A Donor Polymer Based on a Difluorinated Pentathiophene Unit Enabling Enhanced Performance for Nonfullerene Organic Solar Cells

Abstract

AbstractFor oligothiophene‐based donor polymers, changing the number of thiophene rings in each repeating unit is a simple and effective approach to tune their optical and electronic properties. While bithiophene‐, terthiophene‐, and quaterthiophene‐based polymers have been widely known and studied, pentathiophene‐based ones are rarely reported. Here, the pentathiophene‐based polymer PT5T, which yields extremely poor device performance with an efficiency of only 0.37%, is synthesized. However, after the introduction of two fluorine atoms to the center thiophene unit, the resulting polymer (PT5T‐2F) yields dramatic improvement in device performance with the fill factor and the power conversion efficiency improved to 70.8% and 9.69%, respectively. The morphological study shows that the difluorinated pentathiophene building block introduces enhanced molecular aggregation and crystallinity of PT5T‐2F, which facilitates the formation of bulk heterojunction blends with higher domain purity. Most importantly, despite its high crystallinity, PT5T‐2F can still produce reasonably small domain sizes in the polymer‐based blend. The results indicate that the difluorinated pentathiophene is a promising building block to construct donor polymers for nonfullerene organic solar cells.