Observing and Directing Crystalline Morphology in Organic Photovoltaic Materials

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Organic Photovoltaic (OPV) materials are of great interest as a low-cost material the purposes of achieving wide spread, solar energy adoption. However, a limiting factor in materials development is the ability to proactively determine the active layer thin film morphology that largely informs device performance. Thus, understanding how materials design and solution processing parameters impact the morphology formation is of particular value. In this work, morphology characterization methods such as grazing incidence wide-angle x-ray scattering (GIWAXS) are used to characterize the morphology directing qualities of materials design variations, and solvent and processing additive variations during spin-coating. Chapters 2 and 3 present work where the use GIWAXS analysis in concert with other morphological characterizations to elucidate the morphologically directing impact that molecular design variations have on the crystalline structures in the active layers of OPV and thin film transistor devices. Three design motifs are examined: 1) how comonomer structure can determine the levels of backbone periodicity in high performance polymers for transistor applications; 2) how side chain optimizations influence the self-assembled crystalline structures; and 3) the impact of polymer molecular weight on the crystallization directing capabilities of polymers in non-fullerene OPVs. Each of these design motifs offers insight into the subtle changes that molecular design variations will impact on the film properties. The impact of side chain variations on small molecule crystallization is then further explored, including mechanistic analysis of how the inclusion of a solvent additive 1,8-diiodooctane (DIO) to the processing slows down the crystallization process allowing the side chains to direct morphology. Chapters 4 and 5 utilize a newly developed in situ GIWAXS setup during the spin coating deposition to characterize the crystallization of archetypal polymer and small molecule films during the film formation process. Depending on the solvent and additive used in the processing, crystallization can range from occurring in < 1s to lasting up to 9 hrs. Differing crystallization dynamics are also observed between the three materials with significant distinctions between the polymers and the small molecule semiconductor. It is found that the divergence between polymer and small molecule evolutions depends on the additive/semiconductor molecular interaction. The differing dynamics between additives with side-chain focused interactions versus those with π-plane focused interactions is examined. Finally, guidelines for choosing the solvent/additive processing conditions based on the desired morphological properties are provided.

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  • 04/29/2019
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