On the Relation between Morphology and FET Mobility of Poly(3‐alkylthiophene)s at the Polymer/SiO 2 and Polymer/Air Interface

The influence of the interface of the dielectric SiO 2 on the performance of bottom‐contact, bottom‐gate poly(3‐alkylthiophene) (P3 A T) 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 SiO 2 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 SiO 2 surface. While an apparent alkyl side‐chain length dependent mobility is observed for films directly spin‐coated onto the SiO 2 dielectric (with mobilities of ≈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 SiO 2 /film interface. A comparison with NEXAFS on crystalline P3 A T nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3 A T crystals suggest a close to crystalline polymer organization at the P3 A T/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.