In this paper we analyze the system proposed by Trupke T, Green MA, Wurfel P. [improving solar cell efficiencies by up-conversion of sub-band-gap light.J Appl Phys 2002: 92: 4117-22] to increase solar cells efficiency. The system consists in adding to the cell a so-called up-converter, which is a device able to convert the low-energy (sub-band gap) incident solar photons into photons of higher energy. The main novelty of the present model consists in taking into account appropriately the refractive index of solar cell and converter materials. Two configurations are studied: cell and rear converter (C-RC) and front converter and cell (FC-C), respectively. The main conclusions of this work are as follows: (1) The maximum solar energy conversion efficiency increases in case of the C-RC system as compared to the efficiency of a solar cell operating alone, especially at higher values of the concentration ratio; (2) the solar energy conversion efficiency of the C-RC system increases by increasing both the cell and the up-converter refractive indices; (3) the energy conversion efficiency does not increase by adding a front up-converter to the cell, whatever the value of the concentration ratio is. (c) 2008 Elsevier Ltd. All rights reserved.