UC Santa Cruz

Organic photovoltaic (OPV) devices utilize thin films of organic semiconductors that can be used for low-cost solution processed solar cells for harvesting solar energy as a source of renewable energy. The active layer of OPV devices is a bulk heterojunction (BHJ) thin film blend of two or more organic semiconductors (OSCs) including conjugated polymers and small molecules that have chemical structures characterized by alternating single and double bonds that absorb photons in the visible-spectrum. Fullerene derivatives have been relied on as a quasi-spherical small molecule acceptor that facilitates isotropic intermolecular charge transport, however, fullerene derivatives have relatively low optical absorption cross-sections and therefore contribute negligibly to device photocurrent. Recent progress and record power conversion efficiencies in BHJ based organic photovoltaic devices have utilized anisotropic non-fullerene small molecules in conjugated polymer blends. Despite the dramatic progress in device performance, there is a fundamental gap of knowledge in relating the small-molecule chemical structure and intermolecular interactions to the hierarchical device morphology and corresponding optoelectronic processes. The focus of this dissertation develops a deeper understanding of how the Ångstrom scale chemical structure of small molecule organic semiconductors influences the BHJ microstructure on the ~100 nm scale. With experiments designed around several systematically designed chemical series, this thesis interrogates the influence of multiple avenues of chemical structure dimensionality and the resulting evolution in BHJ microstructure using a combination of synchrotron X-ray scattering and electron microscopy techniques. Optical spectroscopy and device measurements are used to probe the device-relevant charge separation and transport processes. The correlation of the optoelectronic and microstructural characterization supports the claim that Ångstrom scale details of the small molecule chemical structure and intermolecular interactions can lead to a prescribed BHJ morphology for controlling the device relevant photophysical properties. The results of this dissertation demonstrate that both coarse changes in molecular geometry, subtle modifications of the number and connectivity of peripheral functional groups, and the geometry of the linking core between interacting side-chains can have an enormous influence intermolecular packing leading to dramatic changes on the nanometer scales of phase separation and self-assembly as well as optical and charge transport properties of both small-molecule and polymer networks. The conclusions of this thesis provide insight for the future design of next-generation organic semiconductors for light harvesting applications in polymer-based solar cells.