Transient optical studies of exciton dynamics in organic solar cells

There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.