Single-wall carbon nanotube (SWCNT) films are ideal components for thin, flexible coatings in solar cells and electronic devices. In this context, we will examine the precise role of the nanotube in SWCNT-silicon heterojunctions, with a particular focus on the impact of purity, doping and alignment. Through an analysis of the light and dark transport characteristics of the junctions as a function of bias and temperature, we extract a coarse picture of the width and location of the depletion layer. We also consider specific examples where SWCNT type or chirality plays a critical role in the response. Finally, we will consider some examples of true nanoscale heterojunctions made entirely from colloidal nanomaterials.