Effect of the phenyl ring orientation in the polystyrene buffer layer on the performance of pentacene thin-film transistors

The phenyl groups of polystyrene (PS) thin films untreated and thermally treated at 80 and 120 °C assume tilt angles of 27°, 39° and 62°, respectively. The PS films were inserted between SiO2 and organic semiconductors as buffer layers for organic thin-film transistors (OTFTs). The results showed that a flat orientation of phenyl ring at the surface of the PS films optimized the surface energy of PS film, resulting in higher crystallinity of pentacene films deposited onto it and an improved interconnection between the pentacene crystalline domains. The device with the PS film thermally treated at 120 °C showed superior performance, affording a mobility as high as 4 cm2/V s, an on/off ratio of about 107–108 and a threshold voltage of about 6.5 V in the saturation region.