A combination of reduced graphene oxide (rGO) nanosheets grafted with regioregular poly(3-hexylthiophene) (P3HT) (rGO-g-P3HT) and P3HT-b-polystyrene (PS) block copolymers was utilized to modify the morphology of P3HT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) active layers in photovoltaic devices. Efficiencies greater than 6% were acquired after a mild thermal annealing. To this end, the assembling of P3HT homopolymers and P3HT-b-PS block copolymers onto rGO-g-P3HT nanosheets was investigated, showing that the copolymers were assembled from the P3HT side onto the rGO-g-P3HT nanosheets. Assembling of P3HT-b-PS block copolymers onto the rGO-g-P3HT nanosheets developed the net hole and electron highways for charge transport, thereby in addition to photoluminescence quenching the charge mobility ( h and e ) values increased considerably. The best charge mobilities were acquired for the P3HT 50000 :PC71BM:rGO-g-P3HT 50000 :P3HT 7000 -b-PS 1000 system ( h = 1.9 × 10 −5 cm 2 V -1 s -1 and e = 0.8 × 10 −4 cm 2 V -1 s -1 ). Thermal annealing conducted at 120 ∘ C also further increased the hole and electron mobilities to 9.8 × 10 −4 and 2.7 × 10 −3 cm 2 V -1 s -1 , respectively. The thermal annealing acted as a driving force for better assembly of the P3HT-b-PS copolymers onto the rGO-g-P3HT nanosheets. This phenomenon improved the short circuit current density, fill factor, open circuit voltage and power conversion efficiency parameters from 11.13 mA cm −2 , 0.63 V, 62% and 4.35% to 12.98 mA cm −2 , 0.69 V, 68% and 6.09%, respectively. RESULTS AND DISCUSSION Synthesis P3HT homopolymerFourier transform IR (FTIR) spectra of P3HT-Br and 1 H NMR spectra of P3HT 7000 and P3HT 50000 are shown in Figs S1(a)-S1(c), Polym Int 2019; 68: 1292-1302