A series of polymer solar cells (PSCs) were fabricated with narrow band gap polymer poly([4,9-dihydro-4,4,9,9-tetra(4-hexylbenzyl)-s-indaceno[1,2-b:5,6-b0]-dithiophene-2,7-diyl]-alt-[2,3-bis(3-(octyloxy) phenyl)-2,3-dihydro-quinozaline-2,20-diyl] (PIDTDTQx), small molecule material 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIB-SQ), or their blend as electron donor, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor. The champion power conversion efficiency (PCE) of ternary PSCs arrives to 6.49% with a short circuit current density (J(sc)) of 11.56 mA/cm(2) and a fill factor (FF) of 66% when the DIB-SQ doping ratio in donors is 9 wt%. The champion PCE values of binary PSCs with PIDTDTQx:PC71BM or DIB-SQ:PC71BM as the active layers are 5.47% or 1.78%, respectively. An apparent PCE improvement of 18.6% was obtained from the optimized ternary PSCs compared with binary PSCs with PIDTDTQx:PC71 BM as the active layers. The underlying reason of PCE improvement was investigated from the absorption spectral complementary, photoluminescence emission quenching, intermolecular charge transfer, and the balance of charge carrier transport in ternary active layers. (C) 2015 Elsevier B.V. All rights reserved.