Poly-thieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) is a promising low bandgap donor polymer for organic solar cells. In this study we employ two different approaches to improve the device efficiency via optimisation of the morphology of the active layer: tuning of the molecular weight of the polymer and thermal annealing. In the former case, a higher molecular weight was found to yield a more intermixed morphology, resulting in enhanced exciton dissociation and charge separation, as confirmed by atomic force microscopy, and photoluminescence and transient absorption spectroscopies. In the later case, thermal annealing prior to metal electrode deposition increased the photon conversion efficiency to as high as 6.6%, with this enhanced efficiency being maintained even with prolonged annealing (240 hours at 80 °C). This enhancement in performance with thermal annealing was correlated with increased polymer crystallinity.