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Abstract: Pentacene for p-channel organic field effect transistors (OTFTs) and perfluoropentacene for n-channel OTFTs have attracted strong attentions to be used as CMOS type organic semiconductor materials for flexible organic displays. n-channel OTFTs with different gate insulators of polyimide (PI) and SiO2 and perfluoropentacene as a semiconductor layer were fabricated, and their instability of the transistor characteristics measured in air, vacuum and oxygen was investigated. The results show that both of the transistors exhibit a significant increase of mobility when measured in vacuum. However, the mobility of the transistors is significantly reduced when measured in pure oxygen, and the threshold voltages continuously increase. These phenomena may be related to water diffusion into the perfluoropentacene/gate insulator interface in air and oxidation of perfluoropentacene in oxygen. [Copyright &y& Elsevier]