 460–490 nm, respectively. The fluorene or carbazole containing POFB-PF and POFB-PK showed blue photoluminescence peaks at 470 and 460 nm, respectively. The single-layer light-emitting diode was fabricated in a configuration of ITO (indium-tin oxide)/polymer/Al. Electroluminescence (EL) emission of POFB-PF and POFB-PK were shown at 475 and 458 nm, respectively, corresponding to the pure blue emissions. And, a dialkoxyphenyl containing POFB-MEH-PPV showed greenish blue light at 494 nm. But, LED devices from synthesized polymers showed poor device performance and high turn on voltage. So, we fabricated light-emitting diodes (LEDs) from blend polymers composed of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and POFB-MEH-PPV (POFB-PF or POFB-PK) as the emitting layers. The EL emission maxima of each blend polymers were in the range of 573–591 nm, which indicates that the emission is mainly due to MEH-PPV and POFB-MEH-PPV (POFB-PF or POFB-PK) contributes to the enhancement of the luminescence. And each blend polymers exhibited higher EL quantum efficiency compared with MEH-PPV at the same current density.