Energy level offset at organic semiconductor heterojunctions

We present an investigation via ultraviolet photoemission spectroscopy of the electronic structure of three organic-organic heterojunctions formed between the standard electron-transport emissive material tris(8-hydroxy-quinoline) aluminum (Alq₃) and two hole-transport materials, i.e., 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), and N,N′-diphenyl-N,N′- bis(l-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD). We measure directly the energy offsets between highest occupied molecular orbitals during the formation of the interfaces. We show that the relative positions of the highest occupied and lowest unoccupied molecular orbitals across the Alq3/PTCDA and Alq₃/α-NPD interfaces are qualitatively different and explain, in part, the difference in the performance of electroluminescent devices based on these heterojunctions. We demonstrate the existence of charge transfer-induced dipoles which shift the molecular levels of one organic with respect to the other and invalidate the usual assumption of vacuum level alignment across organic heterojunctions. Finally, we show that the molecular level alignment is independent of the deposition sequence of the organic films and that transitivity applies to these organic "band offsets."