Donal Donat Conor Bradley

Precursor-route poly(p-phenylenevinylene): polymer characterisation and control of electronic properties

The authors review the results of extensive studies that have been reported by the author and by other workers on the preparation and properties of the conjugated polymer poly(p-phenylenevinylene) (PPV). The use of the sulphonium polyelectrolyte precursor-route synthesis gives considerable flexibility for materials processing and it is shown that high-quality samples of controlled morphology, crystallinity, orientation and length of uninterrupted conjugated sequences can be obtained through a suitable choice of preparation conditions. Where applicable, the experimental results are compared with those obtained from theoretical calculations and any discrepancies are discussed.

Increase in chain conjugation length in highly oriented Durham-route polyacetylene

Polyacetylene prepared by the Durham precursor route can be obtained in a highly oriented form if the transformation region is carried out with the polymer in a stress field. The authors report here a comparison of the stretched and unstretched films. For stretched films the resonant Raman phonon frequencies are lower, the DC conductivity is a factor of 1000 higher and the spins detected in ESR experiments have a room temperature linewidth of 4.25 G; approximately one half the linewidth measured for unstretched films. They interpret these results as indicating that the chain conjugation length in stretched films is significantly longer than in unstretched films. Disorder and chain entanglement in the precursor polymer, which can be reduced by the stretching process, is therefore the principal factor limiting the conjugate length of polyacetylene prepared by this route.

Light-emitting polymer LEDs

Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.

Electroluminescent devices made with conjugated polymers

In this paper we discuss the principles of operation of polymer electroluminescent devices, and identify the factors which limit device efficiency. We identify how efficiencies can be improved by careful control of the polymer system, and by the use of multilayer structures to confine holes within the device. Using these techniques we can achieve efficiencies of better than 1% photons per electron in devices based on poly(p-phenylenevinylene). We also describe the use of induced absorption techniques to identify the excited states present within an electroluminescent device and to estimate their concentrations.

Electroluminescence from multilayer conjugated polymer devices--spatial control of exciton formation and emission

We have constructed electroluminescent diodes using several layers of conjugated polymers with differing energy gaps; these provide a range of different color light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene), PPV, and derivatives have been used, with indium tin oxide as hole-injecting electrode and calcium as electron-injecting electrode. For this selection of materials, we show that the sequence of the polymer layers allows control of the color of device emission. Emission from more than one layer can be produced simultaneously. The position and breadth of the light-emitting region of the device provides information about the mechanisms of charge transport and of exciton motion. Various models for multilayer emission are discussed in the paper.

Electronic structure of poly(p-phenylenevinylene) and poly(2,5-thienylenevinylene)

Abstract The electronic structure of highly-oriented poly(p-phenylenevinylene) (PPV) and unoriented poly(2,5-thienylenevinylene) (PTV) were investigated by high-resolution electron energy-loss spectroscopy (EELS). Performing a Kramers-Kronig analysis, the momentum-dependent dielectric functions could be derived. For the oriented PPV samples, the dielectric functions parallel and perpendicular to the chain axis were determined. In order to study the influence of the vinylene linkages on the electronic properties of these materials, the results were compared with previous measurements on polyparaphenylene (PPP) and polythiophene (PT).