Organic semiconductors: a theoretical characterization of the basic parameters governing charge transport

Abstract

Organic semiconductors based on pi-conjugated oligomers and polymers constitute the active elements in new generations of plastic (opto)electronic devices. The performance of these devices depends largely on the efficiency of the charge-transport processes; at the microscopic level, one of the major parameters governing the transport properties is the amplitude of the electronic transfer integrals between adjacent oligomer or polymer chains. Here, quantum-chemical calculations are performed on model systems to address the way transfer integrals between adjacent chains are affected by the nature and relative positions of the interacting units. Compounds under investigation include oligothienylenes, hexabenzocoronene, oligoacenes, and perylene. It is shown that the amplitude of the transfer integrals is extremely sensitive to the molecular packing. Interestingly, in contrast to conventional wisdom, specific arrangements can lead to electron mobilities that are larger than hole mobilities, which is, for instance, the case of perylene.

Figure 1

Illustration of the bonding–antibonding interactions between the HOMO/LUMO levels of two ethylene molecules in a cofacial configuration; we also illustrate the formation of the valence and conduction bands when a large number of stacked molecules interact.

Figure 2

Evolution of the INDO-calculated electronic splittings of the HOMO and LUMO levels in a cofacial dimer made of two sexithienyl molecules as a function of the intermolecular separation.

Figure 3

Evolution of the INDO-calculated electronic splittings of the HOMO and LUMO levels in a dimer formed by two sexithienyl molecules separated by 4.0 Å as a function of the degree of translation of one molecule along its main chain axis.

Figure 4

Illustration of the LCAO bonding–antibonding pattern of the HOMO (Upper) and LUMO (Lower) levels in the sexithienyl molecule. The color and size of the circles are representative of the sign and amplitude of the LCAO coefficients, respectively; the dashed lines delimit half a monomer unit.

Figure 5

Evolution of the INDO-calculated HOMO and LUMO splittings in cofacial dimers formed by two oligothiophenes separated by 3.5 Å as a function of the number of thiophene rings in the chains.

Figure 6

Evolution of the INDO-calculated HOMO and LUMO splittings in cofacial stacks made of sexithienyl chains separated by 4.0 Å as a function of cos[π/(n + 1)], with n = the number of molecules in the stack.

Figure 7

Illustration of the herringbone packing of pentacene molecules (from ref. 55).