Doping improves performance. N- or B-doped carbon nanotubes (CNTs) uniformly dispersed in the active layer of P3HT/PCMB (poly (3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester) bulk-heterojunction solar cells selectively enhance electron or hole transport and eventually help carrier collection. Specifically, the incorporation of 1.0 wt% B-doped CNTs results in balanced electron and hole transport and accomplishes a power conversion efficiency improvement from 3.0% (without CNTs) to 4.1%.