Theoretical modelling of suns-V-oc characteristics of silicon solar cells in extreme conditions of surface recombination and illumination intensity predicts departures from the standard I-V characteristic curves. These departures result in ideality factors that can be either higher or lower than unity, depending on the conditions. Analytical proof of such non-idealities is also given in the paper, showing that they are due to the finite diffusivity of carriers towards the surfaces. (C) 2008 Elsevier B.V. All rights reserved.