The electronic properties of multicrystalline solar silicon materials are dominated by low-lifetime defect regions containing recombination-active grain boundaries and dislocations. Besides reducing the carrier collection probability, these regions increase the dark saturation current density J(01), which governs the open circuit voltage. By applying lock-in thermography with spatial deconvolution it is shown that the dominant contribution to J(01) comes from recombination-active grain boundaries and to a lower degree from intra-grain defects like dislocations. (C) 2012 Elsevier B.V. All rights reserved.