Thumbnail image of graphical abstract

A new carbazole-based electron accepting unit, 5-(2,7-dibromo-9H-carbazol-9-yl)ben-zo[a]phenazine (CBP), is newly synthesized as the acceptor part of donor-acceptor type low band-gap polymers for polymer solar cells, as presented by Ji-Hoon Kim, Hee Un Kim, Chang Eun Song, Moo-Jin Park, In-Nam Kang, Won Suk Shin, and Do-Hoon Hwang on page 2354. The CBP is copolymerized with electron-donating monomers, and produces two alternating copolymers, PBDT-CBP and PBDTT-CBP. An alternating copolymer (PBDT-CBZ) consisting of 9-alkylcarbazole (CBZ) and BDT units is also synthesized for comparison. CBP containing polymers (PBDT-CBP and PBDTT-CBP) show a significantly red-shifted broad absorption between 300 and 850 nm compared with that of PBDT-CBZ. The principle described here could be extended to other types of accepting units toward the design of new photoactive materials.