New benzotrithiophene derivative with a broad band gap for high performance polymer solar cells

A new planar conjugated polymer, poly{benzo(1,2-b:3,4-b':5,6-d '') trithiophene-alt-4,4'-dihexyl-2,2'-bithiazole} (BTT-BTz), with a broad band gap and higher V-oc based on benzotrithiophene and bithiazole units, was designed and synthesized. This copolymer possesses a deeper HOMO (-5.65 eV), and after 1,8-diiodoctane treatment, the power conversion efficiency of the photovoltaic device based on the BTT-BTz/PC71BM photoactive layer reaches 5.06% with 0.81 V of open-circuit voltage, 10.9 mA cm(-2) of short-circuit current and 0.57 of fill factor, which is the highest one among the conjugated polymers based on BTT or BTz derivatives. This new conjugated copolymer seems a promising candidate for the application in tandem solar cells.