Mapping the fluorescence decay lifetime of a conjugated polymer in a phase-separated blend using a scanning near-field optical microscope

We have studied a blend of the polymers poly(9,9-dioctylfluorene) (F8) dispersed in an inert matrix of polystyrene (PS) using time-resolved scanning near-field optical microscopy (SNOM). On spin-casting, phase separation occurs between the two polymers resulting in a thin film characterized by an F8-rich phase and a PS-rich phase. By spatially mapping the intensity of photoluminescence from the film, we find that there is a low concentration of F8 trapped within the PS-rich phase. We find that the fluorescence emission lifetime (measured at 440 nm) of F8 trapped within the PS-rich phase is significantly longer than that from the F8-rich phase (290 ps compared to 235 ps). Furthermore, spectral measurements indicate that the F8 emission from the PS-rich phase is characterized by a reduced fraction of emission from fluorenone defect states. Taken together, our measurements suggest that in the PS-rich phase interchain exciton diffusion between F8 molecules is suppressed significantly by the effect of dilution.