In this paper the effect of an in depth variation of the Ga/(In+Ga) ratio in GIGS based thin film solar cells is presented. The conclusions made are based on a review of earlier publications, theoretical considerations and results from a large set of new devices. For standard devices with normally thick CIGS films (1.5-2 mu m) deposited at a relatively long deposition time (60 min) an improved efficiency of around 0.4% units for the devices with an increased Ga/(In+Ga) ratio towards the back contact is observed. This improvement is due to a field assisted carrier collection resulting in an improved QE response at long wavelengths. When the CIGS thickness is reduced the importance of the increased Ga/(In+Ga) ratio towards the back contact is enhanced and at a GIGS thickness of 0.5 mu m a gain of 2.5% units is obtained. The gain is due to an improved V proportional to and FF. The main reason for the improvement is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. Also for pure CIS a significant improvement of the device performance is obtained by introducing an increased Ga concentration towards the back contact. This improvement is, however, more related to the introduction of Ga itself than the gradient of the Ga-concentration. From many simulations the largest gain is predicted for an increased Ga/(In+Ga) ratio towards the GIGS surface. However, neither in the literature nor from our own experiments we can find evidence for an improved device performance due to an increased Ga-concentration towards the GIGS surface. (c) 2004 Elsevier B.V. All rights reserved.