The effect of single-walled carbon nanotube (SWNT) incorporation within bulk heterojunction photovoltaic devices based on poly(3-hexylthiophene) − [6,6]-phenyl-C61-butyric acid methyl ester (P3HT−PCBM) (1:1 w/w) active layers was investigated. Both full-length and shortened SWNTs were introduced within the P3HT−PCBM layer at loadings in the range of 0−2 wt%. For full-length SWNTs, it was found that device efficiency decreased at all SWNT loading levels and annealing temperatures, which ranged from 80 to 225 °C. The highest average external efficiencies in the absence of SWNTs reached approximately 2%, while the best efficiencies in devices incorporating the full-length SWNTs only reached 1.3%. When shortened SWNTs were incorporated, device efficiency was unchanged upon annealing at 160 °C (average values of approximately 2%), but the efficiency improved by nearly 50%, relative to controls when devices were annealed at 70 °C. Active layer analysis by grazing incidence X-ray diffraction indicated that nanotubes did not increase polymer crystallinity. Knowing that shortened SWNTs are good hole conductors, it is postulated that the improved device efficiency is due to improved hole transport through the SWNTs in devices where the hole-transporting polymer has not been allowed to adopt its optimal morphology due to underannealing.