As a high-performing, medium band gap donor polymer achieving over 7% in bulk heterojunction (BHJ) solar cells with a thick active layer, PBnDT-FTAZ has demonstrated unique photovoltaic properties that have not yet been fine-tuned. In this study, three new polymers (PSBnDT-FTAZ, PBnDT-SeFTAZ, and PSBnDT-SeFTAZ) are designed to determine how the FTAZ system would respond to further structural modifications. Specifically, we aimed to answer (a) whether alkylthio substitution could increase the open circuit voltage (V-oc) of the related BHJ device and (b) whether selenophene incorporation could decrease the band gap of the FTAZ polymer and lead to an improved short circuit current (J(sc)), while PBnDT-FTAZ's other desirable attributes (high fill factor and thick active layer) could still be retained. We found that although the V-oc of the alkylthio-substituted polymers (PSBnDT-FTAZ and PSBnDT-SeFTAZ) did not appreciably increase, selenophene-incorporated polymers (PBnDT-SeFTAZ and PSBnDT-SeFTAZ) indeed showed lowered band gaps of 1.7 eV. In particular, the smaller band gap of PBnDT-SeFAZ led to a larger J(sc) in its BHJ solar cells than that of the original PBnDT-FTAZ solar cells. Each polymer reached moderate efficiencies (1.87-5.72%) while retaining a thick active layer (similar to 300 nm), demonstrating the further potential of the FTAZ system for organic photovoltaics.