Using a dicarboxylic ester bithiophene (DCBT) co-monomer, electron-withdrawing ester groups are introduced on the two central thiophene rings of an alternating donor-acceptor polymer (PDPP4T) consisting of diketopyrrolopyrrole (DPP) and quaterthiophene (4T) to increase the oxidation potential and reduce the photon energy loss in solar cells. To counteract the increased solubility of the PDPPDCBT polymers owing to the ester side chains, linear instead of branched side chains were used on the DPP monomer. The length of the ester side chains was varied to study their effect on the optoelectronic properties, morphology and photovoltaic performance of these polymers in bulk-heterojunction blends with PC71BM as acceptor. The molecular weight of PDPPDCBT is limited, because the bisstannyl-DPP monomer could not be completely purified from mono-functional analogues. By using a non-stoichiometric monomer ratio or a branching unit, the molecular weights were improved to ∼35 kDa. A maximum power conversion efficiency of PCE = 5.7% was obtained. Compared to the parent PDPP4T (PCE 7.0%), the photon energy loss for the best PDPPDCBT was reduced from 0.79 to 0.76 eV, but the photon-to-electron quantum efficiency was reduced, as a consequence of a too coarse phase separation, as studied with two-dimensional grazing-incidence X-ray scattering and transmission electron microscopy.