Design, Synthesis, and Characterization of Organic Small Molecule and Polymer Semiconductors and Their Use in Organic Photovoltaic Devices

Swick, Steven Mitomu

doi:https://doi.org/10.21985/n2-xd7c-9f85

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Design, Synthesis, and Characterization of Organic Small Molecule and Polymer Semiconductors and Their Use in Organic Photovoltaic Devices

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Organic solar cells (OSC) are a next generation solar energy technology that offers the advantages of scalable fabrication, light weight, flexibility, and earth-abundant starting materials. Despite tremendous advances in OSC power conversion efficiency (PCE) over the last decade, active layer material selection and optimization is still largely empirical. In order to facilitate the rational design and development of high efficiency OSCs, a theoretical framework describing the intra and inter-molecular forces that drive the self-assembly of polymeric and small molecule semiconductors in active layer thin films is essential. In the work presented here, thin film and bulk characterization techniques are used in concert with computational modeling to characterize morphology, investigate the forces controlling morphology, and to understand how molecular packing patterns influence charge transport properties. In Chapter 2, a series of polymeric, p-type semiconductors with varying alkyl chain lengths were synthesized and evaluated for their photovoltaic efficiency. Computational modeling of polymer conformations as a function of alkyl chain length revealed that polymer conformation was strongly dependent on alkyl chain length. Analysis of these conformations allowed for a new thermodynamic parameter (ΔEπ) to be defined, which describes the energetic driving force for domain growth and dopant exclusion. Finally, in combination with advanced thin film characterization techniques, ΔEπ was used to understand how polymer self-attraction influences morphological and photovoltaic parameters, such as domain purity and short circuit current. Chapter 3 presents a study in which two novel n-type semiconductors with π-extended end groups (ITN-C9 and ITzN-C9) based on the indacenodithienothiophene (IDTT) scaffold are synthesized and characterized. Single crystal X-ray analysis reveals that ITN-C9 and ITzN-C9 organize such that their LUMO-rich end groups have intermolecular π-π distances as close as 3.31 Å, with electronic coupling integrals as large as 38 meV, and internal reorganization energies as small as 0.133 eV, which are comparable to or superior to those in the corresponding fullerenes. In Chapter 4, a crystallographic and computational study of nine IDTT-based acceptor crystal structures is presented. A discussion correlating the effect of IDTT side chain length and position, end group polarity, and end group aspect ratio on single molecule conformation, crystal structure packing, electronic structure, and charge transport is presented. In addition, the crystal structure packing motifs and the intra/inter-molecular forces that underlie these unique patterns of molecular organization are documented, analyzed, and interpreted in the context of fullerene and perylene diimide acceptors, two of the most common acceptors used in OSCs. By correlating differences in chemical structure with crystal structure packing (and how that impacts the formation of charge transporting networks), rational design rules for small molecule acceptors are proposed. In Chapter 5 the interplay between π-extension and fluorination of NFAs on OSC device properties, single crystal packing, and charge transport properties (the latter through the use of density functional theory calculations) are investigated through the synthesis and characterization of two novel, fluorinated and π-extended NFAs (ITN-F4 and ITzN-F4). It was of particular interest to see whether π-extension and fluorination could be used in a synergistic fashion to produce new NFAs with low internal reorganization energies and close π-π interactions, potentially promoting efficient electron transport and extraction. Crystallographic and computational analysis reveal that ITN-F4 and ITzN-F4 have small internal reorganization energies and form well connected electrical networks with large electronic coupling values, which likely promote high electron mobilities. Femtosecond transient absorption spectroscopy revealed that both ITN-F4 and ITzN-F4 undergo ultrafast hole transfer (<300 fs) in films with the donor polymer PBDB-TF, despite the formation of excimer states in both neat and blend films. Impedance spectroscopy of illuminated cells in a steady state revealed greater recombination lifetimes in ITN-F4 blends than in ITzN-F4 blends.

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