Influence of Na and H2O on the surface properties of Cu(In,Ga)Se2 thin films

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Abstract
The influence of humidity on the electronic structure of Na-containing polycrystalline Cu(In,Ga)Se2 thin films on soda-lime glass substrates has been investigated by x-ray and UV photoemission as well as by Auger electron spectroscopy. Different interactions between coadsorbed Na, H2O, and the Cu(In,Ga)Se2 surface are revealed at low temperatures and upon annealing at room temperature. Both, reversible and irreversible interactions such as a H2O-induced reduction of the Na surface content and the formation of a Na–O–Cu(In,Ga)Se2 complex are observed. Our findings can be correlated with the influence of ambient conditions on Cu(In,Ga)Se2 solar cell adsorbers and demonstrate the importance of adequate encapsulation of Na-containing Cu(In,Ga)Se2-based thin-film solar cells.