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By introducing single-walled carbon nanotubes (SWNTs) into bulk heterojunction (BHJ) organic solar cells made from blended films of regioregular poly (3-hexylthiophene) (P3HT) and methanofullerene (PCBM), the short circuit current of these devices has been increased up to 14% compared with the device without SWNTs. It is surmised that the increased short circuit current in P3HT:PCBM:SWNT devices is attributed to the increased carrier mobility of the active layer as the semiconducting SWNTs provide lower resistance pathways for carriers directly to the electrodes in contrast to the normal hopping conduction in P3HT:PCBM devices. We also observed the open circuit voltage has increased from 0.69 V in P3HT: PCBM devices up to 0.74 V in P3HT:PCBM: SWNT devices. The increased open circuit voltage might be attributed to the lower effective high occupied molecular orbital (HOMO) by introducing SWNTs. In this study, SWNTs have been shown to be excellent additives to enhance the performance of plastic bulk-heterojunction solar cells. However, we also observe the detrimental effect of the presence of metallic CNTs that causes hole-electron recombination, and as a result degrades the device performance. This calls for a critical need to enrich the semiconducting ones in bulk CNTs either from fabrication or from post-fabrication process.