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

For 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.