Mechanism of charge transport in self-organizing organic materials

Currently there is great interest in the use of organic materials as the active component in opto-electronic devices such as field-effect transistors, light-emitting diodes, solar cells and in nanoscale molecular electronics. Device performance is to a large extent determined by the mobility of charge carriers, which strongly depends on material morphology. Therefore, a fundamental understanding of the relation between the mechanism of charge transport and chemical composition and supramolecular organization of the active organic material is essential for improvement of device performance. Self-assembling materials are of specific interest, since they have the potential to form well defined structures in which molecular ordering facilitates efficient charge transport. This review gives an overview of theoretical models that can be used to describe the mobility of charge carriers, including band theory for structurally ordered materials, tight-binding models for weakly disordered systems and hopping models for localized charges in strongly disordered materials. It is discussed how the charge transport parameters needed in these models; i.e. charge transfer integrals, site energies and reorganization energies, can be obtained from quantum chemical calculations. Illustrative examples of application of the theoretical methods to charge transport in self-assembling materials are discussed: columns of discotic molecules, stacks of oligo(phenylene-vinylene) molecules and strands of DNA base pairs. It is argued that the mobility of charge carriers along stacks of triphenylene and oligo(phenylene-vinylene) molecules can be significantly enhanced by improvement of molecular organization. According to calculations, the mobility of charge carriers along DNA strands is strongly limited by the large charge induced structural reorganization of the nucleobases and the surrounding water.