A three-dimensional (3D) analytical model based on the Green's Function method, is applied on n(+)pp(+) type epitaxial solar illuminated on the front side. This model is implemented through a simulation program in order to determine the device parameters, which optimize the cell's efficiency. A number of cells fabricated on low-cost UMG substrate have been studied and their spectral response is evaluated. The basic design parameter selected for optimization of these cells is the epitaxial layer thickness, through variation of grain size and grain boundary recombination velocity. In addition, 3D spectral response results are compared with experimental data and the 1D model. (C) 2005 Elsevier B.V. All rights reserved.