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

Abstract 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 F···S 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 V oc of 0.78 V, a J sc 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 V oc of 0.89 V, a J sc 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.