Toshihiro Ohnishi

Organic electroluminescent devices having poly(dialkoxy-p-phenylene vinylenes) as a light emitting material

Abstract We have successfully prepared organic electroluminescent, EL, devices having poly(2,5-dialkoxy-p-phenylene vinylene)s, RO-PPV, as light emitting materials, ITO as a transparent electrode and Mg-Ag alloy as a negative electrode. Intensities of photo-, PL, and electro-luminescence for RO-PPVs depended on the chain length of alkoxy side groups. The PL intensity of the RO-PPV increased with increasing the alkoxy chain length, while the EL intensity of the devices using RO-PPV increased until a decyloxy group and then decreased. The EL intensity of the device increased with decreasing the work function of negative electrodes. This result is in good agreement with that reported by Heeger[5]. The luminance reached about 1000cd/m 2 by using both heptyloxy-PPV and MgAg alloy. It is noteworthy that usage of poly(thienylene vinylene), PTV, thin film as a buffer layer between ITO and RO-PPV results in increasing the breakdown voltage and that the luminance of the EL device reaches 1480 cd/m 2 .

Highly conducting poly(phenylene vinylene) derivatives via soluble precursor process

Abstract Highly stretched PPV films were successfully prepared via pyrolysis of a soluble precursor polymer of sulfonium salt with stretched ratio of 10 to 15. The conductivity reached to 4 −5×103 S/cm by H2SO4 doping. The dichroism in polarized ir spectra was found to be the optimum measure for the degree of orientation of the PPV film. The conductivity increment of stretched PPV film doped with H2SO4 was also found to have a good relation to the dichroic ratio of the phenyl group at 1520cm−1. PPV films with dimethoxy- and diethoxy substituents at 2,5-position were prepared by the sulfonium salt process for the first time. By I2 doping, these films showed high conductivity of over 200 S/cm which is far from 10−3 S/cm of non-substituted PPV. Molecular orbital calculations suggest that the introduction of the alkoxy group lowers the ionization potential of PPV and enhances the affinity of PPV to an acceptor dopant.

Marked Enhancement of Photoconductivity and Quenching of Luminescence in Poly(2,5-dialkoxy-p-phenylene vinylene) upon C60 Doping

The absorption spectrum and electrical conductivity of poly(2,5-dioctyloxy-p-phenylene vinylene) (OO-PPV) are hardly influenced by doping of a small amount of C60, suggesting that C60 is not an effective dopant for OO-PPV. On the other hand, photoluminescence has been markedly quenched and photoconductivity has been enhanced by several orders of magnitude upon introduction of several mol% of C60, suggesting photo-induced charge transfer between OO-PPV and C60. These results are discussed in terms of relative electronic energy states of OO-PPV and C60. The photoexcited exciton-polaron (Ex-P) is speculated to migrate along 50~100 monomer units of the polymer main chain in its lifetime and dissociate when encountered with C60.

Color-Variable Light-Emitting Diode Utilizing Conducting Polymer Containing Fluorescent Dye

A color-variable light-emitting diode has been realized utilizing conducting polymer, poly(2,5-dioctyloxy-p-phenylene vinylene) (ROPPV-8), mixed with fluorescent dye, 8-hydroxyquinoline aluminum (Alq3). The electroluminescence of the diode changes from orange to greenish-yellow in color with increasing applied voltage. On the other hand, a light-emitting diode with the two-layer structure of ROPPV-8 and Alq3 shows only light emission from the ROPPV-8 layer. This difference is discussed in terms of the carrier injection process to Alq3.

Highly conductive graphite film prepared from pyrolysis of poly(p-phenylene vinylene)

Pyrolysis of poly(p-phenylene vinylene), PPV, up to 3000 °C was studied in relation to the molecular orientation in the PPV film prepared via decomposition of a soluble precursor sulphonium salt polymer and simultaneous biaxial stretching. The conductivity of PPV pyrolysed at 3000 °C was strongly influenced by biaxial stretching and reached 104 S/cm as it was and 105 S/cm by SO3 doping at three-fold stretching, while conductivities of non-stretched PPV were 103 S/cm and 104 S/cm, respectively. The conductivity obtained for the stretched PPV is comparable to that of a highly oriented pyrolytic graphite (HOPG) or natural graphite. X-ray diffraction analyses for the stretched PPV and the pyrolysed products suggest that the biaxial stretching induces planer orientation in PPV and facilitates its graphitization. It is noteworthy that PPV film is pyrolysed to give a highly oriented graphite maintaining the film form. The Raman spectrum analysis indicated that the film is converted into a pure graphite above 2750 °C.

