A 3,3-Difluoro-2,2-Bithiophene Based Donor Polymer Realizing High Efficiency (>17%) Single Junction Binary Organic Solar Cells

In this study, two novel donor-acceptor (D-A) copolymers are designed and synthesized, DTBT-2T and DTBT-2T2F with 2,2 '-bithiophene or 3,3 '-difluoro-2,2 '-bithiophene as the donor unit and dithienobenzothiadiazole as the acceptor unit, and used them as donor materials in non-fullerene organic solar cells (OSCs). Due to enhanced planarity of polymer chains resulted by the intramolecular FS noncovalent interactions, the incorporation of 3,3 '-difluoro-2,2 '-bithiophene unit instead of 2,2 '-bithiophene into the polymers can enhance their molecular packing, crystallinity and hole mobility. The DTBT-2T:L8-BO based binary OSCs deliver a power conversion efficiency (PCE) of only 9.71% with a Voc of 0.78 V, a Jsc of 20.69 mA cm-2, and an FF of 59.67%. Moreover, the introduction of fluoro atoms can lower the highest occupied molecular orbital levels. As a result, DTBT-2T2F:L8-BO based single-junction binary OSCs exhibited less recombination loss, more balanced charge mobility, and more favorable morphology, resulting in an impressive PCE of 17.03% with a higher Voc of 0.89 V, a Jsc of 25.40 mA cm-2, and an FF of 75.74%. These results indicate that 3,3 '-difluoro-2,2 '-bithiophene unit can be used as an effective building block to synthesize high performance polymer donor materials. This work greatly expands the selection range of donor units for constructing high-performance polymers. A novel donor-acceptor (D-A) copolymer by incorporating 3,3 '-difluoro-2,2 '-bithiophene is synthesized. Introducing 3,3 '-difluoro-2,2 '-bithiophene unit introduces intramolecular FS noncovalent interactions, which can enhance the planarity of the polymer chain, thereby improve molecular packing, crystallinity, and hole mobility. Finally, a photoelectric conversion efficiency (PCE) of 17.03% is achieved. This work can provide a new strategy for the design and synthesis of polymer donor materials. image