Controlling interchain interactions in conjugated polymers: Studies of the physical and electronic properties of PPV-derivatives for application in plastic optoelectronic devices

Conjugated polymers (CPs) are semiconductors with the processing advantages and mechanical properties of plastics. CPs have high photoluminescence quantum yields, can be synthesized to emit in the visible region, and are soluble in common organic solvents or in water. Thus, CPs offer a wide range of applications in novel optoelectronic devices such as flexible, large area displays. Despite these vast potential applications, CP-based devices are still far from commercialization due to their low efficiency and short operational lifetimes. This thesis focuses on the photophysics and electrical properties of CPs in order to improve their application in light-emitting diodes.; One of the controversies about the photophysics of CPs concerns the nature of interactions between polymer chains. The questions we address include: Do interchain species exist? If so, are they best thought of as “aggregates”, “excimers”, or “polaron pairs”? How do interchain interactions affect polymer photophysics and hence the performance of CP-based devices? Are there ways to control these interchain interactions for desired applications? Our studies focus on two typical CPs: poly(2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) and poly(2,5-bis[N-methyl- N-hexylamino] phenylene vinylene) (BAMH-PPV). We find that the polymer conformation changes in different solution environments, leading to interchain interactions because of chain aggregation. When films are cast from these solutions, the aggregates survive the casting process and carry into the films. Near-field scanning optical and atomic force microscopy studies on MEH-PPV films prepared from different solutions reveal a direct correlation between chain packing and the electronic properties of a CP film. Evidence from solvatochromism experiments also suggest that multiple interchain species with various degrees of charge separation exist in annealed MEH-PPV films. We show that the film morphology and the degree of interchain interactions can be controlled by a number of factors: changing the solvent and polymer concentration of the solution from which the films are cast, heating or adding acid to the polymer solutions, and annealing polymer films. The film morphology affects the polymer photophysics, such as photooxidation and exciton-exciton annihilation rates, excited state lifetimes, the performance of CP-based devices: interchain interactions in CP films promote good carrier transport but also reduce the luminescence quantum yield. Thus, much controversy in the literature can be resolved by noting that the polymer samples in different studies were prepared differently, thus have different degrees of interchain interactions. Taking advantage of polymer/mesoporous silica composites, we also discovered that intrachain energy migration occurs more slowly than interchain energy transfer in MEH-PPV. Overall, controlling polymer morphology through film processing conditions has allowed us to improve the performance of CP-based optoelectronic devices.