A rigorous mathematical approach was used to find a relation between the transparent-conductive-oxide(TCO)sheet resistance rho(s)(Omega/square) of a thin-film solarcell and the parameter R (Omega) that describes the TCO resistance in a two-dimensional circuit model. Additionally, the mathematical relationship that connects experimentally derived series resistance R(s)(Omega cm(2)) of the solar cell to the TCO sheet resistance rho(s)(Omega/square) and the bulk semiconductor resistivity rho (Omega cm) was derived. It was found that the fill factor of the solar cell is governed by a reduced dimensionless TCO sheet resistance that depends only weakly on the type and quality of the solar cell. Parameters corresponding to thin-film Cu(In,Ga)Se(2), two-junction a-Si, and an ideal solar cell were used as concrete examples. (C) 2008 Elsevier B.V. All rights reserved,