Conjugated Materials for Organic Electronics

This thesis describes the design and synthesis of novel conjugated materials for applications in organic electronics. The first half of the thesis focuses on conjugated polymers to be used as p-type (hole-transporting) donor materials in organic solar cells. I was first interested in studying how the open-circuit voltage of an organic solar cell could be increased by lowering the HOMO level of the donor polymer via functional group substitution. I chose to study the benchmark poly(3-hexylthiophene) structure and substituted the 3-hexyl chain with a less-electron donating 3-thiohexyl group. In Chapter 2, I synthesized an equally proportioned diblock copolymer containing 3-hexylthiophene and 3-thiohexylthiophene blocks, and found that the open-circuit voltage of the block copolymer device was as high as that of the device containing only the lower HOMO polymer. This inspired me to investigate the minimum amount of the lower HOMO unit required to reach this maximum voltage and thus I synthesized a series of statistical copolymers, presented in Chapter 3. For this new system I was also interested in investigating the effect of polymer sequence on device performance and thus switched from a block to statistical architecture. In the second half of the thesis, I focus on novel n-type (electron-transporting) materials based on arylene diimides. In chapter 4, I synthesized a one donor-two acceptor random terpolymer containing naphthalene diimide (NDI) and perylene diimide (PDI) as the two electron-deficient units and 2,7-carbazole as the electron-rich unit. I varied the ratio of the two acceptor units and found that while the optical and electrochemical properties varied with acceptor composition, the composition dependence was not evident in device performance. Rather, any amount of the NDI monomer reduced the device efficiency to the value of the one donor-one acceptor NDI parent copolymer due to a pinning of the short-circuit current at the lowest value. In chapter 5, I studied the effect of the single atom substitution of oxygen with sulfur on the optoelectronic and self-assembly properties of a series of small molecule naphthalene diimides. The low LUMO levels and strong intermolecular interactions of these thionated molecules make them ideal candidates for use in n-channel organic thin film or micro/nanowire transistors.Ph.D