Fluorescence scanning near-field optical microscopy of polyfluorene composites

Fluorescence scanning near-field optical microscopy (SNOM) is used to investigate binary polyfluorene-based composites of varying composition. The samples investigated contain blends of the polymer poly(9,9'-dioctylfluorene-cobenzothiadiazole), F8BT, with similar polyfluorenes of wider band gap. Images acquired from a film containing 50% by weight F8BT exhibit a high degree of correlation between the topography and fluorescence, with an F8BT-rich phase which protrudes from the surface of the film forming isolated regions with sizes from hundreds of nanometres to several micrometres. A film containing 10% by weight F8BT also has micrometre-size F8BT-rich regions, but also present are small and locally varying proportions of F8BT in the other polyfluorene component phase, indicating a hierarchy of phases within this sample. The fluorescence and topographic images of a third sample studied, containing 90% by weight F8BT, display no correlation, demonstrating that it is not always appropriate to use topographic information to determine the phase structure within polymer blends. The fluorescence SNOM images acquired from these samples are able to assist our understanding of the photovoltaic efficiency of devices fabricated from these films, which are governed by the extent of the interfacial area between these two constituent polymers.