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.