Applications of conjugated polymers in photovoltaics and displays drive the need to understand how morphology affects emission and charge migration. A detailed analysis of the interactions that lead to aggregate-induced fluorescence quenching in conjugated polymers is challenging due to the heterogeneity of their structures. This drives recent efforts to analyze the effects of chain-chain interactions on the emission spectra via a detailed study of PPV- and thiophene-based oligomers and their aggregates.  These are formed in solution by 'poisoning' a solution of the oligomers in a good solvent with a small amount of a poor solvent. They exhibit remarkably uniform spectral properties that vary reproducibly with the oligomer chain length and amount of poor solvent used in the preparation.  This is verified by dispersed fluorescence microscopy of single aggregates. A more detailed probe of aggregate morphology is provided by fluorescence lifetime imaging (FLIM).  The aggregate images are found to be consistent with the formation of �core-shell� structures in which the chains at the aggregate core are tightly packed and the chains at the surface are weakly interacting.  This structural model of the single chain-aggregate interface rationalizes the bulk spectroscopic and lifetime data as well as our data on the corresponding polymers.  Our recent ultrafast studies probing the wavelength dependence of exciton-exciton annihilation dynamics and charge separation are also consistent with this morphological picture.

Optical properties of conjugated materials and their aggregates: Towards imaging of films and devices

248th National Meeting (2014)

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