The performance of polymer solar cells is strongly dependent on the morphology of the photoactive layer, which can be optimized by tuning the polymer side chain pattern. Whereas most studies focus on length and bulkiness, the side chain density receives much less attention. In this work, the effect of the number of side chains on PCPDTQx(2F) low bandgap copolymers on material properties and solar cell characteristics is investigated. The active layer morphology is strongly affected, affording more favorable finely intermixed blends when decreasing the side chain density. As a result, the efficiency increases to a maximum of 5.63% for the device based on the copolymer with intermediate side chain density. Moreover, removal of the side chains also has a positive effect on device stability under prolonged thermal stress. A single structural parameter—alkyl side chain density—is hence used for simultaneous enhancement of both solar cell efficiency and lifetime.