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(a) Structure of the molecules used in this work: TTF and TMTSF act as a donor and TCNQ as an acceptor. (b) Schematic representation of a device used to study transport at charge-transfer interfaces. The broken line represents the interfacial region where mobile charge carriers are present. (c) Optical microscope image of a device based on a TMTSF–TCNQ interface, including the scheme of the measurement configuration.
Panel (a) and (b) show the curves of TMTSF–TCNQ and TTF–TCNQ interfaces measured at room temperature in a two-terminal and a four-terminal configuration. (c) Histogram of the resistivity values of TMTSF–TCNQ and TTF–TCNQ interfaces measured in a four-terminal configuration, on more than 20 interfaces for each system. In both cases, the spread in values is approximately one order of magnitude.
The four-terminal curves of (a) TMTSF–TCNQ and (b) TTF–TCNQ interfaces measured at different temperatures ranging from 300 K to 200 K in 20 K steps. (c) The Arrhenius plot of the resistivity for both systems. The resistivity of TMTSF–TCNQ is thermally activated with activation energy (ranging between 70 and 120 meV; for the device shown here).
Schematic of the simplified band diagrams of (a) TMTSF–TCNQ and (b) TTF–TCNQ interfaces. For a TMTSF–TCNQ interface, the Fermi level lies in the gap between the HOMO of TMTSF and the LUMO of TCNQ, and charge transfer from TMTSF to TCNQ is thermally activated. In a similar diagram for a TTF–TCNQ interface, the HOMO of TTF is higher in energy than the LUMO of TCNQ, and charge transfer occurs spontaneously. In all materials, the Fermi level away from the interface lies in the middle of the HOMO-LUMO gap, as it should be, since the molecular crystals are intrinsic semiconductors.
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