Controlling solid-state order of conjugated polymers through synthetic design remains a longstanding challenge. We investigated the various effects of polymer structure on solid-state packing, especially regarding polymer-packing orientation (edge-on vs. face-on). Furan-containing diketopyrrolopyrrole (DPP) polymers with similar optoelectronic properties were synthesized with either linear hexadecyl or branched 2-butyloctyl side chains. Differences in polymer solubility were observed and attributed to variation in side chain size and polymer backbone curvature. The least soluble polymer, PDPP3F-C16, was found to provide the highest organic field-effect transistor (OFET) hole mobility of 2.25 cm2/V�s. Analysis of the films by AFM and grazing incidence X-ray diffraction (GIXD) reveal that less soluble polymers with linear side chains exhibit larger crystalline domains, pack considerably more closely, and show a greater preference for edge-on orientation. Characterization of polymer solutions prior to spin-coating shows a correlation between early onset nanoscale aggregation and the formation of films with highly aligned polymer-packing. Corroborating this relationship, adding non-solvent MeOH to PDPP3F-BO solutions was shown to induce aggregation and result in films with enhanced solid-state order and field-effect hole mobilities. In order to investigate the effects of backbone structure on polymer-packing, phenyl moieties within isoindigo polymers (PII) were systematically replaced with thiophenes to achieve polymers with increasing degrees of backbone planarity (PTBII, PTII). Increasing thiophene content resulted in more planar structures, based on UV-vis absorption spectroscopy, electrochemistry, and density functional theory (DFT) simulation. GIXD analysis of the thin films revealed that more planar polymers favored face-on orientation, and provided more efficient organic photovoltaic (OPV) devices (up to 4.79% PCE). Meanwhile, polymers that were less soluble or consisted of a less coplanar backbone showed a preference for edge-on orientation. Altogether, these studies highlight new synthetic strategies for designing semiconducting polymers with rationally engineered solid-state properties and enhanced performance for either OFETs or OPVs.

Synthetic control of solid-state order and polymer-packing orientation for enhanced organic electronic device performance

248th National Meeting (2014)

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