The Influence of Morphology and Molecular Orientation on the Efficiency and Lifetime of Organic Solar Cells
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One of the reasons for the low efficiency of organic solar cells (OSCs) is because the relationship between morphology and molecular orientation of the organic semiconductors and the performance and lifetime of the OSC is not well understood. Theoretically, the morphology and molecular orientation of organic semiconductors influence the three major processes that determine the performance of an OSC. These processes are i) light absorption, ii) exciton dissociation and iii) charge transport. The general objective of this research work is to understand how the morphology of the active layers in an OSC affects these three processes with the aim of gaining insights that can be used to improve OSC efficiency and lifetime. The relationship between device physics and morphology is studied by analyzing the photovoltaic performance, external quantum efficiency, UV/visible absorption, charge conductivity and surface morphology of the fabricated OSCs and relevant organic semiconductor active layers. Chapters 5 and 6 in this work study the impact of varying the donor-acceptor ratio in OSCs. The best OSC performance was found to occur at high fullerene concentrations due to increased fullerene aggregation resulting in larger absorption and exciton dissociation in the OSC. Building on this insight, OSCs comprising a mixed layer with high fullerene content followed by a neat fullerene layer, was identified to possess the highest performance of all other device architectures due to balanced charge generation and charge collection. Chapters 7 and 8 in this work study the impact of varying the molecular orientation of phthalocyanine donor materials using templating layers on the performance and lifetime of planar heterojunction OSCs. Templating layers force the phthalocyanine molecules to lie flat on the substrate affecting the three main OSC processes. Depending on the combination of the templating layer and phthalocyanine, different aspects of the OSC device physics are altered leading to large differences in performance gains obtained from templating. Finally, it was found that the morphology of the donor material affects the morphology of fullerene acceptor deposited on top, which results in substantial variation in the lifetimes of phthalocyanine/fullerene planar heterojunction OSCs.