Electronic structure and current injection in zinc phthalocyanine doped with tetrafluorotetracyanoquinodimethane: Interface versus bulk effects

The p-type doping of zinc phthalocyanine (ZnPc) with the highly electronegative tetrafluorotetracyanoquinodimethane (F₄-TCNQ) is investigated via direct and inverse photoemission spectroscopy and in situ current-voltage (I-V) measurement. The electron affinity of F₄-TCNQ and the ionization energy of ZnPc are found to be energetically compatible with an electron transfer between the highest occupied molecular orbital (HOMO) of the host and the lowest unoccupied molecular orbital of the dopant. The Fermi level is near mid-gap in undoped ZnPc, and drops to 0.42 and 0.18 eV above the HOMO in the 0.3% and 3% doped films, respectively, consistent with efficient p-doping. The dependence of the Au/ZnPc:0.3%F₄-TCNQ/Au I-V characteristics on the thickness of the organic film is analyzed in terms of injection-limited versus space-charge-limited current. The analysis demonstrates that the large doping-induced increase in hole current is primarily due to improved carrier injection via tunneling through the narrow interface space charge layer.