  In highest quality organic single-crystal field-effect transistors, electron
transport occurs in the band-like regime, with the carrier mobility increasing
upon lowering temperature. Neither the microscopic nature of this regime, nor
why it occurs only in a small number of materials is currently understood.
Here, comparative studies of closely related materials, exhibiting high-quality
reproducible transport properties are needed. We performed a study of electron
transport in single-crystals of different TCNQ (tetracyanoquinodimethane)
molecules, combined with band structure calculations. We show that F2-TCNQ
devices exhibit very high electron mobility and an unprecedented increase in
mobility upon cooling, whereas in TCNQ and F4-TCNQ the mobility is
substantially lower and decreases upon cooling. We analyze the crystal and
electronic structures of these materials and find that F2-TCNQ crystals are
indeed ideal to achieve outstanding transport properties. Our analysis also
shows that to understand the difference between the three materials, studying
their band structure is not sufficient, and that the electron-phonon coupling
needs to be investigated as well. Besides the outstanding transport properties
of F2-TCNQ, a key result of our work is the identification of the Fx-TCNQ
family as a paradigm to investigate the most fundamental aspects of electronic
transport in organic crystals.
