Excited State Processes in Solar Energy Materials

This dissertation covers studies of excited state processes in two types of solar energy materials: alternating polyfluorene polymers and their blends with fullerenes in the active layer of plastic solar cells, and bis-tridentate RuII-polypyridyl complexes to be used as sensitizer in systems for artificial photosynthesis.

The polymer:fullerene blends were studied by transient absorption and time-resolved fluorescence measurements in order to investigate the role of the charge transfer (CT) state in charge formation. Previous studies have proposed that hot CT states is a necessary requirement for efficient charge formation in some active layer materials. However, in these studies relaxed CT states were shown to act as an intermediate state for at least ~20 % of the charges formed in the studied blends. This suggests that it is possible to achieve efficient charge formation without excess energy, which can lead to the development of solar cells with reduced energy losses.

Excited state properties of three bis-tridentate RuII-polypyridyl complexes with large variations in room-temperature lifetimes were studied by density functional theory (DFT) and time-dependent DFT calculations. Potential energy surfaces (PESs) calculated for the lowest triplet state were able to capture the decay channels responsible for the observed lifetime. The obtained activation energies for these decay processes were in reasonable agreement with experimental values. The PES calculations furthermore illustrated the importance of other features than the activation barriers in order to obtain a long room-temperature lifetime, in particular entropic factors seem to have significant contributions in some long-lived complexes. Improved understanding of the relation between chemical structure and room-temperature lifetime can lead to successful synthesis of long-lived complexes using other metals.