We propose a first principles method of calculating the photo-component of tunneling conductivity of a specific superlattice (SL) photovoltaic structure: an InP/InGaAs p-i(SL)-n solar cell. The method we follow is based on the generalized Greenwood-Kubo formulation and on the causal form of the Green's function. The vertical conductivity (along the growth direction of the device). under illumination, involves a group of parameters directly connected to the geometry of the device (hence it is on one's disposal) and the quantum size effects in the quantum wells of the superlattice. The latter is incorporated in the intrinsic region of p-i-n solar cells and bring along the advantage of carrier tunneling, which is possible if thin layers of wide-gap material (InP at 25 A/layer) are grown successively on narrow band-gap material (lattice-matched In0.53Ga0.47As at 7 nm/layer) to form a sequence of quantum wells. (C) 2004 Elsevier B.V. All rights reserved.