Thickness-dependent thermal transition temperatures in thin conjugated polymer films

We report the effects that geometrical confinement has on both the glass transition temperature, Tg, and the crystalline phase transition temperature, Tc, for two widely studied conjugated polymers, namely poly(9,9-dioctylfluorene) [PFO] and poly(9,9-dioctylfluorene-co- benzothiadiazole) [F8BT]. The Tg and Tc values were determined for thin film samples via temperature-dependent ellipsometry measurements. The thickness-dependent Tg (Tc) behavior is characterized by three regimes, namely, (i) large d or bulk samples for which Tg = Tgbulk (Tc = Tc bulk). (ii) intermediate d samples for which Tg > Tgbulk (likewise for Tc), and (iii) ultrathin samples for which Tg drops again (likewise for Tc). The intermediate regimes occur for 160 nm > d > 60 nm and 300 nm > d > 80 nm for PFO and F8BT, respectively. The higher-than-bulk Tg and Tc values offer the potential to design more robust and thermally stable polymer optoelectronic devices, including light-emitting diodes, lasers, and solar cells. © 2006 American Chemical Society.