Thickness-dependent thermal transition temperatures in thin conjugated polymer films

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