  Interfaces formed by two different organic semiconductors often exhibit a
large conductivity, originating from transfer of charge between the constituent
materials. The precise mechanisms driving charge transfer and determining its
magnitude remain vastly unexplored, and are not understood microscopically. To
start addressing this issue, we have performed a systematic study of highly
reproducible single-crystal interfaces based on rubrene and Fx-TCNQ, a family
of molecules whose electron affinity can be tuned by increasing the fluorine
content. The combined analysis of transport and scanning Kelvin probe
measurements reveals that the interfacial charge carrier density, resistivity,
and activation energy correlate with the electron affinity of Fx-TCNQ crystals,
with a higher affinity resulting in larger charge transfer. Although the
transport properties can be described consistently and quantitatively using a
mobility-edge model, we find that a quantitative analysis of charge transfer in
terms of single-particle band diagrams reveals a discrepancy ~ 100 meV in the
interfacial energy level alignment. We attribute the discrepancy to phenomena
known to affect the energetics of organic semiconductors, which are neglected
by a single-particle description, such as molecular relaxation and band-gap
renormalization due to screening. The systematic behavior of the
Fx-TCNQ/rubrene interfaces opens the possibility to investigate these phenomena
experimentally, under controlled conditions.
