In the present work, the effect of incorporation of graphene on the poly(3-hexylthiophene) quantum dot (CIS QD)-based solar cell has been studied. For this purpose, the concentration of graphene is varied from 0 to 0.01% w/w in P3HT–CIS (1:0.5) film. It is found that graphene does not deteriorate the absorption of the composite film. It assists in dissociating the photogenerated excitons (both in P3HT and QDs) owing to its two-dimensional structure and high electron affinity as is evident by photoluminescence (PL) quenching. At 0.01% w/w concentration of graphene about of PL is quenched. The electrical characteristics show that the incorporation of graphene enhances the efficiency of the device by establishing interconnected conducting pathways in the volume of polymer matrix. The maximum efficiency is observed to be 1.5% at 0.005% w/w content of graphene. However, at higher concentration, i.e., 0.01% w/w, the device starts deteriorating.