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

Abstract

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structureThe influence of humidity on the electronic structure of Na-containing polycrystalline Cu͑In,Ga͒Se 2 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, H 2 O, and the Cu͑In,Ga͒Se 2 surface are revealed at low temperatures and upon annealing at room temperature. Both, reversible and irreversible interactions such as a H 2 O-induced reduction of the Na surface content and the formation of a Na-O-Cu͑In,Ga͒Se 2 complex are observed. Our findings can be correlated with the influence of ambient conditions on Cu͑In,Ga͒Se 2 solar cell adsorbers and demonstrate the importance of adequate encapsulation of Na-containing Cu͑In,Ga͒Se 2 -based thin-film solar cells.

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

“…This shift is associated with a decrease in E bin and hence with increasing NaF thickness, we find an upward band bending with respect to the Fermi energy. Our finding is in contrast to the results of Heske et al 6,49 and Klein et al, 50 who report a E bin increase and, consequently, a downward band bending with increasing Na content on the CIGSe surface. One explanation for this variance could be the differing natures of the Na supplies.…”

“…One explanation for this variance could be the differing natures of the Na supplies. While in our case the Na (provided as NaF precursor layer) diffuses through the CIGSe absorber layer forming a very complex chemical surface structure (see discussion in section D), the experiments of Heske et al 6,49 and Klein et al 50 are based on the in situ deposition of (metallic) Na on sputter-cleaned CIGSe samples and UHV-cleaved CuInSe 2 single crystals, respectively.…”

Incorporation of alkali metals in chalcogenide solar cells

“…There are other disputed topics regarding the influence of Na in CIGSe: the promotion of MoSe 2 by Na [38,[49][50][51][52]; the exact location and effect of Na in the grain boundaries or in the bulk [43,53]; and the relation between oxidized compounds with sodium [5,[54][55][56][57]. The reason for these numerous contradictory results is likely connected with the different growth processes used for the deposition of CIGSe.…”

In situ X-ray photoelectron spectroscopy study of the oxidation of CuGaSe2

“…Therefore we attribute this third peak to Na x O. The binding energy of E B = 1072.0eV for the Na1s peak of the as prepared and thermally oxidized samples (see table 4) corresponds to the value reported for an oxygen passivated Na specie at the surface of air exposed Cu(In,Ga)Se 2 thin films (see table 4 and [31]). The formation of Na 2 O cannot directly be concluded from our experiments as we did not measure the Na(KLL) spectrum to determine the Auger parameter and as the energy values for Na1s, Na2s and O1s peak all deviate from the reported values for Na 2 O (see table 4).…”