For heterojunction solar cells with a spike in one of the bands at the junction, intra-band tunneling will enhance the current through the interface. We have incorporated this phenomenon in SCAPs, a publicly available one-dimensional solar cell device simulator. The thermionic-field emission boundary conditions at the interfaces are formulated based on the WKB approximation and we discuss the changes to the equations used in our model. We have taken care to make our model self consistent and point out differences with previous attempts to incorporate intra-band tunneling in a numerical device simulator. We show new simulations on our model of the Cu-In-S on Cu-tape (CISCuT) solar cell. At the interface between the CuI buffer layer and the CulnS(2) absorber a spike is present in the valence band. We perform simulations with and without the inclusion of intra-band tunneling and conclude that the effect of the spike on the current transport properties of the CuI/CuInS2 interface poses no limitations on cell efficiency. (C) 2006 Elsevier B.V. All rights reserved.