Medium Bandgap Polymer Donor Based on Bi(trialkylsilylthienyl‐benzo[1,2‐b:4,5‐b′]‐difuran) for High Performance Nonfullerene Polymer Solar Cells

A new 2D-conjugated medium bandgap donor–acceptor copolymer, J81, based on benzodifuran with trialkylsilyl thiophene side chains as donor unit and fluorobenzothiazole as acceptor, is synthesized and successfully used in nonfullerene polymer solar cells (PSCs) with low bandgap n-type organic semiconductor (n-OS) 3,9-bis(2-methylene- (3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4- hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]- dithiophene (ITIC) and m-ITIC as acceptor. J81 possesses a lower-lying highest occupied molecular orbital (HOMO) energy level of −5.43 eV and medium bandgap of 1.93 eV with complementary absorption in the visible–near infrared region with the n-OS acceptor. The PSCs based on J81:ITIC and J81:m-ITIC yield high power conversion efficiency of 10.60% and 11.05%, respectively, with high V oc of 0.95–0.96 V benefit from the lower-lying HOMO energy level of J81 donor. The work indicates that J81 is another promising polymer donor for the nonfullerene PSCs.