Two novel A-D-A type molecules IDT-2BM and IDTT-2BM with extended fused-ring indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) unit as core and strong electron-withdrawing unit 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as end-capping group were synthesized and investigated as electron acceptors in solution-processed polymer solar cells (PSCs). IDT-2BM and IDTT-2BM exhibited strong and broad absorption from 300 to 800 nm, and apporpriate LUMO (-3.8 eV) and HOMO (-5.5 to -5.6 eV) levels matching with the classical polymer donor PBDTTT-C-T. IDT-2BM and IDTT-2BM films exhibited intrinsic electron mobilities of about 3.7 X 10-6 and 1.0 X 10-5 cm2 V-1 s-1, respectively. Fullerene-free PSCs employing PBDTTT-C-T as donor and IDT-2BM or IDTT-2BM as acceptor afforded power conversion efficiencies of 4.26% and 4.81%, respectively.