Defect Emission from PDAFs

To account for the formation of the defect sites in pristine 103, they proposed that, during the polymerisation, some of the alkylfluorene units were reduced by the nickel(O) to monoalkylfluorenyl anions 105, which 

Other approaches that have been found to improve the stability of blue emission from PDAFs are to attach bulky substituents such as cyclohexyl groups [214] or polyhedral siloxanes [215-217] to the ends of the alkyl chains. These presumably work by reducing exciton migration to defect sites. 

The blue emission from PDAFs is unstable, with the appearance of a strong emission band around 530 nm after annealing or upon running an EL device [14,15]. Initially this long wavelength emission band was believed to be due to emission from excimers [63,64], but subsequent work has shown that it is due to emission from fluorenone defects within the PDAF chain [15]. 

Other approaches have involved reduction of interchain interaction either by cross-linking the polymers using vinyl endgroups [ 71 ] or by attaching bulky groups, e.g., dendrimers [72,73] or silsesquioxanes [74,75], as end- or side-groups. There is evidence that reduction of fluorene by calcium cathodes is implicated in the formation of ketone defects during LED operation, and this can be suppressed by introduction of a buffer layer between the polymer and the cathode [76]. 

Another approach to avoiding defect formation is to have substitution at C9 which cannot be oxidised to form a ketone. Replacement of the carbon with a nitrogen bridge (carbazole) or a silicon (silafluorene) is covered in other chapters to which the reader is referred. Replacement of the alkyl groups at C9 with aryl groups has proved to be a successful way to obtain stable blue emission from PFs and is the subject of the next section. 

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