THE EFFECT OF MONOMER SEQUENCE ON THE PROPERTIES AND DEVICE PERFORMANCE OF CONJUGATED ORGANIC SEMICONDUCTORS

Although sequence effect can potentially be a powerful tool to tune the photophysical properties of conjugated oligomers and copolymers prepared from donor and acceptor monomers, nearly all research to date focuses on the alternating structure. Little is known about the properties of other sequences and sequence has not been exploited as a tool for tuning these materials for specific applications. In order to explore this potentially powerful tool, a series of sequenced phenylene-vinylene oligomers was synthesized and investigated both experimentally and computationally. Using Horner-Wadsworth-Emmons (HWE) chemistry, dimers, trimers, tetramers, pentamers, and hexamers were prepared from two building block monomers, a relatively electron-poor unsubstituted p-phenylene-vinylene and an electron-rich dialkoxy-substituted p-phenylene-vinylene. UV-Vis absorption/emission spectra and cyclic voltammetry demonstrated that the optoelectronic properties of these oligomers depended significantly on sequence. To further understand the influence of monomer sequence on the properties and solar cell performance of donor-acceptor conjugated oligomers, a library of dimers, trimers, and tetramers were prepared from phenylene and benzothiadiazole monomers linked by vinylene groups. Optical and electrochemical studies established the influence of sequence on both the max and redox potentials of this series of structurally related oligomers. The effect of end groups (cyano, bromo, and alkyl) was also demonstrated to be important for the properties of these oligomers. Bulk heterojunction (BHJ) solar cells fabricated with selected tetramers as the donor exhibited power conversion efficiencies that varied by a factor of three as a function of sequence. The sequence effect on donor-acceptor conjugated polymers was also studied. Two trimeric isomers, comprising dialkoxy phenylene vinylene, benzothiadiazole vinylene, and alkyl endgroups with terminal olefins, were synthesized. Sequence effects were evident in the optical/electrochemical properties and thermal properties. The trimers were used as macromonomers in an ADMET polymerization to give sequenced polymers. The optical and electrochemical properties were similar to those of their trimer precursors—sequence effects were still evident. These results suggest that sequence control is important for tuning optoelectronic properties and photovoltaic performance of these structurally related conjugated oligomers. The polymerization of oligomeric sequences is a practical approach for the incorporation of sequence into polymers