Carbazole side-chained benzodithiophene based two-dimensional D-A conjugated photovoltaic polymers

In this work, one novel carbazole side chained benzodithiophene (BDT) monomer is designed and synthesized, and this new donor monomer based two D−A conjugated polymers with electron-withdrawing acceptor 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) (P5) and benzodithiophene-4,8-dione (BDD) (P7) are prepared. The polymers show 1.65–1.85 eV band gap, which is similar to the polymer with same backbone. The P5 based polymer solar cells show acceptable PCE around 8% and low enegy loss around 0.7 eV, which is similar to the fluorene side-chained polymer. P7 based polymer shows 1 V open-circuit voltage in both fullerene based and fullerene-free polymer solar cells. Molecular conformations, photophysical/electrochemical properties and film morphology of the device active layer blend films were also systematically investigated to reveal the structure−property relationships. This work suggests that carbazole unit is a promising π-conjugated side chain group for BDT monomer to construct efficient two-dimensional (2D) D−A conjugated photovoltaic polymers.