With the aid of optical modeling, the internal quantum efficiencies of organic Bulk Heterojunction (oBHJ) photovoltaic devices based on low band gap polymer of poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2', 3'-d[silole)-2.6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (Si-PCPDTBT) blended with the acceptors of 1-(3-Methoxycarbonyl) propyl-1-phenyl [6,6] C61 (PCBM) and bis-adduct (bis-PCBM) are determined. The Si-PCPDTBT:bis-PCBM devices show considerably lower short circuit current density (J(sc)) as compared to the Si-PCPDTBT:PCBM devices. The results show that 30% of this smaller J(sc) is due to the lower optical absorption of bis-PCBM, while the major losses originate from the electrical losses. It is found that for the best Si-PCPDTBT:bis-PCBM devices with an active layer thickness in the range of 70-100 nm, the inefficient charge generation within the bis-PCBM domains is the major contribution to the whole losses. Increasing the active layer thickness of Si-PCPDTBT:bis-PCBM device significantly enhances recombination losses in polymer/bis-fullerene matrix. (C) 2011 Elsevier B.V. All rights reserved.