Synthesis and Properties of Conjugated Polymers for Photonic Devices
Doctoral thesis, 2003
The synthesis and properties of new conjugated polymers for photonic devices, such as light-emitting diodes and solar cells is dealt with here. Much work was devoted to increasing the efficiencies of the devices by controlling the properties in thin films of the polymers. A series of polythiophenes was prepared first, in which the polymer backbones are separated by bulky substituents. The polymers have high regioregularity and are highly ordered in the solid state. Photoluminescence efficiency of 26% was reached for one of the polymers in the series, and the highest efficiency in LEDs, 0.36%, was reached for another polymer.
Another aspect treated is the synthesis of both insoluble and soluble polythiophenes with covalently linked fullerenes that were evaluated for use in solar cells. The attachment of the fullerenes prevents large scale phase separation in the films. Despite the inclusion of the bulky fullerenes, the soluble polymers retain their high order in films. The polymer with the highest amount of incorporated fullerenes showed the best performance in solar cells; it also showed a power conversion efficiency of 0.6% under low intensity monochromatic light illumination.
Furthermore, polyfluorene copolymers with red-shifted absorption were synthesised and used in solar cells. The copolymers have donor-acceptor segments that induce a partial charge-separation, which lowers the band gap of the polymers and extends the coverage of the solar spectrum. A series of polymers with the same backbone, but different alkyl side-chains was prepared. High-efficiency solar cells, with power conversion efficiencies up to 2.6% at simulated solar illumination, were prepared from blends of the polymer and a fullerene derivative, PCBM. Finally, copolymers with hydroxyl groups, on the side-chains, which can form hydrogen bonds with PCBM, and thereby offering improved morphology in the blends, were prepared. Solar cells, with power conversion efficiencies up to 2.4% under simulated solar illumination, were prepared from blends of these polymers and PCBM.