Synthesis of Conjugated Polymers: Photoactive Substances for Photonic Devices

Doktorsavhandling, 2001

Synthesis and characterization of conjugated polymers are the subject of this thesis, the goal of which is to synthesize polymers with properties that will make them suitable as the active materials in applications such as light-emitting diodes and microcavity laser devices. The two classes of conjugated polymers studied are poly(p-phenylenevinylenes) (PPVs) and polyfluorenes (PFs). By careful design of the chemical structure of a green emitting PPV, it is shown here that the resistance to photooxidation can be increased significantly. Further investigations of this PPV showed that the content of structural defects could be decreased by a lower polymerization temperature. As a consequence, the luminescence efficiencies in the solid state were increased dramatically. The highest photoluminescence yield for this polymer film was 62 %. The use of copolymers was shown to be another effective way to increase the photoluminescence yields. Using a copolymer with two polymers of different band-gaps resulted in materials with higher photoluminescence quantum yields than for any of the pure polymers. Polyfluorenes with segments that have a narrower band-gap were synthesized and used successfully as the active material in a microcavity laser device. The synthetic procedure was designed to allow the incorporation of only one, if any, segment per polymer chain. Energy transfer from the fluorene part of the polymers to the segments did occur. As a consequence, the undesirable self-absorption was reduced and all of the emissions in the solid state were detected from the segments, with photoluminescence yields up to 68 %.