Band alignment at organic-inorganic semiconductor interfaces: α-NPD and CuPc on InP(110)

We present a photoemission study of the electronic properties of organic-inorganic semiconductor heterojunctions formed between the two hole transport materials, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD) and copper-phthalocyanine (CuPc), and InP(110). The highest occupied molecular orbital of α-NPD (CuPc) is found to be 0.2 eV below (0.2 eV above) the InP valence band maximum, leading to a staggered (nested) energy level configuration. The α-NPD/InP and CuPc/InP interfaces exhibit 0.4 and 0.65 eV interface dipole barriers, respectively, that indicate a displacement of negative charge from the organic film to the inorganic substrate. This charge displacement is found to be consistent with the relative ionization energies and electron affinities of the materials.