Highly Conducting Poly(P-Phenylene Vinylene) Prepared From Sulfonium Salt

Abstract Highly conducting poly(p-phenylene vinylene), PPV, has been obtained via pyrolysis of the soluble precursor polymer derived from p-xylylene bis(diethyl sulfonium bromide) which is processed to film. The highly oriented PPV film is obtained by stretching the precursor film up to ten fold at a higher temperature. The films show extremely high conductivity of 2780 S/cm and 491 S/cm by doping AsF 5and SO3 respectivly, with high electrical anisotropy.

Doping effect of buckminsterfullerene in poly(2,5‐dialkoxy‐p‐phenylene vinylene)

Photoluminescence has been markedly quenched and photoconductivity has been enhanced by more than one order of magnitude upon introduction of several mol% of buckminsterfullerene (C60) to poly(2,5‐dialkoxy‐p‐phenylene vinylene) (RO‐PPV), especially at excitations about 2.2 eV, corresponding to the band gap energy of RO‐PPV and also in bands at 1.8 and 3.5 eV, which correspond to optical excitation of C60 molecules, suggesting that photo‐induced charge transfer occurs between RO‐PPV and C60. On the other hand, absorption spectrum and electrical conductivity of RO‐PPV have been scarcely influenced by doping of small amount of C60, suggesting that the ground state charge transfer between C60 and RO‐PPV is not effective, contrary to the case of poly(3‐hexylthiophene). These results are discussed by taking relative electronic energy states of RO‐PPV and C60 into consideration. The photo‐excited exciton‐polaron (Ex‐P) in RO‐PPV is interpreted to migrate along about 100 monomer units along a polymer main chain i...

Resonance Raman spectra of SO3-doped poly(p-phenylenevinylene)

Abstract The resonance Raman spectra of SO 3 -doped poly( p -phenylenevinylene) (PPV) with two different electrical conductivities of 1 and 10 S cm −1 have been observed and analyzed by referring to the data of the radical cations of the model compounds. Both SO 3 -doped PPV films with electrical conductivities of 1 and 10 S cm −1 are considered to contain positive polarons having a more or less uniform lengths. The difference in the formation of polarons on the two sides of the doped film, i.e., one shiny side and the other dull side, is observed.

Endor spectroscopy of poly(paraphenylene vinylene)

Summary form only given. Polarons and other elementary excitations in poly(paraphenylene vinylene) (PPV) with nondegenerate ground-state structure have been extensively studied in relation to the mechanism of intense electroluminescence observed in this polymer. The spatial extension, or wave function in other words, of these excitations is an important physical quantity in identifying the existence of the excitations. Recently we have reported the first high-resolution ENDOR (electron nuclear double resonance) spectra of paramagnetic species existing in the dark state of undoped PPV using stretch-oriented samples [1]. The ENDOR spectra exhibit a clear anisotropy of a /spl pi/-electron on the conjugated chain. The maximum value of the spin density reaches 0.09, corresponding to the half-width of the spatial extension of the spin over several phenyl rings. We show that the observed dark spins are trapped polarons from the comparison of dark and photo-induced ESR as well as reported optically-detected ESR (ODESR) spectra. The detail of the spin density distribution will be discussed from the resolved structures in the ENDOR spectrum of the stretch direction.

Preparation of graphite film by pyrolysis of polymers

Abstract Pyrolysis of aromatic polymers up to 3000°C was studied in relation to the molecular structure and orientation. Poly(p-phenylene vinylene), PPV, was pyrolyzed to give highly oriented graphite in keeping film form. Raman spectrum analyses indicated that PPV was converted into pure graphite above 2750°C. The graphite film obtained showed 104 S/cm as it was and 105 S/cm by SO3-doping, which are comparable to those of the highly oriented pyrolytic graphite (HOPG). X-ray diffraction analyses and scanning electron micrographs for the pyrolyzed products suggest that the biaxial stretching facilitates graphitization of PPV and orientation of the graphite plane. Aromatic polyimide was also converted into graphite and showed 104 S/cm after pyrolysis at 3000°C, while aromatic polyamideimide and polyamide showed 103 S/cm at most after pyrolysis at the temperature.