Electronic and thin film stacking structure of Organic Semiconductors

Abstract

Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing.

A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM.