Carrier Transport in Organic Semiconductors and Insulators

1. Overview And Chapter Objectives

In this chapter we focus our attention on the electronic transport Models of organic semiconductors and insulator materials, which is an active research topic nowadays. Indeed, it may seem strange to talk about charge transport in insulators, and organic semiconductors, which are also inherently insulators. However, if one generates charge carriers in such materials, for example by optical excitation, they may move with a mobility that is comparable to that of conventional semiconductors or more. Obviously, an insulator can be converted into a semiconductor if free charge carriers are generated by either injection, or doping, or by optical excitation.

As we pointed out so far, some organic semiconductor crystals, such as polyacenes (anthracene, tetracene, and pentacene), and fullerenes (bucky balls), can conduct electricity and have attractive characteristics for electronic and optoelectronic devices. Such organic semiconductors are already used in photocopiers. They are also employed in biosensors, light emitting diodes (OLEDs) in flat panel displays, and about to enter the solar cell market. In fact, almost all main players in the electronics industry work on organic displays, nowadays. Technically speaking, organic semiconductors are relatively low cost materials, their fabrication techniques are quite simple (no need for clean-room or high temperature processing), and they can be deposited on various types of substrates.

Organic semiconductors (OSCs) can be classified into two main categories: small molecules or oligomers and polymers. A polymer is a macromolecule that is composed of many repeating units, the monomer units. The properties of OSCs differ from conventional inorganic crystalline semi-conductors in many aspects. A central point is the mechanisms related to charge transport. In order to understand charge transport in OSCs, we need to elaborate on the electronic structure of organic solids. As shown in Figure 1, the organic solids such as polymers, are made of molecular subunits.

Figure 1.

Schematic illustration of the two basic families of organic materials

Conductive OSCs are hydro-carbon molecules with a backbone of unsaturated carbon atoms (like --CH-CH-CH--, each atom is threefold coordinated). These chains may contain other elements, including H, N, and O. Table 1 shows the chemical formulas of conducting polymers commonly used in electronic applications. Note that a polymer is a large molecule (macromolecule) composed of repeating structural units. The polymer subunits are usually connected by covalent bonds.