The study investigates the impact of thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM). The electron mobility in the as-cast devices is low, while the hole mobility is significantly affected by thermal annealing, increasing by more than three orders of magnitude. Annealing also causes a red-shift in the absorption spectrum of P3HT:PCBM blends, improving spectral overlap with solar emission and increasing the rate of charge-carrier generation by over 60%. The enhanced hole mobility is the primary factor leading to a significant increase in short-circuit current (JSC) and fill factor (FF), resulting in a tenfold increase in power conversion efficiency (η). Numerical simulations indicate that the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, explaining the high quantum efficiencies observed in P3HT:PCBM blends. These findings are valuable for the design and improvement of organic photovoltaic devices.The study investigates the impact of thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM). The electron mobility in the as-cast devices is low, while the hole mobility is significantly affected by thermal annealing, increasing by more than three orders of magnitude. Annealing also causes a red-shift in the absorption spectrum of P3HT:PCBM blends, improving spectral overlap with solar emission and increasing the rate of charge-carrier generation by over 60%. The enhanced hole mobility is the primary factor leading to a significant increase in short-circuit current (JSC) and fill factor (FF), resulting in a tenfold increase in power conversion efficiency (η). Numerical simulations indicate that the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, explaining the high quantum efficiencies observed in P3HT:PCBM blends. These findings are valuable for the design and improvement of organic photovoltaic devices.