Studies on Molecular Design and Synthesis of Furan-Containing Diketopyrrolopyrrole Derivatives for Solution-Processed Organic Solar Cells

Abstract

In these days, solution-processed bulk heterojunction (BHJ) type organic solar cells have drawn great attention in order to develop alternative energy system for the future. I have been particularly interested in the structure of diketopyrrolopyrrole (DPP) unit, which is a widely used electron-withdrawing building block in the synthesis of organic semiconducting materials owing to its superior optical and electrical properties. Among the recent works, the DPP-based small molecules with well-organized π-conjugated structure of A-D-A or D-A-D type framework have been developed rapidly, exhibiting promising solar cell efficiencies. However, they inevitably encounter a challenging issue about solubility attributed to the strong intermolecular hydrogen bonding capability of DPP cores. Instead of introducing long and branched alkyl side chains, here I adopted alternative strategy that is the incorporation of furan unit in the conjugated backbone, as a novel design strategy of solution-processable small molecules for the highly efficient organic solar cells. Several furan-containing DPP-based small molecules were successfully synthesized, characterized and fabricated for the BHJ organic solar cells. In the chapter 2, I designed a novel A-D-A type small molecule as a donor material named F2, where A is furan-containing DPP and D is electron-donating dithienothiophene (DTT). I demonstrated that furan moiety can replace thiophene, while maintaining the level of decent photovoltaic properties of thiophene derivatives. It was demonstrated that F2 showed enhanced solubility and Voc value, with more simple synthetic procedure. Consequently, the device of F2 and PC61BM exhibits a PCE of 2.1% with the improved Voc of 0.75 V when fabricated with 1 vol% of DIO. In addition, I also discussed about the effects of furan moiety on molecular properties, systematically. In the chapter 3, I designed four different small molecules with A-D-A-D-A type structures depending on the number and position of furan. This study is intended to demonstrate that the furan-containing DPP-based small molecules are also available for the BHJ organic solar cells as acceptor materials, exhibiting promising photophysical properties.