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

• Published in: Advanced functional materials Vol. 24; no. 14; pp. 1994 - 2004
• Main Authors: Oosterbaan, Wibren D, Bolsee, Jean-Christophe, Wang, Linjun, Vrindts, Veerle, Lutsen, Laurence J, Lemaur, Vincent, Beljonne, David, McNeill, Christopher R, Thomsen, Lars, Manca, Jean V, Vanderzande, Dirk JM
• Format: Journal Article
• Language: English
• Published: 01.04.2014
• Subjects: Backbone; Crystal structure; Dielectrics; Laminates; Organizations; Orientation; Silicon dioxide
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201303298

The influence of the interface of the dielectric SiO sub(2) 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 SiO sub(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 sub(2) surface. While an apparent alkyl side-chain length dependent mobility is observed for films directly spin-coated onto the SiO sub(2) dielectric (with mobilities of approximately 10 super(-3) cm super(2) V super(-1) s super(-1) or less) for laminated films mobilities of 0.14 plus or minus 0.03 cm super(2) V super(-1) s super(-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 sub(2)/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. The orientation and organization of semiconducting polymer at the polymer/(gate dielectric) interface has a strong influence on OFET hole mobilities. For poly(3-alkylthiophene) thin films, both from chlorobenzene and from nanofiber dispersions, orientation and OFET mobility are compared for the interfaces formed at bare SiO sub(2) and at air (then laminated on bare SiO sub(2)), that is, at two extremes of the polarity scale.