In this work, we propose a novel photovoltaic device constructed from the junction of two carbon nanotubes, where the top of the valence band is mainly localized on one of the nanotubes' sides, while the bottom of the conduction one is mainly localized on the other side. This allows the separation of electrons on the side displaying the lower conduction band, from the holes, located at the other side. On the basis of self-consistent charge-density functional tightbinding calculations, we propose that this kind of device can be formed by joining two consecutive semiconducting zigzag carbon nanotubes. We have shown that, for these systems, the existence of the required band energy differences is related to the kind of variations in energy gap with the tube diameters, observed for the zigzag nanotubes. To illustrate the formation of the proposed junction, we have chosen to study a (10,0) tube joined linearly to a (11,0) one through a single pentagon-heptagon defect.