The present study demonstrates the effect on photovoltaic performance of regioregular poly (3-hexylthiophene)(rr-P3HT) grafted oxide graphene (GO) on in situ doping of cadmium sulfide (CdS) quantum dots (QDs) (GO-rr-P3HT-CdS). Firstly, CH2OH-terminated rr-P3HT of different molecular weights (MW) and distributions synthesized by Grignard metathesis (GRIM) method was grafted onto carboxylic groups of GO via esterification reaction. Then, the GO-rr-P3HT-CdS was prepared with an oleylamine solution composed of 1 mmol of cadmium and 1 mmol high pure sulfur in the presence of GO-rr-P3HT. The covalent linkage and the strong electronic interaction between the rr-P3HT and graphene moieties in GO-rr-P3HT were confirmed by spectroscopic analyses (XPS and FTIR). The photovoltaic properties of as prepared nanocomposites are evaluated by UV-Vis spectroscopy, photoluminescence, and electrochemical measurements, and are found to be strongly affected by MW, which influences the behavior of the bulk heterojunction organic solar cells based on this material. It is found that the optical and electrochemical properties of the resultant GO-rr-P3HT-CdS nanocomposite are relatively better than that of conventional composite, in which the pristine graphene, CdS, and rr-P3HT (or GO-rr-P3HT) are physically mixed together. The significant PL quenching is attributed to additional decaying paths of the excited electrons through the CdS. Bulk heterojunction photovoltaic devices with a thin film of GO-rr-P3HT-CdS containing higher molecular weights of rr-P3HT show an increase in the power conversion efficiency by about three times with respect to their counterparts based on GO-rr-P3HT/CdS and GO/rr-P3HT/CdS due to improvement in contact between GO-rr-P3HT and CdS and enhancement in the device parameters like fill factor and open-circuit voltage (V-OC).