## 3 Results and Discussion  
The lattice constant of the F\({}_{8}\)CuPc crystal is consistent with those reported in [21]. The crystal structure is shown in Figure 2. F\({}_{8}\)CuPc forms a one-dimensional regular chain along the \(a\)-axis with an interplanar distance of 3.30 A, which is smaller than the sum of the van der Waals radii of sp\({}^{2}\) carbons, suggesting a strong \(\pi\)-\(\pi\) intermolecular interaction. As the charge transport in organic semiconductors occurs via the \(\pi\)-electrons, a strong \(\pi\)-\(\pi\) intermolecular interaction in the crystalline state is desirable, which can be evaluated by the overlap integral in the conduction path consisting of \(\pi\)-orbitals. Using an extended Huckel calculation method (The extended Huckel calculation was performed using \(CAESAR\) 2 software developed by PrimeColor Software, Inc. (Raleigh, NC, USA) Default parameters were used for the calculations), the overlap integral between the \(\pi\)-orbitals of adjacent F\({}_{8}\)CuPCs in the one-dimensional chain along the \(a\)-axis were estimated: the overlap integral between the LUMOs was found to be 5.4 \(\times\) 10\({}^{-3}\), which is comparable to that of molecular conductors consisting of MPCs [24], whereas that the between HOMOs was 2.9 \(\times\) 10\({}^{-4}\). In addition to the optimal position of the LUMO level, the \(\pi\)-\(\pi\) overlap appears to favor electron transfer over hole transfer. The carrier mobility of an organic thin film depends on the molecular orientation in the film. Figure 3 shows the X-ray diffraction pattern of the F\({}_{8}\)CuPc film deposited on a SiO\({}_{2}\)/n-Si substrate. A peak at 2\(\theta\) = 6.56\({}^{\circ}\), corresponding to (001) plane was observed, indicating that the crystallographic \(c\)-axis of F\({}_{8}\)CuPc in the as-deposited film is perpendicular to the substrate surface. This orientation is favorable for charge transport between the source and drain electrodes of an OFET (vide infra), because the \(\pi\)-\(\pi\) overlap between adjacent F\({}_{8}\)CuPc molecules in the one-dimensional chain lies on the substrate. An OFET composed of F\({}_{8}\)CuPc was fabricated using a SiO\({}_{2}\)/n-Si substrate and Al electrodes. Figure 4 shows a schematic of the fabricated transistor and its current-voltage characteristics measured  
Figure 3: X-ray diffraction pattern of an F\({}_{8}\)CuPc film deposited on a SiO\({}_{2}\)/n-Si substrate. Figure 2: One-dimensional molecular arrangement of F\({}_{8}\)CuPc along the \(a\)-axis. in air. F\({}_{8}\)CuPc shows an n-type semiconducting behavior. However, parabolic current-voltage curves exhibiting a large hysteresis were obtained, although the first half of each measurement was consistent with the current-voltage characteristics reported previously [20]. Furthermore, the results could not be reproduced in repeated measurements. Figure 5 shows the current-voltage characteristics measured in vacuum (\(\sim\)10\({}^{2}\) Pa). Compared to the measurement in air, it is obvious that the instability is rather suppressed. These features indicate that the F\({}_{8}\)CuPc transistor is unstable in air, and therefore, it is difficult to evaluate the transistor characteristics under air. In fact, we could not observe the reported ambipolar characteristics under air [20,21] even when Au was used as the source and drain electrodes. On the other hand, Figure 6 shows the current-voltage characteristics measured under high vacuum (under a pressure less than 10\({}^{-2}\) Pa). F\({}_{8}\)CuPc shows a typical, stable n-type semiconducting behavior in vacuum, and the field effect electron mobility in the thin film transistor was calculated to be 7.9 \(\times\) 10\({}^{-4}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) (on/off ratio being 3.6 \(\times\) 10\({}^{3}\)). Obviously, the p-type characteristics could not be found in high vacuum, irrespective of the source and drain electrodes.