Effects of Bithiophene Imide Fusion on the Device Performance of Organic Thin-Film Transistors and All-Polymer Solar Cells

Two new bithiophene imide (BTI)-based n-type polymers were synthesized. f-BTI2-FT based on a fused BTI dimer showed a smaller band gap, a lower LUMO, and higher crystallinity than s-BTI2-FT containing a BTI dimer connected through a single bond. s-BTI2-FT exhibited a remarkable electron mobility of 0.82 cm² V^-1 s^-1 , and f-BTI2-FT showed a further improved mobility of 1.13 cm² V^-1 s^-1 in transistors. When blended with the polymer donor PTB7-Th, f-BTI2-FT-based all-polymer solar cells (all-PSCs) attained a PCE of 6.85 %, the highest value for an all-PSC not based on naphthalene (or perylene) diimide polymer acceptors. However, s-BTI2-FT all-PSCs showed nearly no photovoltaic effect. The results demonstrate that f-BTI2-FT is one of most promising n-type polymers and that ring fusion offers an effective approach for designing polymers with improved electrical properties.