Two n-type semiconducting polymers with alternating arylene (thiophene or selenophene)-tetraazabenzodifluoranthene diimide (BFI) donor-acceptor architecture have been investigated as new electron acceptors in polymer/polymer blend solar cells. The new selenophene-linked polymer, PBFI-S, has a significantly smaller optical band gap (1.13 eV) than the thiophene-linked PBFI-T (1.38 eV); however, both polymers have similar HOMO/LUMO energy levels determined from cyclic voltammetry. Blends of PBFI-T with the thiazolothiazole-dithienylsilole donor polymer (PSEHTT) gave a 2.60% power conversion efficiency (PCE) with a 7.34 mA/cm(2) short-circuit current. In contrast, PBFI-S:PSEHTT blends had a 0.75% PCE with similarly reduced photocurrent and external quantum efficiency. Reduced free energy for charge transfer and reduced bulk electron mobility in PBFI-S:PSEHTT blends compared to PBFI-T:PSEHTT blends as well as significant differences in bulk film morphology are among the reasons for the large loss in efficiency in PBFI-S:PSEHTT blend solar cells.