(Invited) The Effect of Molecular Dynamics on Charge Transport in Organic Semiconductors

Organic semiconductors are studied extensively for their incorporation in next generation large-area, low-weight flexible optoelectronic applications. Librational motion is ubiquitous in such materials; this motion, however, has been difficult to connect with experimental observations of a material’s electronic properties. Here we present the effect of libration on charge transport in monomolecular compounds and binary charge-transfer (CT) complexes through a combined experimental and theoretical approach. The case study of trans-stilbene (STB) and its CTs with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB:F4TCNQ) and 7,7,8,8-tetracyanoquinodimethane (STB:TCNQ) will be discussed. It will be shown that the change in amplitude of the torsional oscillation (libration) of the double bond connecting the C7 and C7a carbons in the stilbene molecule results in a crossover from thermally activated to a temperature-independent charge transport. This pedal-like (crankshaft) motion, is responsible for inter-conversion between the two possible conformers. At low temperature the libration amplitude is not sufficiently large to mediate this rotation, generating thermodynamic non-equilibrium states and resulting in freezing in of orientational disorder. The transport can take place only via tunneling, therefore being temperature-independent. At higher temperatures, when inter-conversion motion is fast, a transition to temperature-activated hopping transport occurs.

The following people contributed to this work: K. P. Goetz, A. Fonari, D. Vermeulen, P. Hu, H. Jiang, P. J. Diemer, J. W. Ward, M. E. Payne, C. S. Day, C. Kloc, V. Coropceanu, L.E. McNeil, and O. D. Jurchescu.