By systematically moving the alkyl chain branching position away from the polymer backbone, two new thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTT-T) polymers are presented. When used as donor materials in polymer : fullerene solar cells, efficiencies exceeding 7% could be achieved without the use of processing additives. Effects of the alkyl chain branching point position on the thin film morphology are investigated using x-ray scattering techniques as well as the effects on photovoltaic and charge transport properties. For both solar cell and transistor devices, moving the branching point further from the backbone is beneficial. This is the first time that this effect has been shown to improve solar cell performance. Strong evidence is presented for changes in microstructure across the series which is most likely the cause for the photocurrent enhancement.