Structure, morphology and ultrafast dynamics of photoexcited states in pi-conjugated polymers.

摘要

Since the discoveries of metallic and super-conductivity, photovoltaic effect and electroluminescence π-conjugated polymers (PCP) should be regarded as organic semiconductors. However, a typical pristine polymer film has optical properties of a set of disordered organic molecules. Observed optical transitions characterize only discrete portions of polymer chains, called chromophores. The energy transfer to most aligned chromophores takes time of order of a few picoseconds and is similar to that observed in photosynthetic reactions. Therefore, the degree of structural order of PCP is a key parameter governing both photophysics and transport properties. The subject of the present study is the interplay between structure, morphology and ultrafast photophysics in systems of π-conjugated molecules.; Intraband excitons are recognized as the primary optically excited states in all studied polymers and organic crystals. Exciton states follow the symmetry of isolated chains or chain aggregates. Direct band-to-band transitions are of much less importance.; In heat- or vapor-treated films a new conformational phase appears. Increased chain planarity results in larger rod-like chromophores. Close contacts between such π-conjugated chromophores enables geminate polaron pairs generation via exciton dissociation. Polaron pair generation is enhanced in films with increased intrachain order and their diffusion-like recombination kinetics is faster.; In films with increased interchain coupling the excitons acquire much larger interchain component. In such films photogenerated geminate polaron pairs with ultrafast dynamics are observed. The recombination rate of polaron pairs permits to estimate the mobilities in self-organized two-dimensional π-conjugated stacks.; Exciton migration plays an important role in the process of laser action in highly luminescent PCPs. While the amplified spontaneous emission is the fastest relaxation process in PCPs with isolated chain morphologies, bimolecular recombination effects prevail in polymers with stronger interchain interactions.