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Donor doping of thin films of pentacene (PEN) with alkaline metals such as sodium, potassium, and rubidium was carried out. Drastic changes in conductivities of PEN films from an insulator to an n‐type semiconductor due to the doping was observed. Maximum conductivity of 2.8 Ω−1 cm−1 was shown in the film doped with rubidium. Alkaline metal doping of thin films of other acene compounds such as tetracene and hexacene was also performed.

Structural studies on highly conductive thin films of pentacene (PEN) were carried out by x‐ray and electron diffraction methods. The ultrathin film before and after doping with iodine was confirmed to consist of single crystalline domains, and the lattice parameters were determined. Drastic elongation along the c axis as a result of doping was observed, while the a‐b plane did not change during doping. The intercalation of iodine molecules between molecular layers of PEN was confirmed.

Studies on electrical properties of highly ordered and amorphous thin films of pentacene (PEN) doped with iodine have been made. Highly ordered film heavily doped with iodine (PEN1I2.2) showed electrical conductivity of 150 Ω−1 cm−1 with a hole mobility of 0.2 to 1 cm2 V−1 s−1 at room temperature. Metallic temperature dependence of the conductivity of the film from 240 to 4 K was observed as the first metallic transport phenomenon ever noted in an acenic compound. In contrast, amorphous film of PEN doped with iodine showed a conductivity of 10−3 Ω−1 cm−1 with an estimated hole mobility of 2×10−5 cm2 V−1 s−1 and semiconductive transport. These results revealed that the electrical properties of PEN thin films are strongly correlated with the degree of molecular orientation.