Synthesis and photovoltaic properties of carbazole-based conjugated polymers

Two kinds of copolymers based on dihydroindolo[3,2-b]carbazole in combination with dithieno[3,2-b:2',3'-d]silole (P1) and cyclopenta[2,1-b:3,4-b']dithiophene (P2) were designed and synthesized. The structures and properties of the copolymers were fully characterized. Compared with P1 film, P2 film showed a slighted red-shifted absorption spectrum. The two polymers had similar lowest unoccupied molecular orbital energy levels (3.6-3.7 eV); however, there were differences in highest occupied molecular orbital energy level: -5.54 eV for P1 and -4.92 eV for P2. These results indicate that it is a simple and effective approach to tune the bandgap in a conjugated polymer by tune the bridge atom.