Impact of Solar Cell Processing Conditions on Active Layer Morphology and Charge Generation Dynamics in Organic Solar CellsPublic
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Increasing industrialization and the resulting negative environmental impacts highlight the need to develop alternative renewable energy sources. The Sun is a massive source and organic solar cells are a growing field of study. As new materials are synthesized, the efficiencies of organic solar cells continue to grow, but without an understanding of the fundamental processes of current generation, improvements in solar cells are limited to high throughput screening of new materials and ad hoc use of a variety of processing conditions. In this thesis, we demonstrate the effects of processing conditions and molecular structure on the photoactive layer morphology and charge carrier generation in organic solar cells based on small molecule and polymeric materials. Chapters 2 and 3 address the effects of solvent additives on polymer and small molecule aggregation in solution and the effect on thin film morphology. We demonstrate in two active layer solutions, (1) PTB7 and PC61BM and (2) PBTIBDT and PC71BM, that the additive, 1,8 diiodooctane, completely dissolves the fullerene component of the active layer solution allowing for interpenetration into the polymer matrix and a large mixed fullerene-polymer domain in the thin film. This results in improved charge generation, but also increased bimolecular recombination demonstrating that the active layer morphology must be delicately balanced between pure domains and mixed domains to achieve the highest solar cell efficiency. The focus of Chapter 4 is the structure of small molecule donors in organic solar cells. In solar cells based on small molecule donors, the crystal structure and the resulting charge transport abilities of the crystal have a large impact on the final solar cell efficiency. In this study, we present a series of small molecule donors with an acenedithiophene core unit of varying lengths and find that longer and more planar molecules create more tightly packed crystals that create small, but highly crystalline domains in thin film. We further demonstrate that the longer molecules show increased mobility in both neat thin films and in films blended with the fullerene PC61BM.
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