The study investigates charge transport and photocurrent generation in poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) bulk-heterojunction solar cells. The effect of thermal annealing on the performance of these devices is analyzed. The results show that thermal annealing significantly improves the charge transport properties of the P3HT:PCBM blends. The electron mobility in the P3HT phase of the blend increases dramatically after annealing, while the hole mobility in the P3HT phase increases by more than three orders of magnitude. This improvement in hole mobility leads to a significant increase in the short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (η) of the solar cells. The study also shows that the absorption spectrum of the P3HT:PCBM blends undergoes a red-shift after annealing, which improves the spectral overlap with solar emission and increases the rate of charge-carrier generation. The results indicate that the most important factor leading to the enhancement of the efficiency is the increase in hole mobility in the P3HT phase of the blend. Numerical simulations show that the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90% under short-circuit conditions, which explains the large quantum efficiencies measured in P3HT:PCBM blends. The study also shows that the performance of the solar cells is strongly limited by the buildup of space-charge in the device when the difference in electron and hole mobility is too large. However, at optimal annealing temperatures above 110°C, the difference in electron and hole mobility is reduced to a factor of 20, leading to a more balanced transport and higher fill factors. The results demonstrate that thermal annealing significantly improves the performance of P3HT:PCBM solar cells by enhancing the hole mobility and reducing the space-charge limitation. The study also shows that the P3HT-based devices have similar charge transport properties to MDMO-PPV:PCBM devices, but with a higher maximum rate of charge-carrier generation and a higher fill factor. The power conversion efficiency of P3HT:PCBM cells is significantly higher than that of MDMO-PPV:PCBM cells due to the increased current and fill factor, despite the lower open-circuit voltage.The study investigates charge transport and photocurrent generation in poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) bulk-heterojunction solar cells. The effect of thermal annealing on the performance of these devices is analyzed. The results show that thermal annealing significantly improves the charge transport properties of the P3HT:PCBM blends. The electron mobility in the P3HT phase of the blend increases dramatically after annealing, while the hole mobility in the P3HT phase increases by more than three orders of magnitude. This improvement in hole mobility leads to a significant increase in the short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (η) of the solar cells. The study also shows that the absorption spectrum of the P3HT:PCBM blends undergoes a red-shift after annealing, which improves the spectral overlap with solar emission and increases the rate of charge-carrier generation. The results indicate that the most important factor leading to the enhancement of the efficiency is the increase in hole mobility in the P3HT phase of the blend. Numerical simulations show that the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90% under short-circuit conditions, which explains the large quantum efficiencies measured in P3HT:PCBM blends. The study also shows that the performance of the solar cells is strongly limited by the buildup of space-charge in the device when the difference in electron and hole mobility is too large. However, at optimal annealing temperatures above 110°C, the difference in electron and hole mobility is reduced to a factor of 20, leading to a more balanced transport and higher fill factors. The results demonstrate that thermal annealing significantly improves the performance of P3HT:PCBM solar cells by enhancing the hole mobility and reducing the space-charge limitation. The study also shows that the P3HT-based devices have similar charge transport properties to MDMO-PPV:PCBM devices, but with a higher maximum rate of charge-carrier generation and a higher fill factor. The power conversion efficiency of P3HT:PCBM cells is significantly higher than that of MDMO-PPV:PCBM cells due to the increased current and fill factor, despite the lower open-circuit voltage.