The efficiency of a solar cell is given by its average electrical parameters. On inhomogeneous materials and especially on large-area solar cells the inhomogeneity of the short circuit current, the open circuit voltage and the fill factor are important factors to reach high and stable efficiencies and may limit the overall performance of the device. A locally increased dark forward current (shunt) reduces the fill factor and the open circuit voltage of the whole cell. The inhomogeneity of the forward current in a solar cell can be measured using lock-in thermography. The quantitative and voltage-dependent evaluation of these thermographic investigations of various solar cell types on mono- or multi-crystalline silicon enables the classification of the different shunting mechanisms found. By further microscopic investigations the physical reasons for the increased dark forward currents can be determined. It turns out that a high density of crystallographic defects like dislocation tangles or microdefects can be responsible for an increased dark forward current. Unexpectedly, grain boundaries in solar cells on multicrystalline silicon do not show any measurable influence on the local dark forward current, In most cases shunts caused by process-induced defects are dominating the current-voltage characteristic at the maximum power point of the solar cell. In commercial solar cells shunts at the edges are most important, followed by shunts beyond the grid lines. (C) 2002 Elsevier Science B.V. All rights reserved.