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

We present a photoemission study of the electronic properties of organic-in organic semiconductor heterojunctions formed between the two hole transport materials, N,NN'-diphenyl-N, N'-bis(1-naphthyl)-1-1'biphenyl-4,4 " diamine (alpha-NPD) and copper-phthalocyanine (CuPc), and InP(110). The highest oc cupied molecular orbital of alpha-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 alpha-NPD/InP and CuPc/InP interfaces exhib it 0.4 and 0.65 eV interface dipole barriers, respectively, that indicate a displacement of negative charge from the organic film to the inorganic sub strate. This charge displacement is found to be consistent with the relativ e ionization energies and electron affinities of the materials. (C) 1999 Am erican Institute of Physics. [S0021-8979(99)09309-3].