Studies of conjugated polymer thin film morphology: effect on emission and charge transport
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Since their discovery, semiconducting conjugated polymers have shown great promise as active materials for a range of electronic devices. Initially desired for their high quantum yield, conjugated polymers have become popular due to their low cost and potential to be transferred to existing technology. Conjugated polymers are liquid crystalline, packing into well ordered domains upon thermal annealing of the films, which often leads to complex polymer interactions that can affect their semiconducting properties such as charge transport, emission color and ultimately device efficiency. Film morphology is difficult to characterize, with the order often varying on the nanoscale within a film. Near field scanning optical microscopy (NSOM) combined with Atomic Force Microscopy (AFM) can probe the degree of order of a film on the nanoscale and correlate it to topography; this can then be related to changes in luminescence emission and device characteristics to infer how charges are moving within a film. The effect of morphology on device function can vary between polymer systems; for example, di-alkyl polyfluorenes (PFs), a popular blue emitter for LEDs, undergo fluorescence degradation from ketone-based defects. Ordering of PF films containing some chemical defects increased the energy transfer from pristine chains to defects, increasing the defects’ degrading effect on the film emission. In comparison, the air-stable di-alkyl polyphenylene ethynylenes (PPEs) have numerous chain interactions in the amorphous pristine film, but show evidence of fewer interactions between these chains after ordering the film rather than more interactions. PPE polymers with varied lengths of sidechains produce dissimilar electroluminescence intensities, due to differences in their morphologies that affected how charges moved and recombined within the films. Understanding the effect of changes in polymer film morphology on luminescence and charge movement will help future efforts in understanding more complex polymer interactions, such as seen in blended polymer films.