Electrical characteristics of single-component ambipolar organic field-effect transistors and effects of air exposure on them

We investigated the electrical characteristics of single-component ambipolar organic field-effect transistors (OFETs) by controlling the device structure and preparation and the measurement conditions. Six organic semiconductor materials (copper-phthalocyanine, tris-(8-hydroxyquinoline)aluminum $(Alq3)$⁠, alpha-sexithiophene, $4-4′$-bis-styrylphenyl, 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene, or a photopolymerized polydiacethylene derivative (PDA) were used as the active layer, and all were found to transport both holes and electrons. The PDA-based FETs had the highest hole and electron mobilities (0.12 and $0.025cm2/Vs$⁠, respectively). We also investigated the effect of air exposure on the OFETs. The hole mobility was barely affected by the exposure while the electron mobility was significantly affected. The threshold voltage for $p$-channel operation was shifted by the exposure while that for $n$-channel operation was not, indicating that the hole density in the active layer is increased by air exposure whereas the electron density is independent of air exposure. Furthermore, we prepared an $Alq3$-based $p$-channel OFET and investigated the effect of air exposure on it. While its operation was stable in vacuum, air exposure degraded its characteristics. These behaviors indicate that irreversible chemical reactions occur between cationic $Alq3$ species and oxygen or water molecules.