The effects of the incorporation of semiconducting single-walled nanotubes (sc-SWNTs) with high purity on the bulk heterojunction (BHJ) organic solar cell (OSC) based on regioregular poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C-61-butyric acid methyl ester (rr-P3HT:PCBM) are reported for the first time. The sc-SWNTs induce the organization of the polymer phase, which is evident from the increase in crystallite size, the red-shifted absorption characteristics and the enhanced hole mobility. By incorporating sc-SWNTs, OSC with a power conversion efficiency (PCE) as high as 4% can be achieved, which is approximate to 8% higher than our best control device. A novel application of sc-SWNTs in improving the thermal stability of BHJ OSCs is also demonstrated. After heating at 150 degrees C for 9 h, it is observed that the thermal stability of rr-P3HT: PCBM devices improves by more than fivefold with inclusion of sc-SWNTs. The thermal stability enhancement is attributed to a more suppressed phase separation, as shown by the remarkable decrease in the formation of sizeable crystals, which in turn can be the outcome of a more controlled crystallization of the blend materials on the nanotubes.