Polymers

Conjugated Polymers and Molecular Solids

Professor R H Friend, Dr Neil Greenham, Dr Franco Cacialli, Dr Henning Sirringhaus, & numerous postdoctoral fellows & research students

Conjugated polymers with delocalized -electron systems behave as model organic semiconductors. A broad programme of research on this topic exists in the Cavendish and Department of Chemistry (Dr A B Holmes). Activities range from design and synthesis of new polymers through to their use in a variety of devices. Our main interest in the Cavendish is the semiconductor physics of these materials, which is very different from that of inorganic semiconductors and which gives strong electro-optical and non-linear optical responses. A range of device-related projects are in progress. (a) Field-effect devices show large changes in optical properties as a consequence of charge injection, an effect that can be used in waveguide optical modulators. (b) Polymer light-emitting diodes (LEDs) were first made in Cambridge using poly(phenylene vinylene), and the performance of these LEDs has risen to surpass that of visible emitting inorganic devices. (c) Photovoltaic and photoconductive diodes can show high efficiency if heterojunctions between polymer layers are used to achieve charge separation. (d) Sub-picosecond time-resolved spectroscopy is used to study formation and evolution of polaronic electronic excitations.

Molecular solids provide a range of materials with which to study Peierls instability and strong Coulomb interactions occurring as a result of reduced dimensionality. Directional bonding between molecular units can be arranged to give either one-dimensional (stacks) or two-dimensional (sheets) properties. Work in the Cavendish is in collaboration with chemistry groups at the Royal Institution and the University of Wales, Bangor. Interests include the study of ferromagnetism, anti-magnetism, and the spin-Peierls transition, in addition to metallic and superconducting properties.