The Influence of Quasiplanar Structures of Partially Oxygen-Bridged Triphenylamine Dimers on the Properties of Their Bulk Films

Abstract

Abstract The relationship between molecular orientation and charge-transporting properties in vapor-deposited films of two doubly oxygen-bridged triphenylamine dimers, which differ in their connection mode, was examined. A comparison of the fluorescence spectra obtained from these films, as well as from solution and the crystalline state revealed that the dimers retain two different states in amorphous films, i.e., a monomer-like random orientation and a crystalline-like π-stacking orientation. Regarding the crystalline-like π-stacking orientation in the films, one dimer forms a one-dimensional π-stacking structure, whereas the other dimer forms a two-dimensional π-stacking structure, thus reflecting the difference in packing structure in the crystals. Time-of-flight measurements showed a strong dependence of the charge mobilities in the film of the dimer with a one-dimensional π-stacking structure on the electric field. The film of the dimer with a two-dimensional π-stacking structure exhibited ambipolar charge-transport properties with high electron and hole mobilities. When these dimers were used as hole injection layers in organic light-emitting diode devices, the external quantum efficiencies improved by a factor of 1.2 relative to standard devices.