The mechanism of action of nanotube-silicon heterojunction solar cells is under discussion with literature reports suggesting either p-n or Schottky junction characteristics. The crux of the issue is whether the nanotubes contribute to the observed photocurrent or not. In order to further understand the mechanism of action of these solar cells, devices were fabricated using nanotubes sorted by (n,m) species, so that the excitonic transition is well defined and is outside the range of absorption of silicon and such that any contribution to the photocurrent from the nanotubes should be easily resolved from that of the silicon by analysis of the photocurrent spectrum. The devices exhibited the photocurrent spectra of silicon only, indicating that the nanotubes do not contribute to the photocurrent. However, by changing the back contact electrode material, results were obtained that appear to show such a contribution. Carbon nanotube-silicon solar cells have recently been reported with 15% conversion efficiency; however, the mechanism of action of such devices is still uncertain. The role of the nanotubes in the carbon nanotube-silicon design is examined, using nanotube material highly enriched in small or large diameter semiconducting nanotubes, yielding unexpected results.