M. Onoda

Electrical and optical properties of poly(3-alkylthiophene)

Abstract Poly(3-alkylthiophene) has been found to be fusible at relatively low temperature. Films and fibers prepared by molding and melt-spinning methods, respectively, demonstrate also an insulator-metal transition upon doping. Reversible absorption spectrum change of poly(3-alkylthipphene) films with temperature and anomalous temperature dependence of luminescence have been also observed. By the application of hydrostatic pressure, the spectrum at high temperature turns into that of low temperature. Drastic change of electrical conductivity and optical properties have been also found at the melting point. These novel characteristics are tentatively explained in terms of the remarkable conformation change and its influence on the conjugation system.

Photoinduced charge separation in photovoltaic cell with heterojunction of p- and n-type conjugated polymers

A poly(p-pyridyl vinylene) (PpyV)/poly(3-hexylthiophene) (P3HT) heterojuction photovoltaic cell was fabricated and its photoirradiation effects have been investigated. Quenching of photoluminescence both in the PPyV and P3HT layers has been observed in the PPyV/P3HT heterojunction film. The photovoltaic characteristics of the heterojunction photovoltaic cell are also greatly improved from those in a P3HT monolayer photovoltaic cell. Its photoresponse gives evidence of the photoinduced charge transfer between PPyV and P3HT. These results are discussed by taking account of the difference in electronic states of both PPyV and P3HT.

Instability of conducting polymer, poly (3-alkylthiophene) gel with solvent, temperature and doping and effect of alkyl chain length

Abstract Volume, shape and color of poly(3-alkylthiophene)s change drastically with solvent composition and temperature, which is interpreted as characteristic behaviour of conducting polymer gels. Volume shrinkage ratio is larger in a chemically prepared sample than in an electrochemically prepared sample and also depends on alkyl chain length. Drastic shrinkage of poly(3-alkylthiophene)s in chloroform is also observed by adding iodine, which are explained by the formation of apparent cross-linking between polymer main chains by doping. These instabilities are reversible and also are considered to be candidates of molecular actuators.

Donor polymer (PAT6) — acceptor polymer (CNPPV) fractal network photocells

Abstract Unique characteristics such as quenching of photoluminescence and improvement of photovoltaic effect were observed in donor polymer-acceptor polymer composites, PAT6-CNPPV system and PDPATPSi-CNPPV system. Electronic energy structures of these polymers were determined and these characteristics are interpreted in terms of photoinduced charge transfer between donor polymer (PAT6 and PDPATPSi) and acceptor polymer (CNPPV) and formation of fractal network .

Electroluminescence in conducting polymers based on poly(phenylene ethynylene)

Abstruct Electroluminescence (EL) in various conducting polymers (CP) based on poly(phenylene ethynylene) (ROPPE) which have C-C triple bond in their main chains were studied. EL in Al/CP/ITO structure utilizing copolymer based on ROPPE and pyridine as CP was blue-green and stronger than that in same structure utilizing well-known poly(dialkoxy-p-phenylene vinylene) (ROPPV). However, weaker red EL was observed in same structure utilizing copolymer based on ROPPE and anthracene. These result suggests that the increased band gap energy and improve exciton confinement efficiency, due to shortened conjugate length, were realized by introduction of C-C triple bonds in main chain of conducting polymers such as ROPPV, while these effects of C-C triple bonds are suppressed by introduction of electron-rich moiety such as anthracene which should increase effective conjugation length. Electrochemical studies on these copolymers also confirmed this interpretation.

Novel properties of conducting polymers containing azobenzene moieties in side chain

Photochromic characteristic and optical molecular reorientation in conducting polymers such as polyacetylene and poly(p-phenylenevinylene) derivatives containing photochromic azobenzene moieties in the side chain have been studied. Upon irradiation of a linearly polarized light on these films, accompanying with trans-cis isomerizasion of azobenzene, optical anisotropy is induced.

Influence of hydrostatic pressure on electrical and optical properties of fusible conducting polymer at around melting point

Abstract Remarkable changes of electrical and optical properties have been observed in poly(3-alkylthiophene) with the hydrostatic pressure. The melting point increases by pressure with a rate of 20 °C/kbar. Electrical conductivity in the solid phase was enhanced by pressure but not in the liquid state. The blue shifted absorption peak at high temperature shifted again to lower energy and comes back to the original low temperature spectrum. These results are explained in terms of introduction of torsioning of some bonds between thiophene rings at high temperature and its suppression under hydrostatic pressure.

Electrochemical preparation of conducting poly(trans-1,2-di(2-thienyl)ethylene) and poly(1,4-di(2-thienyl)-1,3-butadiene) and their properties: comparison with polythiophene

The electrochemical and optical properties in poly(trans-1,2-di(2-thienyl)ethylene), (trans-PTE), and poly(1,4-di(2-thienyl)-1,3-butadiene) (PTB), in comparison with those of polythiophene (PT), during electrochemical p-type doping have been investigated by cyclic voltammetry and in situ optical-absorption-spectrum measurements. The evolution of localized states is evidenced by the spectral change associated with doping. These results are discussed in terms of polaron and/or bipolaron models. The band gap of trans-PTE and PTB was evaluated to be both about 1.9 eV and smaller than that of PT by about 0.2 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighbouring thiophene rings. The polaron states in trans-PTE appear in the band gap, about 0.65 eV above the valence band but about 0.6 eV below the conduction band. The polaron states in PTB appear at about 0.7 and 0.6 eV below and above the conduction and valence bands respectively. The top of the valence band of both trans-PTE and PTB is located at a higher-energy state than that of PT by about 0.2 eV. This result satisfactorily explains the stability of p-type dopants in trans-PTE and PTB compared with PT. The bottom of the conduction band of both trans-PTE and PTB is in almost the same position as that of PT and successful n-type doping in trans-PTE and PTB is expected.

Photoluminescence of poly(3-alkylthiophene) under hydrostatic pressure

Photoluminescence of poly(3-alkylthiophene) in the liquid state is very noticeably enhanced under hydrostatic pressure. However, at high pressures above the liquid-solid transition, the luminescence intensity decreases and the emission peak shifts to a lower energy. These results are tentatively explained in terms of a change of the torsion angle at the bond between thiophene rings and the resulting change of effective conjugation length under pressure.

A photoelectron emission study of polythiophene derivatives

The difference between the energy of the top of the valence band in polythiophene and those in poly(3-alkylthiophene)s has been evaluated by photoelectron spectroscopy in air. The top of the valence band becomes higher in energy on gradually increasing alkyl chain length. This indicates that an acceptor dopant is relatively stable in poly(3-alkylthiophene)s with longer alkyl chain lengths, as observed. In doped polythiophene and poly(3-methylthiophene), the difference in the threshold energy of photoelectron emission is explained tentatively by electron emission from the polaron and bipolaron levels formed by the dopant.