After an extensive scientific effort spent on the understanding of conjugated polymers and molecules, these material are on the border of commercialisation. Even though highly efficient devices can be made with conjugated polymers the underlying physics is far from being fully understood. In this thesis, photoluminescence is used for studying excited state events inconjugated polymers. The work is relevant for a number of applications, such as photodiodes, lasers and light emitting devices.

A close co-operation with chemists which have synthesised a number of phenyl substituted polythiophenes has made it possible to find correlation's between the chemical structure and the resulting photoluminescence of these polythiophenes. The yield of photoluminescence is set by intrachain properties influencing the geometry of the single chain and interchain properties. By comparing the photoluminescence of the polymers in solution and thin film one can assign the intrachain properties, which are dominant in solution and the interchain properties which act in the thin film. We have found that polymers with planar backbones give the most luminescent polymers in solution. Using interchain interaction to increase the planarity has been shown to increase the PL yield even further. In thin films the interchain interaction between different chains can give both intrachain emitting species, and an increase of exciton transport, which increase the possibility of finding PL quenching sites in the polymer film, thus reducing the PL yield in the thin film.

With a strong electron acceptor, the exciton on the polymer chain can split into its constituents, forming an electron and a hole. A quenching of the photoluminescence occurs. This has been used to determine the exciton quenching range in a polythiophene, close to a strong donor-acceptor interface. A short distance was found (5 nm) which is strongly limiting the efficiency of photovoltaic devices.