Taek Ahn

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 ...

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.

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.

New amorphous semiconducting copolymers containing fluorene and thiophene moieties for organic thin-film transistors

New amorphous semiconducting materials consisting of fluorene-based thiophene copolymers, poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-3-hexylthiophen-2-yl)-5-(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene) P1 and poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-4-hexylthiophen-2-yl)-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene) P2, have been successfully synthesized via a palladium-catalyzed Suzuki coupling reaction. The number-average molecular weights (Mn) of P1 and P2 were found to be 18 300, and 15 800, respectively. These polymers dissolve in common organic solvents such as chloroform, chlorobenzene, and toluene. The UV-vis absorption maxima of P1 and P2 appeared at 436 and 427 nm in solution and 441 and 431 nm in the film state, respectively. X-Ray diffraction (XRD) analysis showed no reflection peaks indicating amorphous collections of randomly oriented polymer chains. Atomic force microscopy (AFM) images of P1 and P2 showed amorphous film morphologies. Field-effect transistor mobilities of stable amorphous OTFTs of P1 and P2 under ambient conditions have been achieved up to 5.4 × 10−4 cm2V−1 s−1 and 1.6 × 10−4 cm2V−1 s−1, respectively. The high stability and mobility of fluorene-based thiophene copolymers in the amorphous state make them a new family of promising candidates for organic thin-film transistors.

Synthesis and electroluminescence properties of ortho-, meta- and para-linked polymers containing oxadiazole unit

A series of electroluminescent π-conjugated polymers containing an oxadiazole group in the backbone was prepared through Hecks coupling or Wittigs condensation reaction, poly[(2,5-bis(5-hexyloxyphenyl)-1,3,4-oxadiazole)-2,2-diylvinylene-alt-1,4-phenylenevinylene] (POOXPV), poly[(2,5-bis(2-hexyloxyphenyl)-1,3,4-oxadiazole)-5,5-diylvinylene-alt-1,4-phenylenevinylene] (PMOXPV) and poly[(2,5-diphenyl-1,3,4-oxadiazole)-2,4-diylvinylene-alt-1,4-(2,5-dihexyloxy)-phenylenevinylene] (PPOXPV). The three polymers were soluble in common organic solvents and showed good thermal stability. The maximum photoluminescence (PL) wavelengths of POOXPV, PMOXPV and PPOXPV appeared at 495, 470, and 510 nm, respectively. The electroluminescence (EL) spectra of POOXPV and PPOXPV showed maximum peaks at 500 and 510 nm, respectively, corresponding to greenish-blue light, with the single-layer light-emitting diodes of Al/polymer/ITO glass fabricated. In blending synthesized polymers with 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM), polymers are also believed to serve as an excellent polymer electron-transporting materials in our devices.

β-Phase formation in poly(9,9-di-n-octylfluorene) by incorporating an ambipolar unit containing phenothiazine and 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran

Novel fluorene-based blue-light-emitting copolymers (F8Rs) in the β-phase are designed and synthesized using the palladium-catalyzed Suzuki reaction. The β-phase can be generated by introducing an ambipolar unit, that contains 10-n-hexylphenothiazine (PTZ) and 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM) (as the donor and acceptor moieties, respectively) on a polyfluorene backbone. By comparing the absorption (peak at 437 ± 1 nm), photoluminescence (PL) (peak at 441 ± 1 nm), and X-ray diffraction (XRD) (peak centered around 2θ = 7.0°) characteristics of the resulting polymers (F8Rs) with those of the polyfluorene homopolymer (PFO), we found that fluorene-based copolymers with ambipolar moieties, such as PTZ and DCM, can prompt the formation of the β-phase after spin-coating without gelling in a poor solvent, exposure to tetrahydrofuran vapor, cooling to liquid-N2 temperature, and reheating. The PL and electroluminescence (EL) efficiencies of the copolymer F8R5 are higher than those of PFO (by factors of two and 15, respectively). The EL spectrum of the copolymer showed almost pure blue emission, with CIE (Commission Internationale de l′Eclairage) coordinates of x = 0.17 and y = 0.10.

