Controlling the work function of ZnO and the energy-level alignment at the interface to organic semiconductors with a molecular electron acceptor

We show that the work function (Phi) of ZnO can be increased by up to 2.8 eV by depositing the molecular electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On metals, already much smaller Phi increases involve significant charge transfer to F4TCNQ. No indication of negatively charged F4TCNQ on ZnO is found by photoemission spectroscopy. This fundamental difference is explained by a simple electrostatic model that identifies the bulk doping and band bending in ZnO as key parameters. Varying Phi of the inorganic semiconductor enables tuning the energy-level alignment at ZnO/organic semiconductor interfaces. DOI: 10.1103/PhysRevB.87.155311