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 (tetraphenylnaphthacene) and F-x-TCNQ (fluorinated tetracyanoquinodimethane), 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 F-x-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 approximate to 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 bandgap renormalization due to screening. The systematic behavior of the F-x-TCNQ/rubrene interfaces opens the possibility to investigate these phenomena experimentally, under controlled conditions.