UC Berkeley

Utilizing extended pi conjugated structures, organic semiconducting materials can effectively transport charges and are imbued with properties unique from inorganic systems. The field of organic electronics has achieved preliminary success in applying these semiconducting small molecules and polymers to intriguing new applications, such as thin-film technology, biologically compatible electronics, flexible devices, and many other areas.

Further advances in organic electronics require the discovery of new materials. For over two decades, fullerene based acceptors have been considered essential for high performance, slowing development of alternative electron deficient materials. Work undertaken in this dissertation focuses on advancing the next generation of non-fullerene acceptor materials. Robust and modular chemistry aids in the successful development of novel, high performance, electron acceptor materials with controllable physical and optoelectronic properties.

This modular and robust synthesis is exemplified by the development of bay-annulated indigo (BAI). Using indigo as a precursor, this stable amide-based withdrawing unit has outstanding charge transport properties, showing one of the highest recorded ambipolar conductivities. In order to better control intermolecular interactions, a method to desymmetrize the BAI core has been developed. Using this new methodology, a donor-acceptor BAI adduct is synthesized which self-assembles into nanowires that are capable of transporting charge. The use of this self-assembling material as an additive for photovoltaic applications gives an improvement in solar cell efficiency of ~11% over the control P3HT/PC[60]BM device.

To enhance the withdrawing character of existing conjugated systems, the 2-(1,3-dithiol-2-ylidene)malononitrile (DTM) group is proposed. Condensation of activated methylene compounds, such as malononitrile, with carbon disulfide produces a nucleophilic dithiolate salt that can participate in SNAr reactions. Incorporation of this withdrawing group is found to significantly alter the optoelectronic properties of 1,2,5-benzothiadiazole (BTD) acceptors. When monomers functionalized with the DTM group are polymerized, the resulting polymers have broadened light absorption, strong thermochromic and solvatochromic behavior, and improved crystallinity compared to a control fluorinated polymer analogue. When used as the active component in organic field effect transistors (OFETs) the DTM modified polymer is imbued with the ability to transport both electrons and holes, whereas the fluorinated polymer is a unipolar hole transport material.

Finally, a wide band gap acceptor is developed with the goal of improving the open circuit voltage (VOC) in solar cells. Utilizing the flagship non-fullerene acceptor ITIC as a template, a new family of molecules are synthesized using weakly withdrawing thiobarbituric acid (TBTA) groups to raise the lowest unoccupied molecular orbital (LUMO) energy. Not only does this result in an increased VOC, but the material outperforms the parent ITIC acceptor. To understand the solid state properties of the TBTA based acceptor, a morphological study is undertaken. The withdrawing TBTA groups are functionalized with a range of solubilizing hydrocarbon chains to provide a meaningful comparison. When applied to solar cells the effect on performance is drastic. Grazing incidence wide angle x-ray scattering (GIWAXS) experiments are performed to examine the crystallinity and intermolecular interactions in this system. Using this information, clear relationships are drawn between molecular functionality and device performance. This underlies the importance of morphological studies and demonstrates a need to understand these complex relationships.