Two conjugated polymers (PBDT86-TQ and PBDT88-TQ) with different side chains, but with the same polymer backbone, have been prepared for polymer solar cells. The side chains in the two polymers induced different steric hindrance. PBDT86-TQ showed the 0-0 absorption peak at similar to 740 nm, and it was blue-shifted to similar to 660 nm for PBDT88-TQ as a result of strong steric hindrance from two dioctyl chains. Further, the HOMO energy levels of PBDT86-TQ (-5.10 eV) and PBDT88-TQ (-5.54 eV) also showed a large difference. The hole mobility of PBDT88-TQ is similar to 70 times higher than that of PBDT86-TQ. The photovoltaic cells based on PBDT86-TQ gave a very low PCE of 0.69-0.80%, and for PBDT88-TQ-based cells, the PCEs were improved to 3.70-4.50% with a V-oc of up to 0.98 V and improved J(sc) and FF. These results show the importance of side chains on the design of conjugated polymers for high performance polymer solar cells. (C) 2015 Elsevier Ltd. All rights reserved.