On the Relation between Morphology and FET Mobility of Poly( 3-alkylthiophene) s at the Polymer/ SiO2 and Polymer/ Air Interface

The influence of the interface of the dielectric SiO2 on the performance of bottom-contact, bottom-gate poly(3-alkylthiophene) (P3AT) field-effect transistors (FETs) is investigated. In particular, the operation of transistors where the active polythiophene layer is directly spin-coated from chlorobenzene (CB) onto the bare SiO2 dielectric is compared to those where the active layer is first spin-coated then laminated via a wet transfer process such that the film/air interface of this film contacts the SiO2 surface. While an apparent alkyl side-chain length dependent mobility is observed for films directly spin-coated onto the SiO2 dielectric (with mobilities of approximate to 10(-3) cm(2) V-1 s(-1) or less) for laminated films mobilities of 0.14 +/- 0.03 cm(2) V-1 s(-1) independent of alkyl chain length are recorded. Surface-sensitive near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements indicate a strong out-of-plane orientation of the polymer backbone at the original air/film interface while much lower average tilt angles of the polymer backbone are observed at the SiO2/film interface. A comparison with NEXAFS on crystalline P3AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3AT crystals suggest a close to crystalline polymer organization at the P3AT/air interface of films from CB. These results emphasize the negative influence of wrongly oriented polymer on charge carrier mobility and highlight the potential of the polymer/air interface in achieving excellent out-of-plane orientation and high FET mobilities.