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 ${\mathrm{SnO}}_2$ thin-film prepared by ion-beam-assisted deposition (IBAD) was investigated. When a buffer layer of 100-nm ${\mathrm{SnO}}_2$ 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 $0.5{\mathrm{cm}}^2∕\mathrm{V}$ that was initially $0.62{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ , 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 $0.35{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ , 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 ${10}^5$ to that of the unprotected devices $(\sim {10}^4)$ , which was reduced from $\sim {10}^6$ before aging. Therefore, the enhanced long-term stability of our encapsulated OTFTs should be attributed to good protection of permeation against ${\mathrm{H}}_2\mathrm{O}$ into the devices with the IBAD ${\mathrm{SnO}}_2$ 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.