We investigated device design parameters for low-bandgap conjugated organic polymer blends of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]: phenyl-C(61)/C(71)-butyric acid methyl ester (PCPDTBT-PC(61)BM and PCPDTBT-PC(71)BM) used as photoactive components in polymer solar cells. The interrelationships between absorption, fill factor (FF), energy conversion efficiency, and current density have been simulated. Our results indicate that optimizing device efficiency is strongly dependent on the simultaneous control of active layer thickness and the charge carrier mobilities. It is found that energy conversion efficiencies have peak values when charge carrier transport is balanced. In addition PCPDTBT-PC(71)BM showed 44% higher efficiencies compared to PCPDTBT:PC(61)BM and yielded power conversion efficiencies of more than 6% when the active layer thickness is 90-100 nm. The results from this study will be used to predict potential efficiencies in PCPDTBT-PCBM organic solar cells. (C) 2010 Elsevier B.V. All rights reserved.