Interface state and dipole assisted hole injection improvement with 1,4,5,8,-naphthalene-tetracarboxylic-dianhydride in organic light-emitting devices

1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) is known to improve hole injection when inserted between the hole transport layer and the indium tin oxide (ITO) anode in organic light emitting devices. To clarify the origin of the improvement, the interfacial electronic structures between N,N′-diphenyl-N, N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′ diamine (NPB, typical hole transport layer) and ITO with a NTCDA insertion layer were explored. The NTCDA layer generates an interface state when it interacts with ITO and also induces large interface dipole. The interface state assists hole transport and the interface dipole pulls entire energy levels of NPB up, reducing the hole injection barrier.