We demonstrate a photoluminescence based,
contactless method to determine the current-voltage
characteristics of the individual subcells in a multi-junction
solar cell. The method relies upon the reciprocity relation
between the absorption and emission properties on a solar
cell. Laser light with a suitable energy is used to excite
carriers selectively in one junction and the internal voltages
are deduced from the intensity of the resulting
luminescence. The IV curves obtained this way on 1J, 2J
and 6J devices are compared to those obtained using
electroluminescence. Good agreement is obtained at high
injection conditions while discrepancies at low injection are
attributed to in-plane carrier transport.