Solution Chemistry and Morphological Properties for Organic Solar Cells : Exploring Alternative Solvents Using Microgravity and Modelling as Tools

Organic photovoltaics (OPVs) have the advantage of the accessibility of energy for all, due to facile and low-cost processing, with its low energy payback time compared to other technologies, therefore promising applications. Research and development have led to power conversion efficiencies of nearly 20% and now catching up to their inorganic counterparts. To enhance the efficiency even further, it is crucial to get an insight into the correlation between the active layer's morphology and the device's performance as well as how to control the morphology of the active layer. This thesis focuses on a molecular understanding of the morphology formation in a thin film of a polymer blend for OPVs. By using Hansen solubility parameters (HSP) and solution chemistry, the thermodynamics of the phase separation of conjugated polymers, both in solution and thin films, is investigated. Furthermore, to get a deeper understanding of the phase separation between the polymers in the active layer, films were prepared under microgravity conditions, as the phase separation is slowed down under such conditions. Atomic force microscopy combined with infrared spectroscopy was used to characterize the morphology of the dry film. Our results show that understanding solvent-solute and solute-solute interactions is key to comprehending morphology formation. Moreover, HSP proves to be a valuable tool for the initial screening of alternative solvents and solvent blends for more environmentally friendly processing and upscaling. It was found that microgravity conditions provide a tool to study the early stages of phase separation, as well as facilitate the study of the dependence of the morphology on the thicknesses of the film. Additional research is needed to separate the complex effects of gravity fluctuations and to eliminate uncertainty concerning the complete drying of the film under the microgravity phase.
Organic photovoltaics have the advantage of the accessibility of energy for all, due to facile and low-cost processing and therefore promising applications. Research and development have led to power conversion efficiencies of nearly 20% and now catching up to their inorganic counterparts. It is crucial to understand the correlation between the active layer's morphology and the device's performance. This thesis focuses on a molecular understanding of the morphology formation in a thin polymer film for application in OPVs. Hansen solubility parameters (HSP) and solution chemistry, the thermodynamics of phase-separation of two photo-active polymers, both in solution and films, were investigated. To probe the phase separation in the active layer, films were made under microgravity conditions, as the separation is slowed down under such conditions. Atomic force microscopy combined with infrared spectroscopy was used to characterize the dry film. Our results show that understanding the solution chemistry is key to comprehending the morphology formation. It is shown that HSP is useful for the initial screening of more environmental-friendly solvents and solvent blends for processing. It is found that microgravity conditions provide means to study the early stages of phase separation.