A study was conducted to demonstrate that treating poly(3-bexylthiophene) (P3HT) with dimethylsulfate improves its photoelectrical properties. Solutions of P3HT in chlorobenzene were reacted with Me2SO4 for 1 hour at room temperature before films were prepared from it through spin-coating of drop casting. The films were also annealed at 120°C for 30 minutes in a glovebox and kept in vacuum for at least 90 minutes before characterizing. The study demonstrated that the treatment increases the chemical stability of the polymer toward photo-oxidation and increases the hole mobility and exciton diffusion length. It was also observed that the exciton diffusion length determines the distance over which light can be harvested in the film.