Development of Direct Arylation Polymerization Methodologies for Conjugated Copolymers and the Synthesis and Characterization of Indacenodithienothiophene Electron Accepting Molecules for Organic Solar Cells

Aldrich, Thomas Jordan

doi:https://doi.org/10.21985/n2-72hk-e480

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Development of Direct Arylation Polymerization Methodologies for Conjugated Copolymers and the Synthesis and Characterization of Indacenodithienothiophene Electron Accepting Molecules for Organic Solar Cells

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Photovoltaic devices containing organic semiconducting chromophores are a promising technology for the conversion of solar energy into electricity. Research into the molecular design and processing of these materials has propelled the power conversion efficiency of laboratory-fabricated organic polymer solar cells (PSCs) to over 17%, which exceeds the 10% threshold deemed necessary for industrial viability. However, improvements in the synthetic methodologies that enable access to high-performance photovoltaic materials as well as in the understanding of structure-properties-performance relationships are still required. In Chapter 1, the fundamental operational principles of PSCs are introduced and general chemical design considerations for high-performance organic photovoltaic materials are outlined. In Chapters 2 and 3, environmentally benign polymerization methodologies are developed for the synthesis of photovoltaic copolymers. In Chapters 4 and 5, structure-property-performance relationships for electron accepting molecules are investigated. In Chapter 6, the compatibility of DARP copolymers and nonfullerene acceptors is assessed. In Chapter 2, a direct C–H arylation polymerization (DARP) approach is optimized for the sustainable synthesis of the archetypal benzodithiophene-alt-diketopyrrolopyrrole copolymer, PBDTT-DPP. Molecular model studies elucidate the effects of DARP reaction conditions on the resulting relative defect formation rates. Copolymer branching arising from non-selective β-C–H arylation at the polymer chain ends is identified as the principal photovoltaically deleterious defect. The optimal low-defect density DARP-derived copolymers achieve superior PSC performance than defect-rich DARP-derived copolymers and rival those derived from toxic Stille polycondensations. In Chapter 3, the DARP methodology is further optimized for the synthesis of high-performance copolymers, including the widely-used benzodithiophene-alt-fluoro-thienothiophene-ester copolymer, PBDTT-FTTE. The DARP-derived copolymers are benchmarked versus Stille-derived counterparts by comparison of physicochemical and NMR spectroscopic properties, all of which indicate great chemical similarity. The DARP- and Stille-derived copolymer and fullerene blend morphologies are characterized by AFM, TEM, and XRD and are virtually indistinguishable, in accord with the similar PSC performances. The PBDTT-FTTE PSC efficiency of 8.4% is a record for PSCs containing DARP copolymers. In Chapter 4, the influence of indacenodithienothiophene-based electron acceptor (ITIC) alkyl substituent length on PSC active layer morphology, charge transport, and photovoltaic performance is investigated. The alkyl substituent length modulates the thermal annealing response of the acceptors and can enhance active layer crystallinity. The ITIC acceptor with n-nonyl substituents achieves the highest performance of 10.2% PCE. In Chapter 5, the effects of ITIC end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, and photovoltaic performance are investigated. The most heavily fluorinated acceptor, ITIC-6F, achieves the highest performance of the series, approaching 12% efficiency in blends with the benzodithiophene-based donor copolymer, PBDB-TF. Additionally, a previously unrecognized solution-phase redistribution process between the end-groups (EGs) of ITIC-related acceptors, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2 ⇌ 2 EG1-IDTT-EG2, is identified and mechanistically studied, and the effects on PSC performance are assessed. In Chapter 6, the optimized DARP methodology is further employed in the synthesis of a benzodithiophene-alt-fluoro-thienothiophene-ester copolymer family, PBDT(Ar)-FTTE, with varied heteroaryl (Ar) backbone substituents. These donor copolymers are then investigated in BHJ PSCs with the IDTT-derived acceptor ITIC-Th. The effects of the copolymer Ar substituent on the photovoltaic performance, BHJ morphology, and photovoltaic performance stability are assessed.

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