Study On Defect Structures, Optical Properties And Stabilization Of Fluorene-Based Conjugated Polymers

Much progress has been made in improving colour purity, efficiency, and stability for organic light-emitting diodes over the past decade; however their device stability and life time are not good enough to industrialization, especially for conjugated polymers. More evidences confirm that these problems are due to the presence of defects in polymer backbone. Thus many of recent research on conjugated polymers focus on defect structures and their relationship with optoelectronic properties in order to increasing stability.Polyfluorene and their derivatives have been extensively studied over the past few decades, for promising candidates of high-efficiency blue emitting polymer. However, one of the difficulties often encountered is that the light emitting diodes (LED) are their low-energy emission band appears in the range between 2.0 to 2.5 eV, which turns the desired blue into green. It was firstly attributed to excimer or aggregate formation. But more powerful evidence indicates that due to emissive ketonic defect site. However, the understanding of these interrelated properties is still debatable, such as the kinetics of degradation and the incompatibility of the concentration of fluorenone and the intensity of low-energy emission, and so on. In this thesis, we study on photo- and thermal- degradation of bridged poly(para-phenylene) derivatives, which focus on defect structure, optical properties and stabilization.Photo- and thermal- degradation of fluorene-based conjugated polymer derivatives in air is studied by spectroscopic methods, which suggest autocatalytic radical-chain reaction leading to emissive fluorenone and non-emissive alkyl ketones. Our results suggest the possibility of the oxidation of the 9-dialkyl fluorene sites. The degradation kinetics strongly depend on the substituents, residual monoalkyl fluorene and palladium catalysts.Strong low energy emission is observed in degraded bridged poly(p- phenylene) derivatives doped in inert matrix where the average distance between adjacent molecules is very far from effective intermolecular interaction. Thus the low energy emission must come from the emission of single fluorenone but not any type of fluorene and/or fluorenone based aggregate or excimer. Alkyl ketones, which play a role as non-emissive quencher, are another very important factor besides fluorenone, for effective quenching of bulk emission also increases the ratio of the low energy emission. This is further testified in degradation in presence of endgroups and palladium catalysts. The concentration dependent emission behavior in PF/non-emissive-quencher /fluorenone blends can simulate degradation kinetics very well.Based on the results in the degradation mechanism and the origin of low energy emission, we have designed the polymer and gold nanoparticle composites to increase stability. Interaction between energy level of polymer and surface plasma resonance of gold nanoparticle effectively increase the colour purity, efficiency and stability. We also discuss this topic in gold nanoparticle with different size and ligands, which related to surface plasma resonance.