Stability of CIGS solar cells under illumination with damp heat and dry heat: A comparison

Unencapsulated CIGS solar cells were simultaneously exposed to either dry or damp heat combined illumination. In-situ monitoring of their electrical parameters demonstrated a rapid decrease of the efficiency for the solar cells exposed to damp heat plus illumination. This decrease was mainly driven by changes in the shunt resistances, affecting also the open circuit voltage and fill factor, while a minor increase in series resistance was also observed, The solar cells exposed to dry heat plus illumination were almost stable during the exposure. The solar cells were studied before and after exposure in order to investigate their material changes. For the dry heat illumination samples, the visual characteristics remained unchanged, while for the damp heat illumination samples, spots that were likely shunted, were observed. For these solar cells, SIMS showed the migration of sodium, likely leading to changes in the shunt resistance and output voltage. A similar, but less severe migration effect was observed for potassium, so this element might also have a small, but comparable role. Migration of hydroxide, likely caused by water ingression, was also observed and probably affected the conductivity of the ZnO:Al front contact. The migration of sodium and hydroxide was especially strong on the spot locations. These changes were not observed for the dry heat illumination samples. From these experiments, it was concluded that unencapsulated CIGS solar cells can degrade rapidly in the presence of humidity, but in the absence of water, these solar cells were stable even under high temperatures and illumination intensities.