In terms of structure, the field of semiconductors spans a wide range, from the perfect order of single crystals to the non-periodic, disordered amorph ous state. The two extremes of this range attract a large amount of inter est. On one side, glamorous novel phenomena are being found which can only occur in specially tailored ultra-perfect periodic lattices. On the other side, the exotic and challenging nature of the amorphous state has triggered a surge of activity in recent years. Po1ycrystall i ne semi conductors are in between. They are among the work horses in the field, useful in many applications, a handy solution to many practical problems and still - they have not received in the past the amount of research interest that they deserve. It is the aim of the present book to improve this situation. The book originated from the lectures and seminars presented at the course on "Po1ycrystall i ne Semi conductors - Physical Properties and Applications" of "the International School on Materials Scien ce and Technology, hel d at the Centre for Sci entifi c Culture "Ettore Majorana" in Erice, Italy, July 1-15, 1984."

This book reviews the current understanding of electronic, optical and magnetic properties of conjugated polymers in both the semiconducting and metallic states. It introduces in particular novel phenomena and concepts in these quasi one-dimensional materials that differ from the well-established concepts valid for crystalline semiconductors. After a brief introductory chapter, the second chapter presents basic theore tical concepts and treats in detail the various models for n-conjugated polymers and the computational methods required to derive observable quantities. Specific spatially localized structures, often referred to as solitons, polarons and bipolarons, result naturally from the interaction between n-electrons and lattice displacements. For a semi-quantitative understanding of the various measure ments, electron-electron interactions have to be incorporated in the models; this in turn makes the calculations rather complicated. The third chapter is devoted to the electrical properties of these materials. The high metallic conductivity achieved by doping gave rise to the expression conducting polymers, which is often used for such materials even when they are in their semiconducting or insulating state. Although conductivity is one of the most important features, the reader will learn how difficult it is to draw definite conclusions about the nature of the charge carriers and the microscopic transport mechanism solely from electrical measurements. Optical properties are discussed in the fourth chapter."