Organic solar cells were fabricated with varying amounts of ZnO-NPs in a buffer layer located over an active layer of P3HT/PCBM incorporating a fixed amount of CuO nanoparticles. The buffer layer serves as an electron transporting layer in the device. Thermal annealing treatment was applied to all the devices at different temperatures (150  o C, 200  o C and 250  o C) to optimize the nanoscale morphology. The samples which were annealed at 200  o C exhibited the best power conversion performance. The enhanced morphological and optoelectronic properties attained by applying thermal annealing increased the power conversion efficiency by 14.6% compared to a reference cell. The ZnO-NPs buffer layer improved the exciton dissociation rate, electron mobility, optical absorption and charge collection at the anode, resulting in higher short circuit currents and external quantum efficiencies. The short circuit current (Jsc) of the optimum device was measured at 8.949 mA/cm 2 compared to 7.62 mA/cm 2 in the reference cell before annealing. Meanwhile, the external quantum efficiency (EQE) increased from 61.8% to 62.9%, after thermal annealing.