Localization and identification of microscopic defects leading to locally enhanced currents across pn-junctions of solar cells

We investigate the nature of paths of locally enhanced currents in solar cells fabricated on multicrystalline silicon (Baysix/sup R/). Local current-voltage characteristics are determined by means of arrays of mesa diodes etched into the p-n-junction. About 1% of the mesas have a reverse current orders of magnitude higher than the average and low values of open circuit voltage. Electron beam induced current (EBIC)-images show marked spotlike contrasts in these diodes when low voltages (<0.5 V) are applied. Removal of the corresponding areas by ion bombardment improves the characteristics of the diodes and thus proves that a localization of "shunts" on the micrometer scale is possible by this EBIC-technique. Microscopical investigations (we employ transmission and scanning electron as well as atomic force microscopy) suggest that inclusions of a few micrometers in diameter are involved in the formation of "shunts" in the selected mesa diodes.