Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: A direct and inverse photoemission study

P-doping of zinc phthalocyanine ~ZnPc! with tetrafluorotetracyanoquinodimethane (F4-TCNQ) is investigated with ultraviolet and x-ray photoemission spectroscopy, inverse photoemission spectroscopy, and in situ current–voltage (I – V) measurements. The electron affinity of F4-TCNQ ~5.24 eV! is found to be equal, within experimental error, to the ionization energy of ZnPc ~5.28 eV!, consistent with efficient host-to-dopant electron transfer. As a result, the Fermi level in doped ZnPc drops from near midgap to 0.18 eV above the leading edge of the highest occupied molecular orbital and a narrow space-charge layer ~,32 Å! is formed at the interface with the Au substrate. In situ I – V measurements show a seven orders of magnitude doping-induced increase in hole current. © 2001 American Institute of Physics. @DOI: 10.1063/1.1424067#