Surface Energy Patterning and Optoelectronic Devices Based on Conjugated Polymers

Abstract: The work presented in this thesis concerns surface energy modification and patterning of the surfaces of conjugated polymers. Goniometry and Wilhelmy Balance techniques were used to evaluate the surface energy or wettability of a polymer’s surface; infrared reflectionabsorption spectroscopy (IRAS) was used to analyse the residuals on the surface as modified by a bare elastomeric stamp poly(dimethylsiloxane) (PDMS). The stamp was found to be capable of modifying a polymer surface. Patterning of a single and/or double layer of conjugated polymers on the surface can be achieved by surface energy controlled dewetting. Modification of a conjugated polymer film can also be carried out when a sample is subjected to electrochemical doping in an aqueous electrolyte. The dynamic surface energy changes during the process were monitored in-situ using the Wilhelmy balance method.This thesis also concerns studies of conjugated polymer-based optoelectronics, including light-emitting diodes (PLEDs), that generate light by injecting charge into the active polymer layer, and solar cells (PSCs), that create electrical power by absorbing and then converting solar photons into electron/hole pairs. A phosphorescent metal complex was doped into polythiophene to fabricate PLEDs. The energy transfer from the host polymer to the guest phosphorescent metal (iridium and platinum) complex was studied using photoluminescence and electroluminescence measurements performed at room temperature and at liquid nitrogen temperature. PSCs were prepared using low-bandgap polyfluorene copolymers as an electron donor blended with several fullerene derivatives acting as electron acceptors. Energetic match is the main issue affecting efficient charge transfer at the interface between the polymers and the fullerene derivatives, and therefore the performance of the PSCs. Photoluminescence, luminescence quenching and the lowest unoccupied molecular orbital (LUMO) together with the highest occupied molecular orbital (HOMO) of the active materials in the devices were studied. A newly synthesized fullerene, that could match the low-bandgap polymers, was selected and used as electron acceptor in the PSCs. Photovoltaic properties of these PSCs were characterised, demonstrating one of the most efficient polymer:fullerene SCs that generate photocurrent at 1 ?m.