Transitions via impurities, in addition to transitions from the valence band to the conduction band, increase the generation rate of electron-hole pairs. By the presence of impurities, however, the recombination rate is also increased. in this study, the recombination process is assumed to be entirely radiative. The probability for photon absorption in generation and for photon emission in recombination depend on the occupation probability of the impurity states. The variation of the occupation resulting in a variation of the optical properties along a current-voltage characteristic is taken into account. With a single impurity state a maximal efficiency of 46% for the AM0 spectrum is found for a band gap of 2.3 eV and an impurity level at 0.88 eV. No improvement is found for band gaps less than 1.2 eV. For two different impurity levels, transitions between these levels must be excluded, otherwise the lower level is mostly occupied and the higher level is mostly empty leading to poor optical absorption for some of the possible impurity transitions. (C) 2002 Elsevier Science B.V. All rights reserved.