Film morphology for organic solar cells : Effect of Solubility and Gravity Conditions

Organic photovoltaics is a promising environmentally friendly technology, due to their printability, light-weight nature and mechanical flexibility, and the possibility to use off-grid. Research and development in this field has resulted in power conversion efficiencies of over 15%. To further improve the efficiency, it is important to understand the connection between the morphology of the active layer and the performance of the device. This thesis focuses on understanding on a molecular level of the morphology formation in a thin coated film of a polymer bend, using two different approaches. One approach will focus on the thermodynamics of conjugated polymers in relation to the morphology, by using the Hansen solubility Parameters (HSP) and solution chemistry. The second approach focuses on understanding phase separation between the two polymers in the active layer. To be able to study phase separation, films were fabricated under microgravity conditions, as previous studies show that in these conditions phase separation mechanism is slowed down. Atomic force microscopy is used to characterize the resulting morphology of the thin films. Preliminary studies in this thesis showed that using HSP is a good tool, to understanding solvent-solute and solute-solute interactions in solution and to guide the final film morphology in relation to solubility. Furthermore, HSP is a good tool for the preliminary screening of alternative solvents and solvent blends for environmentally friendly processing solvents for upscaling. It was also found that dip coating of films under microgravity conditions provides a tool to study the early stages of the phase separation, as well as facilitate the study of the dependence of the morphology on the thicknesses of the coating. More work is needed to be able to separate the complex effect of hypergravity and to eliminate uncertainty concerning if the deposited wet film is completely dried under the microgravity phase