Fermi level determination in organic thin films by the Kelvin probe method

We show that the Kelvin probe method (KPM) is well suited for in situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive, and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc–F16) are presented. The KPM results show that a 2.5% admixture of ZnPc–F16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc–F16 has acceptorlike properties relative to ZnPc.