New low-band gap polymers based on dithienylbenzothiadiazole (DBT) and dithiophene with different bridging atoms have been synthesized and explored in a comparative study on the photochemical stability and photovoltaic performance. Two differently modified DBT units were exploited, namely 5,6-bis(tetradecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT1) and 4,7-bis(4-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT2). In thin films the polymers had optical band gaps in the range of 1.51–1.70 eV where the DBT2 based polymers are red shifted 61–81 nm compared to the DBT1 based polymers indicating greater interchain packing when the side chains are situated on the thienyl groups compared to on the benzothiadiazole unit. The best photovoltaic devices based on blends of polymer and [6,6]-phenyl C₆₁butyric acid methyl ester (PCBM) were prepared with polymers based on the DBT1 unit giving efficiencies up to 2.3%. The photochemical stability was measured by the amount of absorbed photons under 1 sun versus the ageing time for each polymer, which clearly shows that the two polymers containing a 4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) unit are by far the most unstable. Substitution of the bridging carbon atom with silicon results in a significant stability improvement by a factor 5.