Charge transport in disordered organic semiconductors is receiving a great deal of attention because, as with inorganic semiconductors, conductivity in organic semiconductors can be changed through doping. In addition, a few disordered organic semiconductors also have âhighâ carrier mobilities. The discovery of these organic semiconductors marks a major change in the research outlook for these semiconductors, since they have the potential of replacing more expensive single-crystal semiconductors in devices. Organic materials possess other advantages as well. A few organic semiconductors have already been used in industrial applications, such as the productio of light-emitting devices (LEDs), transistor circuits built on flexible substrates, and a variety of thin-film sensors and solar cells. In all of these applications, cleverly devised structures were adopted while, in many cases, advantages in manufacturing and cost effectiveness remained to be exploited. Indeed, some electronic display devices based on disordered organic semiconductors have already been successfully marketed in products such as cell phones and television screens. This chapter focuses on the study of charge transport and the optical properties of disordered organic semiconductors.
17.2 Charge Transport
We first raise the question of how disordered organic semiconductors conduct current. The answer requires that the organic semiconductors possess a substantial number of âfreeâ carriers (free carriers include electrons and holes). In organic semiconductors, the fact that free carriers can be produced from the distributed Ï-bonds in a chain molecule is well known. In addition, the free carriers must be able to move effectively inside the organic semiconductors. This requires the presence of âtransport sites,â which are the locations within the molecules that act as intermediate stopping points as the free carriers move. A proper description of transport sites requires one to include site energies. At thermal equilibrium, the free carriers have a distribution of energies; how they interact with the transport sites depends on the affiliated energy correlation. Free carriers essentially move from one transport site to another with a weighted probability based on energy correlation. This probability may also be affected by atomicâmolecular vibrations.
In addition to the transport sites, free carriers may also be captured by traps that are usually associated with defects andâor grain boundaries. The release of a carrier from a trap involves energy exchange and a time delay. A trapped carrier may at times recombine with a free carrier of the opposite polarity to release the excess energy in the form of heat or light. Assuming that the trap density is small (usually justifiable in the case of a high-quality organic semiconductor), one may consider the density of transport sites to be the density of states. Obviously, such an assumption ignores the effect of the trapped charges on the molecular potentials. In some disordered molecular semiconductors, deformation associated with trapped charges cannot be ignored, and traps are known to create a dipole field extending many atomic radii. When coupled to a self-induced structural deformation, a charge will form an entity known as a polaron. A polaron can therefore be viewed as a charge localized in the potential minimum formed by a molecular deformation. It is interesting to note that polarons (sometimes considered to be âdressedâ charges) are capable of migrating across molecules.