One reason why that CIGS thin-film solar cell achieved
high conversion efficiency includes sodium (Na) effect. Na
incorporated into CIGS absorbing layer is well known to
have widely valuable effects that lead to enhanced CIGS
thin-film solar cell efficiency. The some positive influences
of Na on CIGS polycrystalline thin-film have been claimed
from previous reports. Na passivates or removes the grain
boundary defects such as In antisite on Cu (InCu)
5 and Se
vacancy (VSe).6 The oxygen model was suggested that the
hole density increases due to the neutralization of donor VSe
defects through an enhanced chemisorption of oxygen atom
in the presence of Na.6