Synthesis and LED device properties of carbazole and naphthalene contained conjugated polymers

Abstract Two new fully conjugated alternating copolymers containing both carbazole and naphthalene units were prepared through the Wittig condensation polymerization (carbazole units were linked with napthalene units by 1,4- and 2,6-linkages). The maximum photoluminescence wavelengths of 2,6-PNCPV and 1,4-PNCPV were found at 498 and 545 nm, which correspond to the greenish–blue and the yellow emission region, respectively, depending on the napthalene linkage. The turn-on voltages of 2,6-PNCPV and 1,4-PNCPV were 10.4 and 9.6 V, respectively, when the single-layer light-emitting diodes of Al/2,6-PNCPV or 1,4-PNCPV/ITO glass were fabricated. The electroluminescence spectrum of 1,4-PNCPV exhibited EL emissive band at 553 nm (yellow emission). Interestingly, the EL spectrum of 2,6-PNCPV was tailed to the longer wavelength region from the EL maximum point of 496 nm, which may be caused by formation of the intermolecular excimers. The emission color of 2,6-PNCPV was almost white to the naked eyes due to the tailing of 2,6-PNCPV emission and the weak interchain excimer peak at 750 nm.

Synthesis and properties of new light-emitting polymers containing fluorinated tetraphenyl units

A series of fully conjugated blue light-emitting polymers has been synthesized. The polymers are composed of fluorinated tetraphenyl units, and dialkoxybenzene, carbazole and fluorene containing arylenevinylene units. The synthesized polymers were soluble in common organic solvents and thermally stable up to 300°C. The photoluminescence (PL) peak wavelengths of the polymers were varied from 490 to 468 nm, depending on the polymer structure. Single-layer EL devices using the polymers were fabricated. Light emission from the devices becomes visible between 12 and 22 V, depending on the device structure.

Systematic approach of blue-light-emitting copolymers by introducing various naphthalene linkages into fluorene containing PPV derivatives

Abstract We newly synthesized a series of naphthalene and fluorene containing alternating copolymers, poly[9,9- n -dihexyl-2,7-fluorenediylvinylene- alt -1,4-naphthalenevinylene] (1,4-PNFPV), poly[9,9- n -dihexyl-2,7-fluorenediylvinylene- alt -2,6-naphthalenevinylene] (2,6-PNFPV), and poly[9,9- n -dihexyl-2,7-fluorenediylvinylene- alt -2,3-naphthalene-vinylene] (2,3-PNFPV) through the well known Wittig polycondensation reaction. The conjugation lengths of the polymers were controlled by differently linked naphthalenes in the polymer main chain. The resulting polymers were completely soluble in common organic solvents and exhibited good thermal stability up to 400 °C. The synthesized polymers showed UV–visible absorbance and photoluminescence (PL) in the ranges 352–379 and 465–512 nm, respectively. The maximum emission peak of 1,4-PNFPV was found at 512 nm with green light. But, 2,6- and 2,3-PNFPV showed more blueshifted blue PL emission than that of 1,4-PNFPV at 477 and 465 nm, respectively. This result showed that 2,3- and 2,6-naphthalene linkage of the 2,3- and 2,6-PNFPV reduced π-conjugation length compared to 1,4-naphthalene linkage of the 1,4-PNFPV. The single-layer light-emitting device was fabricated which has a simple ITO (indium–tin oxide)/polymer/Al configuration. Electroluminescence (EL) maxima of 1,4-PNFPV and 2,6-PNFPV were shown at 523 and 484 nm, respectively. The measurement of current vs. electric field showed the threshold voltages of 1,4-PNFPV and 2,6-PNFPV to be about 1.7×10 8 and 2.1×10 8 V/m. The EL powers of 1,4-PNFPV and 2,6-PNFPV were reached 36 and 24 cd/m 2 at 2.3×10 8 V/m (206 mA/cm 2 ) and 2.8×10 8 V/m (190 mA/cm 2 ), respectively.

Synthesis and characterization of sulfur-containing PPV derivatives

Abstract New sulfur-containing poly(2-hexylthio-5-methoxy-1,4-phenylenevinylene) (PMHPV) and poly(2-hexylsulfinyl-5-methoxy-1,4-phenylenevinylene) (PMHOPV) were synthesized through water-soluble precursor route. These polymers were characterized using UV-visible, FT-IR and thermal analyses. The light emitting devices composed with ITO/polymer films/A1 were fabricated and luminescent properties were characterized. The PMHOPV that contains the electron-accepting hexylsulfinyl group showed lower electrical conductivity of 1.2 × 10 −4 S/cm than that of PMHPV (9 × 10 −2 S/cm) and maximum EL emission in the longer wavelength region.