Organic light-emitting diodes based on arylamine molecules and polymers with a fluorene core

A series of soluble arylamine-based hole transporting molecules with a fluorene core and with various ionization potentials have been synthesized. The transport properties of these molecules doped into polystyrene have been measured by time-of-flight experiments and compared to those of analogous compounds with a biphenyl core (TPD). Reorganization energies between the neutral molecules and their cations have been calculated using density functional theory. The effects of bond length and geometry relaxations on the overall reorganization energy in these two classes of molecules are discussed. Molecules from both classes have been doped into polystyrene and used as hole-transport layers (HTLs) in multi-layer light-emitting diodes with the structure ITO/HTL/AlQ₃/Mg:Ag [ITO = indium tin oxide, AlQ₃ = tris(8-hydroxyquinolinato)aluminum]. The electroluminescent properties and lifetime measurements at constant current have been evaluated. Significant variations in lifetime when using different substituents have been observed.