Microstructural investigation of LID sensitive mc-PERC solar cells

Abstract

Light induced degradation can lead to a severe efficiency loss in multi-crystalline PERC solar cells depending on bulk Si material properties and solar cell processing parameters, known as mc-LID or LeTID. Various defect models have been suggested which indicate a clear distinction to BO- and FeB-LID mechanisms. Cu is known to cause light induced degradation and therefore is one of the possible causes of mc-LID in PERC cells. However, until now, a direct microstructural proof of its presence is still missing. In this contribution, we investigate mc-LID sensitive PERC cells which show the typical lateral appearance of mc-LID where structural defects, such as grain boundaries, show a reduced degradation. Investigations of grain boundaries from front and rear side with respect to the recombination activities (LBIC) in correlation to the crystalline structure (XRD Laue mapping) indicate that gettering at grain boundaries reduces degradation. Furthermore, enhanced rear recombination at grain boundaries and scattered local spots of µm size is detected. At these regions with damaged rear passivation, Cu-containing microscopic particles are unveiled by microstructural investigations (SEM) and elemental micro-analysis (EDX). Target preparation (TEM) shows a Cu-filled channel that connects the Cu-containing microscopic particles and the silicon bulk. These observations indicate that the presence, diffusion, and precipitation of Cu might play a role in the mc-LID defect formation.