title: Thickness-dependent glass transition temperature and charge mobility in cross-linked polyfluorene thin films.
abstract: We report thickness-dependent glass transition temperature (T_{g}) and charge mobility in cross-linked thin films made of conjugated polymer poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB). Monotonic T_{g} depressions with reducing film thickness in thermally and UV cross-linked TFB thin films supported on Si-SiOx substrates are observed through ellipsometry measurements, suggesting that a surface mobile layer with enhanced chain dynamics still exists in cross-linked TFB thin films, even with a high cross-linking percentage. Data fitting using a three-layer model shows that the T_{g} in the interface, bulk and surface layer both increases with increasing cross-linking, while the thickness of the interface and surface layer increases and reduces, respectively. Cross-linking of TFB thin film generates traps that hinder charge transport and consequently reduce charge mobility. The charge mobility converges in thick (>140 nm) and thin (<40 nm) TFB films but shows strong thickness dependence in between, reducing from 4.0×10^{-4}cm^{2}/Vs in a 180-nm film to 0.1×10^{-4}cm^{2}/Vs in a 20-nm thin film.
authors.affiliation: School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China.
authors.forename: Dan
authors.initials: D
authors.lastname: Liu
authors.affiliation: School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Qin
authors.affiliation: School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China.
authors.forename: Jinghui
authors.initials: J
authors.lastname: Zhang
authors.affiliation: School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Wang