Correlating Morphology to Performance in Conjugated Polymer Nanocomposite Thin Films

The morphology and performance of thin films that consist of conjugated polymers and nanoparticles are investigated in this thesis. In the first system, the morphology of the nanocomposite that consists of low band gap alternating copolymers with a methano-fullerene are determined by neutron scattering and correlated to their photovoltaic performance as polymer solar cells. These results show that the conjugated alternating copolymers have high miscibility relative to other conjugated polymers. The analysis of the scattering data shows that the morphology of the conjugated polymer-fullerene bulk heterojunction can be described as the formation of aggregates on two length scales. Important parameters obtained via small angle neutron scattering including correlation length and volume fraction of the aggregates provide insight into the photovoltaic efficiency of film. In addition to role of polymer structure on performance, the impact of post-deposition processing by solvent annealing on the power conversion efficiency of a organic photovoltaic active layer consisting of a low band gap polymer and methano-fullerene was also studied. The impact of solvent quality for both the polymer and fullerene on morphology development in the active layer on the morphology and performance is studied. The results show the system that is annealed in chlorophenol, which is a selective solvent for the fullerene showed a maximum increase in photo-efficiency. The diffusion coefficient of the solvent into these bulk heterojunction was also obtained and optimum time for solvent annealing was deduced. Finally, the possibility of improving the mechanical and barrier properties of the hole transport layer in organic photovoltaic cells has been investigated via addition of clay platelets. The impact of the addition of montmorillonite clay to the conductivity of thin film consisting of PEDOT:PSS and clay was studied. The optimum loading of clay in PEDOT:PSS that improves these structural properties, but does not significantly alter electronic performance is found to be between 5 to 10%. The active layer and hole transporting layer of organic photovoltaic cells have also been investigated through imaging techniques to more thoroughly characterize the morphology and structure of the examined conjugated polymer nanocomposites.