Diffusion lengths of silicon solar cells from luminescence images

A method for spatially resolved measurement of the minority carrier diffusion length in silicon wafers and in silicon solar cells is introduced that is based on measuring the ratio of two luminescence images taken with two different spectral filters. Good agreement is observed with the diffusion length distribution obtained from a spectrally resolved light beam induced current (LBIC) map. In contrast to the determination of diffusion lengths from one single luminescence image, the method proposed here gives absolute values of the diffusion length without the need for a separate calibration. It is also much less sensitive to lateral voltage variations across the cell area as caused by local variations of the series resistance. It is shown that measuring the ratio of two luminescence images allows distinguishing shunts or surface defects from bulk defects. The method is applicable in principle to both, photoluminescence and electroluminescence measurements. In this work it is demonstrated experimentally by electroluminescence measurements on a multicrystalline silicon solar cell.