Xiao Hong Yin

Enhanced photoconductivity of C60 doped poly(3-alkylthiophene)

Abstract Photoconductivity of poly(3-alkylthiophene) is found to be remarkably enhanced upon C 60 doping. Excitation profile of photoconductivity shows two peaks in the spectral ranges around 650 nm and 350nm. The former peak is interpreted to be due to ππ ∗ interband excitation of poly(3-alkylthiophene) followed by electron transfer to C 60 , while the latter is due to the optical excitation of C 60 molecule, accompanied by hole transfer to poly(3-alkylthiophene). In both cases, positive polarons (P + ) photogenerated with increased efficiency in poly(3-alkylthiophene) chains are considered to be responsible for enhanced photoconductivity, since geminate recombination of those positive charges (P + ) in poly(3-alkylthiophene) with negative charges (P c − ) in C 60 is suppressed compared to intrachain P + P − recombination in undoped poly(3-alkylthiophene). The decay time of photoconduction becomes shorter upon C 60 doping.

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.

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...

Preparation and Electrical Properties of Conducting Polyaniline-Polycarbonate Composite Films

Conducting polymer polyaniline and insulating polymer polycarbonate composites have been prepared. Electrical conductivity can be controlled in a wide range (more than 15 orders of magnitude) by a small amount of polyaniline. This characteristic can be explained by a percolation model. That is, at the threshold concentration, the electrodes must be bridged by a conducting channel of polyaniline. The percolation threshold and critical exponent of the conductivity parallel to the film surface are different from those perpendicular to the film surface. This suggests the anisotropy of conductivity in the composite. The conductivity of a high polyaniline concentration composite is found to be stable up to 160° C.

Preparation and Electrical Property of Polypyrrole-Polyethylene Composite

Polypyrrole-polyethylene composites have been prepared by pressing the mixture of polypyrrole coated and non-coated polyethylene spheres. Electrical conductivity is enhanced by more than 16 orders of magnitude and its activation energy decreases remarkably at concentration of polypyrrole coated polyethylene above around 10-20%, which corresponds to effective polypyrrole concentration of 0.1-0.2%. These characteristics can be explained by a percolation model. That is, at this concentration electrodes are bridged by conducting channel of doped polypyrrole. Thermoelectric power increases in proportion to absolute temperature and is independent on concentration of polypyrrole coated polyethylene sphere above 30%, which support the percolation model. The electrical property of this polypyrrole-polyethylene composite is found to be stable up to 160°C. The application of this composite to the semiconducting layer of a cable has been proposed.

Percolation conduction in polymer composites containing polypyrrole coated insulating polymer fiber and conducting polymer

Abstract Electrical conductivity of polymer composite containing insulating polymer fiber coated with thin polypyrrole layer has been found to increase remarkably by more than ten orders of magnitude above some threshold concentration. The activation energy of the electrical conductivity also changes drastically above this threshold. Thermoelectric power at concentration above this threshold is nearly the same with that of doped polypyrrole. The threshold concentration has been found to be dependent on the length of fiber remarkably. Longer length of fiber exhibits lower threshold concentration. These results are discussed in terms of percolation theory by taking the shape of fiber into consideration. Percolation conduction is also observed in other composites made of insulating polymers containing conducting polymers such as polyaniline.

Characteristics of buckminsterfullerene doped conducting polymer

Abstract Conducting polymers such as poly(3-alkylthiophene) are effectively doped with buckminsterfullerene (C 60 ). Change of absorption spectrum and drastic quenching of photoluminescence have been observed upon C 60 doping. On the other hand, the slight enhancement of electrical conductivity and the change of ESR by C 60 doping is not so remarkable compared with conventional strong dopants. Photoconductivity of poly(3-alkylthiophene) is enhanced and the response time becomes shorter by C 60 doping, which suggests that C 60 is a weak dopant, providing mainly photoinduced charge transfers between C 60 and the polymer. These results are explained by taking electronic energy states of poly(3-alkylthiophene) and C 60 into consideration at account of polaronic effects in both C 60 and polymer. Small enhancement of electrical conductivity and significant quenching of photoluminescence have been also observed upon C 70 doping, but change of optical absorption is less remarkable than the case of C 60 doping.

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.

Novel photophysical properties of fullerene doped conducting polymers

Abstract Photoluminescence in conducting polymer with non-degenerated ground state (NDGS CP) such as polythiophene derivatives, poly(p-phenylene vinylene) derivatives and polyfluorene derivatives is quenched remarkably upon doping by fullerenes such as C60 and C70. On the other hand photoconductivity in NDGS CP and also in conducting polymer with degenerated ground state (DGS CP) such as polyacetylene derivatives is enhanced remarkably upon fullerene doping at the excitation of the wavelengths of the inter-band transition in conducting polymer and allowed and forbidden optical transitions in fullerene. Remarkable polarity effect is observed in the spectral response of enhanced photoconductivity. Conducting polymer/C60 layer interface exhibits also unique junction characteristics demonstrating sizable photovoltaic effect due to charges photoseparation at the interface. These unique characteristics are discussed in terms of photo-induced charge transfer between conducting polymers and fullerenes, taking int...

Electrical property of polypyrrole-insulating polymer composite

Abstract PMMA small spheres were coated with polypyrrole by chemical polymerization utilizing FeCl 3 as a catalyst. Polypyrrole-PMMA composite has been prepared by the hot press of the mixture of polypyrrole coated PMMA spheres and non-coated PMMA spheres with appropriate concentration. Polypyrrole-PE composite has also been prepared by a similar method. Electrical conductivity has been controlled in the wide range (10 −17 ∼10 1 S/cm) by changing the concentration of coated spheres in non-coated spheres. That is, when the concentration of PPy coated sphere exceeds 10∼20% remarkable enhancement of conductivity by more than 17 orders of magnitude was observed. The activation energy of conductivity also changed drastically at this concentration. Low thermoelectric power of 6μV/K was evaluated for the composite of coated sphere concentration exceeding 20%. This insulator-metal transition like characteristics can be explained by a percolation model. That is, at this concentration electrodes are considered to be bridged by conducting path (polypyrrole). By taking thickness of polypyrrole layer into consideration the concentration of polypyrrole at the percolation threshold is evaluated to be less than 0.1%.