Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a...

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 10⁴ cm^-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (V_OC) of 0.92 V, a short-circuit current (J_SC) of 18.27 mA cm^-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.