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Investigation on the thickness dependent analysis clearly demonstrates that the slow carrier mobility is the main reason limiting the power conversion efficiency of conjugated polymer/fullerene organic solar cells. In order to increase the carrier mobility, single-walled carbon nanotubes (SWCNTs) are introduced into organic solar cells made from blended films of regioregular poly 3-hexylthiophene (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM). It is observed that the power conversion efficiency of the device with low concentration of SWCNTs has been increased significantly compared with the device without SWCNTs. The increased performance is attributed to the presence of semiconducting SWCNTs which provides ballistic pathways for carriers to be extracted and hence boosts carrier mobility in organic solar cells. However, if the concentration of SWCNTs increased up to certain degree, the performance of the device decreased. By both theoretical and experimental analysis on the intensity dependent photocurrent, the decreased performance is attributed to the metallic SWCNTs which cause bimolecular recombination of free carriers.