In this work, we designed, calculated and synthesized a planar electron acceptor IDT-2BR with an A-D-A structure, using indacenodithiophene (IDT) unit as a core and 5-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazo -lidin-4-one (BR) unit as end-capping electron-withdrawing groups. Theoretical calculations revealed that IDT-2BR adopted a nearly flat backbone configuration while the hexylphenyl groups on the IDT moiety exhibited a dihedral angle of ca. 115[degree] to the backbone plane. IDT-2BR exhibited excellent thermal stability, broad and strong absorption from 300 to 750 nm with a maximum extinction coefficient of 1.3 [times] 105 M-1 cm-1 at 636 nm, appropriate LUMO (-3.69 eV) and HOMO (-5.52 eV) energy levels matched with those of P3HT, and relatively high electron mobility of 3.4 [times] 10-4 cm2 V-1 s-1. Fullerene-free PSCs based on P3HT: IDT-2BR blended films gave PCEs of up to 5.12%, which is much higher than that of PC61BM-based control devices (3.71%) and is the highest value