ABSTRACT
There are four general categories of solids distinguished by the levels of bonding: ionic, covalent, metallic, and molecular (i.e., van der Waals). Organic molecules are materials that consist of carbon and hydrogen structural elements and can contain other
elements such as S, N, O, or metal atoms. This classification of organic materials includes polymers, hydrocarbons, alkanes, etc. as outlined in Fig. 2.1 and specifically excludes materials such as C60 and carbon nanotubes (both allotropes of carbon), which belong to the broader category of small-molecular solids which nonetheless exhibit many similar properties of organic molecules and are also of interest in this chapter. Organic and molecular compounds exhibit aspects of both covalent-(intramolecular) and molecular-(intermolecular) bonding. An even more important subsection of molecular compounds, and the focus of this chapter, are conjugated small-molecule compounds that are characterized by highly conjugated π-electron systems. These materials are of particular interest due to the delocalization of the π−π electron system that results in electronic excitation energies in the range of a few electronvolts. These π-electrons are delocalized above and below the plane of the atoms, creating a “localized cloud of electrons” that are essentially confined by the boundary of the molecule and imparting many of the physical (optical and electronic) semiconductor characteristics. The key distinguishing properties for small-molecule semiconductors include (1) high degree of excitation localization and low dielectric permittivity that leads to the presence of excitons, (2) lack of complete band structure where conduction and valence bands are replaced, in name, by molecular orbitals and charge conduction typically proceeds via hopping mechanisms, and (3) notably high absorption coefficients.
