Design, Synthesis and Theoretical Investigation of Small Molecules for Utility in Organic Semiconducting Devices
Solution-processed small molecule bulk-heterojunction solar cells represent a specific subset of organic photovoltaics (OPV). OPV devices rely on materials with appropriately aligned frontier molecular orbitals, bandgaps commensurate with the solar spectrum, and ultimately must self-assemble into a morphology conducive to high device performance. Optical electronic and physical properties in organic materials are highly sensitive to their chemical structure and the conformations of those structures in space. Materials can be engineered to exhibit specific traits; a process referred to as "molecular design." While the molecular design toolbox is ever-expanding, each of these properties requires unique considerations, and indeed vary greatly in the degree of control the synthetic chemist has in producing predictable properties. In order to elucidate the relationship between structure and properties, a class of small molecules was developed adhering to what can be described as a D'ADAD' architecture, where D, D' and A refer to an electron rich core, electron rich end-caps and electron deficient heterocyclic fragments, respectively. These fragments, as well as solubilizing side groups were systematically modified, yielding useful design rules for organic donor materials as well as record breaking small-molecule OPV devices. The top performing material in the group exhibited diminutive performance on the ubiquitous solution deposited substrate PEDOT:PSS due to interfacial chemistry. This led to the development of a new material, p-DTS(FBTTh2)2, which was not susceptible to the interfacial chemistry with PEDOT:PSS, and broke the previous performance record for solution-processed small molecule OPV devices. Four isostructural molecules, including p-DTS(FBTTh2)2 were investigated with single crystal x-ray diffraction. While all four molecules appear topologically equivalent, two types of crystal structure were observed with distinct crystal systems and each with a characteristic molecular geometry. A multi-scale theoretical investigation of simulated isolated molecules and experimentally determined crystal structures offers a clear explanation for the observed lattices, where useful experimental data is unavailable.