Microstructural identification of Cu in solar cells sensitive to light-induced degradation

Light-induced degradation (mc-LID or LeTID) can lead to a severe efficiency loss in multi-crystalline solar cells. The underlying mechanism clearly distinguishes from known mechanisms as B-O-LID and Fe-B-LID. Various defect models have been suggested for mc-LID mainly based on metal impurities, including Cu which is known to cause light-induced degradation. We investigate mc-LID sensitive PERC cells that show an efficiency degradation of 15% rel. The weaker degradation of the grain boundaries (GBs) typical for mc-LID is identified and further investigated from front and rear side with respect to recombination activities. The combination of local electrical measurements (LBIC), target preparation (REM, FIB) and element analysis (EDX, TEM) unveil Cu-containing precipitates at the rear side of the solar cells. They accumulate at grain boundaries and at the rear surface of the Si-bulk material where the passivation stack is damaged. We conclude that Cu originates from the cell material and discuss its relation to mc-LID. [GRAPHICS] LBIC mapping (EQE at fixed wavelength) of a degraded mcSi PERC cell from front and rear side results in qualitatively different appearance of GBs.