Energy structures of triphenylamine oligomers

Energy structures of triphenylamine (TPA) oligomers attractive as hole transport layers for organic electroluminescent (EL) devices were investigated by measuring the ionization potentials and the absorption spectra. The highest occupied molecular orbital levels of the TPA oligomers from trimer to pentamer were found to be lower and closer to the work function of indium-tin-oxide (ITO) electrode as compared with that of the well-known dimer, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1-1′-biphenyl-4,4′-diamine (TPD). The EL devices using TPA oligomers above trimer exhibited a steep increase of injection current with increasing applied voltage, indicating the low barrier height between the ITO electrode and the TPA oligomers. This result is consistent with the low ionization potential of the TPA oligomers. Therefore, the TPA oligomers above trimer have desirable energy structures for hole injection.