A novel fused ladder alternating D–A copolymer, PIDT–DPP, with alkyl substituted indacenodithiophene (IDT) as donor unit and diketopyrrolopyrrole (DPP) as acceptor unit, was designed and synthesized by Pd-catalyzed Stille-coupling method. The copolymer showed good solubility and film-forming ability combining with good thermal stability. PIDT–DPP exhibited a broad absorption band from 350 to 900 nm with an absorption peak centered at 735 nm. The optical band gap determined from the onset of absorption of the polymer film was 1.37 eV. The highest occupied molecular orbital level of the polymer is as deep as −5.32 eV. The solution-processed organic field-effect transistor (OFETs) was fabricated with bottom gate/top contact geometry. The highest FET hole mobility of PIDT–DPP reached 0.065 cm² V⁻¹ s⁻¹ with an on/off ratio of 4.6 × 10⁵. This mobility is one of the highest values for narrow band gap conjugated polymers. The power conversion efficiency of the polymer solar cell based on the polymer as donor was 1.76% with a high open circuit voltage of 0.88 V. To the best of our knowledge, this is the first report on the photovoltaic properties of alkyl substituted IDT-based polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012