Interdiffusion of molecular acceptors through organic layers to metal substrates mimics doping-related energy level shifts

Photoemission measurements reveal energy level shifts toward the Fermi level when a strong electron acceptor (tetrafluoro-tetracyanoquinodimethane, F4-TCNQ) is deposited on pristine layers of 4,4', 4 ''-tris(N,N-diphenyl-amino)triphenylamine (TDATA) or 4,4'-bis(N-carbazolyl)biphenyl (CBP). The shifts of the TDATA and CBP energy levels toward the Fermi level of the Au substrate could, in principle, arise from p-type doping of the intrinsic organic layers. While this indeed takes place in TDATA, doping of CBP by F4-TCNQ, i.e., charge transfer complex formation, does not occur. The shifts observed in CBP arise from the diffusion of F4-TCNQ toward the Au substrate, which modifies the buried metal surface potential, leading to a realignment of the energy levels of the organic overlayer. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3213547]