Copolymers of carbazole and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (dTBT) incorporating thioalkyl (−SR) and alkoxy (−OR) solubilizing groups on the 2,1,3-benzothiazdiazole (BT) unit are synthesized and compared. The introduction of −SR and −OR groups onto the BT unit of the polymer was found to have different effects on the electronic properties of the polymers as well as the conformation of the polymer backbone. Large conformational changes between the ground state (GS) and excited state (ES) geometries of the polymers with −SR groups led to very large Stokes shifts of up to 224 nm. The polymer with −OR groups was found to have approximately double the photovoltaic efficiency at ∼4% compared to the polymers with −SR groups (PCE ∼ 2%). However, polymers with −SR groups were found to give very high open circuit voltages (VOC) of over 1 V. Changing the −SR chain length from ethyl to dodecyl was found to have little influence on the solar cell performance of the polymer or the magnitude of the Stokes shift.