The interface between the single crystal and the Au source/drain electrodes in [7]phenacene single-crystal field-effect transistors (FETs) was modified using 14 electron acceptors with different redox potentials. The effective hole-injection barrier heights (phi(eff)(h)s) for [7]phenacene single-crystal FETs have been plotted as a function of the redox potential (E-redox) of the inserted electron acceptors, showing that the phi(eff)(h) decreases with increasing E-redox. The highest phi(eff)(h) occurs without inserted material (electron acceptors), and this deviates from the otherwise linear relationship between phi(eff)(h) and E-redox. We have investigated the temperature dependence of phi(eff)(h) in an attempt to determine why the phi(eff)(h) value without inserted material is so high, which suggests that no additional barrier, such as a tunneling barrier, is formed in the device. We conclude that the pure Schottky barrier in this FET is lowered very significantly by the insertion of an electron acceptor. The gate-voltage dependence of phi(eff)(h) suggests a slight reduction of Schottky barrier height owing to hole accumulation. Furthermore, the clear correlation between threshold voltage and redox potential suggests a relationship between threshold voltage and phi(eff)(h). Controlling the interface between the single crystal and the source/drain electrodes in this FET produced a very high mu (similar to 6.9 cm(2) V-1 s(-1)) and low absolute threshold voltage, i.e., excellent FET characteristics. The topological characterization of inserted materials on [7]phenacene single crystals are achieved using atomic force microscope (AFM) and X-ray diffraction (XRD). The results show that the single crystals are not completely covered with the inserted materials and the inhomogeneous modification of inserted materials for single crystals effectively leads to the drastic change of hole-injection barrier between source/drain electrodes and single-crystal active layer.