A new conjugated polymer (PBAIIDTT) based on bay-annulated indigo and indacenodithieno[3,2-b]thiophene was designed, synthesized, and characterized. PBAIIDTT shows strong absorption in 400-500 and 600-800 nm, and its HOMO and LUMO energy levels are -5.45 eV and -3.65 eV, respectively. In organic field-effect transistors, the polymer exhibits a relatively high hole mobility of 0.39 cm(2) V-1 s(-1). PBAIIDTT was added to poly(3-hexylthiophene) (P3HT) and phenyl-C-61-butyric acid methyl ester (PC61BM) based organic solar cells. Ternary blend solar cells with 10% PBAIIDTT show an increased short circuit current density due to the broadened photocurrent generated in the near-infrared region, and a power conversion efficiency of 3.78%, which is higher than that of the P3HT:PC61BM binary control devices (3.33%). (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 213-220