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Encyclopedia of Physical Science and Technology En012j-597 July 26, 2001 11:8

646 Polymers, Electronic Properties

Pyroelectricity Change in electric polarization due to
temperature change.
Soliton As applied to conducting polymers, it refers to
a bond alternation defect that results in a particlelike
field pattern.
Triboelectricity Transfer of charge between two solids
due to their contact and separation.

A POLYMER SOLID is composed of a collection of very
long molecular chains that are characterized by strong
chemical bonding within the chain and much weaker inter-
chain bonding. In this sense, a polymer can be considered
to be an assembly of individual chains. Each chain can
5
contain a very large number (∼10 or more) of identical
subunits bonded together. Each subunit can be viewed as
a separate molecule with electronic states consisting of
the molecular orbitals of the molecule. In describing the
electronic states of polymers, the degenerate molecular
orbitals that overlap in a periodic fashion lift their degen-
eracy by forming extended, that is, bands of, electronic FIGURE 1 Representative conductivities of polymers and in-
states. Thus, bonding and antibonding molecular orbitals organic materials. The asterisk denotes that these values are
lead to polymer valence and conduction bands, respec- achieved in doped materials. [From Mort, J., and Pfister, G., eds.
tively. To this degree, polymers can be viewed as organic (1982). “Electronic Properties of Polymers,” Wiley, New York, by
permission.]
semiconductors and, by analogy with the more familiar
inorganic crystalline semiconductors such as silicon, the
concepts of energy band theory can be used to characterize range of conductivities that have been reported in poly-
their electronic states and properties. mer materials compared with the more familiar inorganic
materials.
This article provides a review of the development of
I. CONDUCTIVITY this new area of solid-state science. Emphasis is placed on
key ideas, particularly as they contrast to more traditional
Polymers are a familiar part of everyday life and, because concepts in the solid-state properties of crystalline or or-
of the ability of chemists to tailor-make their properties, dered solids. Topics covered include electronic states in
have found widespread application. Until recently, these polymers, an essential precursor to understanding the elec-
applications capitalized on advantageous properties such tronic properties of polymers; charge storage; piezo- and
as chemical inertness and durability. By contrast, the most pyroelectricity; photoconductivity and electronic trans-
valued electrical property of polymers was their capacity port in polymers (e.g., pendant-group polymers) where
to inhibit conductivity, that is, act as insulators, and lit- localized molecular ion states play a determining role and
tle concerted work was done to examine and understand in polymers (e.g., conjugated polymers) where delocal-
the fundamentals in order to enhance electrical conduc- ization of electronic charge is significant.
tivity in polymers. Beginning in the 1970s, a resurgence Finally, the actual and potential technological applica-
of effort occurred in the study of polymers as electronic tions and commercial use of polymers as electronic mate-
materials. This has been characterized by an interdisci- rials are discussed. Particular stress is placed on how the
plinary approach involving physicists, chemists, materi- contrasting properties of the various types of polymers
als scientists, and device engineers. This renewed interest determine their advantage in specific applications.
was stimulated by the interplay of scientific and techno-
logical motivations. For the scientist, polymers posed new
and often unconventional questions regarding the inter- II. ELECTRONIC STATES
pretation of experimental results. From the technological
perspective, it was an explicit goal to explore the poten- It is convenient for the discussion of this aspect of poly-
tial for combining useful electronic functions of polymers mers to consider separately two classes that have quite dif-
with their unique materials properties. Figure 1 shows the ferent optical and electrical properties. The first consists