Maki Hamaguchi

Polarized electroluminescence from rubbing‐aligned poly(2,5‐dinonyloxy‐1,4‐phenylenevinylene) films

Polarized light emission was observed in conjugated polymer electroluminescent diodes comprising rubbing‐aligned poly(2,5‐dinonyloxy‐1,4‐phenylenevinylene) as the emissive layer. By using 2‐(4‐biphenyly1)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole dispersed in polystyrene as the electron‐transporting/hole‐blocking layer, the electroluminescence efficiency and the dichroic ratio of the electroluminescent light increased, resulting in polarized electroluminescence with a dichroic ratio of 4.0.

COLOR-VARIABLE ELECTROLUMINESCENCE FROM MULTILAYER POLYMER FILMS

Color‐variable emission has been observed from a multilayer polymer electroluminescent device composed of three polymer layers: perfluoropropylated poly(p‐phenylene) (F‐PPP) sandwiched between two emissive polymers, poly (3‐dodecylthiophene) (PDT) and poly (p‐phenylenevinylene) (PPV). Green and red emissions were observed when a negative bias was applied to the PPV and PDT side electrodes, respectively. F‐PPP was supposed to act as an electron‐blocking layer due to its small electron affinity (2.2 eV) compared with those of PPV (2.7) and PDT (3.5), and to facilitate the confinement of the excitons in the negatively biased polymer layer, where the radiative recombination of the charge carriers takes place.

Rubbing-induced molecular orientation and polarized electroluminescence in conjugated polymer

A preferred molecular orientation in a conjugated polymer, poly(2,5-dinonyloxy-1,4-phenylenevinylene) (NO-PPV), can be obtained by a unidirectional rubbing treatment of its thin solid film. Polarized electroluminescence with dichroic ratio of 1.6 as well as anisotropic optical absorption and photoluminescence have been observed for the rubbed NO-PPV film. An analysis of orientation distribution in the rubbed NO-PPV film suggested that radiative decay occurs in both oriented and unoriented layers. Surface geometry measurements suggested that plastic deformation of the polymer along the rubbing direction was the cause of the preferred orientation even in the conjugated polymer with rigid main chain structure.

Liquid crystalline behavior and optical properties of poly(2,5-dinonyloxy-p-phenylenevinylene)

Poly(2,5-dinonyloxy- p-phenylenevinylene) prepared by dehydrochlorination polymerization of 2,5-dinonyloxy- p-xylylenedichloride has been found to begin to soften at about 463 K, and to be in the nematic liquid crystalline state upon melting. By cooling the polymer at a liquid crystalline state to room temperature, an oriented thin film was obtained, whose band-gap energy was substantially lower than that of the (amorphous) film prepared by casting chloroform solution of the same polymer. Anomalous temperature dependences of absorption and photoluminescence spectra of the polymer were also revealed.

Optical Properties of Photochromic Dyes in the Amorphous State

Optical properties of photochromic dyes, 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BFCP) and cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene have been studied in the solid state. These photochromic dyes have been found to form a stable amorphous solid state when they are sandwiched between two glass plates. The time for the transition from the amorphous state to the crystalline state increases with decreasing distance between the two glass plates. The amorphous solid films clearly show photochromism upon light irradiation and also a marked change in photoluminescence intensity. The crystallization of the open-ring BFCP isomer is much faster than that of the closed-ring isomer. It has also been revealed that the photoluminescence spectrum of BFCP changes upon photoisomerization.

Color-Variable Emission in Multilayer Polymer Electroluminescent Devices Containing Electron-Blocking Layer

Color-variable emission in multilayer polymer electroluminescent (EL) devices has been obtained by the use of poly(N-vinylcarbazole) (PVCz) as an electron-blocking layer. The emissive polymers used were poly ( p-phenylenevinylene), poly(3-dodecylthiophene), and poly(2,5-dinonyloxy- p-phenylenevinylene), whose emission colors were green, red, and orange, respectively. When EL devices consisting of two emissive polymer layers separated by a PVCz layer were driven under either forward or reverse bias mode, emission from the negatively biased polymer was observed. PVCz is assumed to act as an electron-blocking layer due to its small electron affinity (2.3 eV) compared with those of the emissive polymers (2.7–3.5 eV), and to facilitate exciton confinement in the negatively biased polymer layer, where radiative recombination of the charge carriers occurs.

Electrical and optical properties of molecularly doped conducting polymers

Abstract Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C60, C70 etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C60-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.

Electrical properties of discotic liquid crystal hexahexyloxytriphenylene

Carrier transport of 2,3,6,7,10,11-hexahexyloxytriphenylene (HHOTP) in the discotic phase and the effects of C60 doping and shear stress have been studied by a time-of-flight method. The relatively high mobility of positive carriers insensitive to temperature and also C60 doping are interpreted to be the hole mobility. The hole mobility decreases on applying shear stress which deforms the columnar structure in the discotic phase. Small and temperature-dependent mobility (with activation energy of 0.3 eV) of negative carriers which are influenced by C60 doping is interpreted to be that of ionic carriers. Electronic band schemes of HHOTP in various phases (solid, discotic and isotropic) have been evaluated by absorption and photoelectron emission spectra measurements.

Novel properties of new type conducting and insulating polymers and their composites

Novel properties of recently developed conducting and insulating polymers and their composites are discussed. Properties of conducting polymer whose main chains are composed of unsaturated /spl pi/-bonds depend strongly on the main chain structure, substituent and also molecular dopants. Various applications of conducting polymers such as electroluminescence (EL) elements, electrolyte capacitors, photoconductors, photovoltaic cells, superconductors and insulators at cryogenic temperature, are discussed by taking effects of molecular dopants such as C/sub 60/ into consideration. A new type of insulating polymer, syndiotactic polypropylene prepared by newly developed metallocene catalysts has been studied and found to exhibit much superior electrical, thermal and mechanical characteristics compared with those of conventional isotactic polypropylene, atactic polypropylene and polyethylene. These excellent characteristics originate from lower crystallinity, smaller spherulites and different crystal lattice than in isotactic polypropylene. Negligible degradation of syndiotactic polypropylene by contact with copper is interpreted in terms of difference of catalysts and suppression of diffusion of copper cation. New types of conducting polymer, insulating polymer composites were prepared. Their conductivity was controlled over more than 10 orders of magnitude by small amounts of a conducting polymer, polypyrrole, which can be interpreted in terms of the percolation model depending on the shape and density of polypyrrole coated insulating polymer particles. Nonlinear current-voltage characteristics were also studied.

Photoluminescence Quenching in Polysilanes by Fullerene Doping and Effective Photoinduced Charge Transfer Depending on Aromatic Side Group

Photoluminescence of polysilanes with aromatic side groups has been found to be strongly quenched upon C60 doping, in contrast to the case of polysilanes with saturated hydrocarbon side groups in which a clear quenching effect has not been observed. These results have been interpreted by considering the role of the aromatic side group as a mediator for photoinduced electron transfer from electronic states of the main chain polysilanes to the C60 molecule.