A new method of diffusion length measurement in n +-p +-p + structure-based silicon solar cells is presented. This method exploits the strong dependence of the collection efficiency of the p-n junction on both the thickness d and the diffusion length L of the minority carriers in the p-base region when the p-n junction is separated from the illuminated surface by the base region. The cell is illuminated from the p + side with monochromatic radiation and the variation in the short-circuit current density J sc with the intensity P in is measured. The diffusion length is determined from the slope of the J sc versus P in curve. The validity of this method is not limited to the cases of d/L >1 and, in practice, it may be suitabale for cells with d/L > 0.6. In the present case it was used to determine L in n +-p-p + silicon solar cells with bifacial symmetry. These cells had d/L > 2.5 and the values of the L determined by the present method showed a remarkable agreement with those determined from the photoresponse measurement for illumination from the n + side.