Efficient design and structural modifications for tuning the photoelectric properties of small-molecule acceptors in organic solar cells

Abstract

Five novel small molecules were designed and synthesized to investigate the relationships between molecular structures and photoelectric properties. Firstly, three A₁–A–A₁ type molecules, incorporating a 2,1,3-benzothiadiazole (BT) unit with no, one or two fluorine atoms as the core, 2-ethylhexyl-substituted phthalimide (PI) as the terminal moiety and ethynyl functionalized thiophene as the π-linker, were synthesized to explore the effect of fluorination on photoelectric properties, namely (PIAT)₂BT, (PIAT)₂fBT and (PIAT)₂dfBT. By introducing fluorine atoms on the molecular backbone, both the HOMO and LUMO energy levels can be tuned efficiently, with minimal influence on optical absorption. Next, on the basis of the BT-based molecules, two extended molecules containing diketopyrrolopyrrole (DPP) were constructed, namely (PIAT)₂DPP and FADPPPI, improving optical absorption successfully, especially for FADPPPI with an absorption edge of 850 nm in the film. Considering the photoelectric properties of the above five molecules, poly(3-hexylthiophene) (P3HT) was chosen as the electron donor material to pair with these molecules, and preliminary device fabrication and investigation of photovoltaic performances were performed. These results demonstrate that our materials show great potential in being small-molecule acceptors for organic solar cells (OSCs), and further research on device fabrication and optimization is in progress in our laboratory.