## 3 Results and Discussion
Figure 4: (**a**) Schematic of an F\({}_{8}\)CuPc film transistor; (**b**) current-voltage characteristics of the F\({}_{8}\)CuPc film transistor measured in air; and (**c**) those obtained from repeated measurements; these were performed immediately after the first. As for an n-channel F\({}_{16}\)CuPc transistor, it has been previously shown that the F\({}_{16}\)CuPc film could be rendered air-stable by the introduction of F atoms [8]; fluorination renders the organic molecules resistant to oxidation, because the energy levels of their frontier orbitals are lowered. It has also been suggested that the conformational orientation of the F\({}_{16}\)CuPc molecules in the film with the F-atoms being exposed to air prevents the penetration of moisture into the film [8]. Photoemission spectroscopies revealed that the energy levels of the frontier orbitals of F\({}_{8}\)MPc are positioned between those of MPc and F\({}_{16}\)MPc [15; 25]. Therefore, they are clearly lower than that of MPc, and higher than that of F\({}_{16}\)MPc. Consequently, F\({}_{8}\)CuPc could be more readily oxidized compared to F\({}_{16}\)CuPc. Furthermore, the un-substituted H-atoms in the benzene ring of F\({}_{8}\)CuPc and the attendant changes in the molecular arrangement in the crystal structure might permit moisture to penetrate through the thin film. Apparently, these factors are responsible for the air instability of the fabricated n-channel F\({}_{8}\)CuPc film transistor. The field effect electron mobility of 7.9 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) is almost 10 times lower than that of the F\({}_{16}\)CuPc film transistor; [PERSON] et al. reported a field effect electron mobility of 5 \(\times\) 10\({}^{-3}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) for the F\({}_{16}\)CuPc thin film transistor [8], and we too obtained a similar value with our measurement system. As for the electron transport, the overlap of the LUMOs is responsible for the transport. In the case of F\({}_{16}\)CuPc, the overlap integral between the LUMOs of adjacent F\({}_{16}\)CuPcs in the one-dimensional chain could be estimated to be 2.1 \(\times\) 10\({}^{-3}\), according to the crystal data in [26]. Despite the larger overlap integral between the LUMOs, the F\({}_{8}\)CuPc film transistor shows smaller electron mobility compared to that of the F\({}_{16}\)CuPc film transistor. The lower electron mobility in the F\({}_{8}\)CuPc film transistor could be attributed to the relatively low crystallinity in the as-deposited film, as indicated by the broad diffraction peak observed in Figure 3, whereas sharp diffraction peaks suggesting high crystallinity were observed in the F\({}_{16}\)CuPc film [8]. When a single crystal of F\({}_{16}\)CuPc was used, the field effect electron mobility of the F\({}_{16}\)CuPc transistor increased by two orders of magnitude [9]. Figure 5: Current-voltage characteristics of the F\({}_{8}\)CuPc n-channel transistor in vacuum (\(\sim\)10\({}^{2}\) Pa). Figure 6: Current-voltage characteristics of the F\({}_{8}\)CuPc n-channel transistor in high vacuum (less than 10\({}^{-2}\) Pa). This implies that higher field effect electron mobility could be obtained for F\({}_{8}\)CuPc, if a single crystal is used. Indeed, a recent study on a single-crystal transistor of F\({}_{8}\)CuPc demonstrated that the field effect electron mobility of F\({}_{8}\)CuPc is comparable to that of F\({}_{16}\)CuPc [21], even though the measurements were performed in air.