Electronic and Optical Properties of Conjugated Polymers

Electronic and Optical Properties of Conjugated Polymers , William Barford , Oxford U. Press, New York, 2005 .$112.50 (262 pp.). ISBN 0-19-852680-6

Conjugated polymers became a major research field in solid-state physics when Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa discovered in 1977 that it was possible to dope poly-acetylene (PA) to high electric conductivity. The three were later awarded the Nobel Prize in Chemistry in 2000. In the 1980s research on conjugated polymers mainly concentrated on the conductive and charge-storage properties of those materials. The discovery in 1990 of electroluminescence in poly(para-phenylene vinylene), or PPV, by Richard Friend’s group at the University of Cambridge’s Cavendish Laboratory in England reawakened interest in conjugated polymers.

William Barford’s Electronic and Optical Properties of Conjugated Polymers is a theoretical complement to the high experimental culture of British research on conjugated polymers. Currently, the semiconducting properties of undoped, conjugated polymers are the focus of that research interest. Scientists are driven by the desire to understand the electronic and optical properties of the phenyl-based, light-emitting polymers in the hope that those materials will be exploited in a range of technologies. Such technologies include cheap and flexible light-emitting displays, photovoltaic devices, optical switchers, FETs, and all-polymer integrated circuits. Moreover, conjugated polymers are active components in many biological processes, such as the collection of light during photosynthesis and vision.

Interest in conjugated polymers is based not only on possible applications but also on the fact that in low-dimensional solids, the electron–electron and electron–lattice interactions are of particular importance to the electronic structure of solids. Understanding the complex effects of those interactions is one of the cutting-edge fields in condensed matter research. Conductive polymers belong within those low-dimensional systems because they are quasi-one-dimensional; therefore, their electronic properties are quite different from inorganic semiconductors such as silicon.

Barford’s book starts with a treatise of noninteracting π-electrons in conjugated polymers. Then, step by step, the author includes electron–lattice and electron–electron interactions. He discusses the influence of those interactions on the ground-state dimerization and on dipole-allowed and dipole-forbidden excited states. In particular, Barford covers the weak-coupling limit, the strong-coupling limit, and the relevant intermediated range of electron–electron interactions. He explains how those interactions determine the character and the energetic order of excited states. He also describes why in PA the lowest excited state is dipole forbidden, which leads to a nonluminescent polymer, while the lowest excited states in PPV and in poly(para-phenylene), or PPP, are dipole connected to the ground state, thus allowing the conjugated polymers to be used for light-emitting diodes.

The book has excellent chapters on the dynamics and binding energy of excitons in conjugated polymers; the interactions between polymer chains in real materials; linear and nonlinear optical properties; and strategies for enhancing the singlet exciton yield, which determines the internal electroluminescent quantum efficiency of organic light-emitting diodes, by interconversion from the triplet states. At the end of the book, the author compares the experimental results for nonluminescent, conjugated polymers (PA, for example) and luminescent, conjugated polymers (such as PPP and PPV) with theoretical results calculated within the framework of correlated electrons, including strong electron–lattice effects. I feel the author could have also included experimental results on the exciton dispersion from electron energy-loss spectroscopy and the charge gap from photoemission spectroscopy.

Barford, who teaches at the University of Sheffield in England, is a well-regarded scientist whose research is principally focused on the theoretical and computational modeling of the correlated electronic structure in low-dimensional solids, such as conjugated polymers, and, to a lesser extent, in the field of high-T _c superconductors.

Throughout his book the electronic models of conjugated polymers are developed in the second-quantization representation. His approach makes the text clear and instructive, but readers need some prior knowledge of the theory for the electronic structure of condensed matter. But the author helps less-informed readers by including appendices that explain the second-quantization representations and other theoretical tools.

What makes Electronic and Optical Properties of Conjugated Polymers as a whole so enjoyable to read is that it gives a complete overview of the influence of correlation effects on the ground and excited states of those materials. It is a comprehensive treatise aimed at theoretical physicists and chemists working in the field and at graduate students and other researchers who need to analyze their data in terms of theoretical models. The book is long overdue.