Bulk heterojunction binary blends of poly(3-hexylthiophene) (P3HT) and poly(2,3-dihexyl thieno(3,4-b)pyrazine) (pC(6)TP) and ternary blends of poly(3-hexylthiophene) (P3HT), poly(2,3-dihexyl thieno(3,4-b)pyrazine) (pC(6)TP) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) [6,6]-phenyl-C61-butyric acid methylester (PCBM) have been prepared and studied. Previous work has shown that pC(6)TP lies in a select class of semiconducting polymeric materials capable of producing photocurrent at photon energies below 1.1 eV. Here we report a fundamental study to probe the role of pC(6)TP in ternary blend devices with P3HT and PCBM and develop an equivalent circuit model for device function based on the concept of separate parallel binary devices. The excellent agreement between the calculated and measured open circuit voltages of the ternary blend devices validates the parallel circuit model and provides a pathway to the design of dedicated low band gap materials for these devices. (C) 2013 Elsevier B.V. All rights reserved.