The performance of organic solar cells is substantially enhanced by introducing high-mobility conjugate polymers as additives for the first time. The most pronounced effect is observed in one type of device with the average power conversion efficiencies increased from 8.75% to 10.08% after the addition of a high-mobility polymer for only 0.5 weight percent in the active layers, which is mainly attributed to the increased hole mobility and carrier lifetime in the devices. Besides the hole mobility, the energy band structure of the additive is also found to be critical to the enhancement of the device performance. This work demonstrates a novel approach for improving the efficiencies of organic solar cells. ; Department of Applied Physics ; Department of Electronic and Information Engineering