An Alternating Copolymer Derived from Indolo[3,2-b]carbazole and 4,7-Di(thieno[3,2-b]thien-2-yl)-2,1,3-benzothiadiazole for Photovoltaic Cells

An alternating narrow bandgap conjugated copolymer (PICZ‐DTBT, Eg = 1.83 eV) derived from 5,11‐di(9‐heptadecanyl)indolo[3,2‐b]carbazole and 4,7‐di(thieno[3,2‐b]thien‐2‐yl)‐2,1,3‐benzothiadiazole (DTBT), was prepared by the palladium‐catalyzed Suzuki coupling reaction. The resultant polymer absorbs light from 350–690 nm, exhibits two absorbance peaks at around 420 and 570 nm and has good solution processibility and thermal stability. The highest occupied molecular orbital (HOMO) energy level and lowest unoccupied molecular orbital (LUMO) level of the copolymer determined by cyclic voltammetry were about −5.18 and −3.35 eV, respectively. Prototype bulk heterojunction photovoltaic cells from solid‐state composite films based on PICZ‐DTBT and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC71BM), show power conversion efficiencies up to 2.4% under 80 mW · cm−2 illumination (AM1.5) with an open‐circuit voltage of Voc = 0.75 V, a short current density of Jsc = 6.02 mA · cm−2, and a fill factor of 42%. This indicates that the copolymer PICZ‐DTBT is a viable electron donor material for polymeric solar cells. An alternating narrow bandgap conjugated copolymer (PICZ‐DTBT, Eg = 1.83eV) derived from 5,11‐di(9‐heptadecanyl)indolo[3,2‐b]carbazole and 4,7‐di(thieno[3,2‐b]thien‐2‐yl)‐2,1,3‐benzothiadiazole, is prepared by a palladium‐catalyzed Suzuki coupling reaction. The copolymer absorbs light from 300–690 nm, exhibits two absorbance peaks at around 420 and 574 nm, and has good solution processability and thermal stability. Photovoltaic cells based on a PICZ‐DTBT/PC71BM blend show power conversion efficiencies up to 2.4% under 80 mW · cm−2 illumination (AM 1.5) with a light response extending to 750 nm.