Two novel donor–acceptor conjugated polymers comprising benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione and an extended TTBTBTT system (thiophene—T, benzothiadiazole—B) with different alkyl side chains are synthesized and investigated. It is found that the length of alkyl side chains has a strong influence on energy levels, film morphology, and charge‐carrier mobility of the designed polymers. Organic solar cells based on polymer P2 with bulky 2‐hexyldecyl side chains demonstrate significantly improved photovoltaic performances as compared to the devices based on P1 bearing relatively small 2‐ethylhexyl solubilizing groups.