Tuning the band gap of conjugated polymers is very crucial to achieve high performance organic photovoltaic (OPV) devices. The band gap can be controlled by introducing electron-withdrawing and/or electron-donating atoms on the polymer backbone. We investigate the effect of oxygen and fluorine substitution on the frontier molecular orbital energies of polymers PCPDT-BT, PCPDT-DFBT and PDTP-DFBT using time dependent density functional calculations. Torsional dependence of orbital energy and the band gap of these polymers are compared. In the polymer limit, the band gap of these polymers follows the order: PCPDT-DFBT > PCPDT-BT > PDTP-DFBT.