Poly(3-hexylthiophene) (P3HT) is a p-type organic semiconductor and is intrinsically a donor material. It is one of the most attractive polymers because of its high electrical conductivity and solubility in various solvents. However, its carrier mobility is considered low when compared to that of inorganic semiconductors. In the current study, it will be shown how the addition of different graphene (G) contents tailors the principal optical parameters of P3HT such as the bandgap, the hole collection, the carrier mobility, and the refractive index and the extinction coefficient. In particular, the hole collection and the carrier mobility are enhanced, and the bandgap reduced with increasing graphene content. The optical constants n (refractive index) and κ (absorption coefficient) of the nanocomposite with 1 wt% G have been fitted to different analitical functions with good agreement between the experimental an the theoretical values. The application of these nanocomposites in a specific organic solar cell (OSC) will be assessed. Finally, the simulation of the photocurrent in this OSC and the comparison with the OSC comprising only pristine P3HT is detailed.