Passivation effects on the stability of pentacene thin-film transistors with SnO₂ prepared by ion-beam-assisted deposition

The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO₂ thin-film prepared by ion-beam-assisted deposition (IBAD) was investigated. When a buffer layer of 100-nm SnO₂ film had been thermally deposited to reduce ion-induced damage prior to the IBAD process, our encapsulated organic thin-film transistors (OTFTs) showed somewhat degraded field-effect mobility of 0.5 cm²/V that was initially 0.62 cm²/V s, while the OTFTs without a buffer layer showed a 60% reduction in field-effect mobility after the IBAD process. However, surprisingly, the mobility was sustained up to one month and then gradually degraded down to 0.35 cm²/V s, which was still three times higher than that of the OTFT without any encapsulation layer after 100 days in air. The encapsulated OTFTs also exhibited superior on/off current ratio of over 10⁵ to that of the unprotected devices (∼10⁴), which was reduced from ∼10⁶ before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs should be attributed to good protection of permeation against H₂O into the devices with the IBAD SnO₂ thin film, which was identified as having a dense amorphous microstructure with lots of OH groups. Passivation effects on the electrical properties of OTFTs are discussed in terms of the physical and chemical properties of the barrier films.