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 dithieno[2,3-b;7,6-b]carbazole-based polymers (P1 and P2) were synthesized.
• P1 showed a larger band gap (2.07 eV) and a slightly deeper HOMO level (−5.2 eV).
• The P1-based photovoltaic devices showed a power conversion efficiency up to 2.51%.
• P1 has a less curved backbone and a higher absorbance in the visible range.

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.

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