Synthesis of alternating copolymers consisting of N-2-octyldodecyldithieno[2,3-b;7,6-b]carbazole and N-octylthieno[3,4-c]pyrrole-4,6-dione derivative units for photovoltaic applications

Two conjugated polymers consisting of alternating dithieno[2,3-b;7,6-b]carbazole and thieno[3,4-c]pyrrole-4,6-dione either without (P1) or with (P2) thiophene spacers were synthesized via Stille coupling. P1 and P2 showed a band gap of 2.07 and 1.80 eV and a highest occupied molecular orbital (HOMO) level of −5.2 and −5.1 eV, respectively. The P1-based solar cells showed the best power conversion efficiency (PCE) of 2.51% when 1.0% diiodooctane was mixed into the active layer of P1/PC71BM (1/1). In contrast, the PCE of P2-based solar cells was much lower, due to its lower molecular weight, more curved backbone, smaller absorption coefficient, and slightly higher HOMO level.