Cu-accumulation at the interface between sputter-(Zn,Mg)O and Cu(In,Ga)(S,Se)2 - A key to understanding the need for buffer layers?

Zn0.85Mg0.15O buffer layers can replace both i-ZnO and CdS in n-ZnO/i-ZnO/CdS/Cu(In xGa1-x)(S ySe1-y)2/Mo/glass (CIGSSe) solar cells without significant loss of efficiency. We found that the efficiency of Zn0.85Mg0.15O buffered solar cells decreased with increasing sample temperature when we sputter-deposited Zn0.85Mg0.15O directly onto the CIGSSe absorber surface, e.g. from 9.5% without deliberate heating to 6.5% at 240 deg. C. To find an explanation for this behavior we sputter-coated bare, KCN-etched CIGSSe absorbers with about 420 nm of Zn0.85Mg0.15O at different sample temperatures and subsequently removed the Zn0.85Mg0.15O layers by wet-chemical etching with dilute acetic acid. The exposed CIGSSe surfaces were examined by surface-sensitive X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). We found a strong increase in the [Cu]/([In] + [Cu]) ratio compared to a bare, Acetic-acid-etched CIGSSe reference. The surface of samples that had been sputter-coated at 150 deg. C changed from being initially Cu-poor to Cu-rich. The chemical shift of Cu Auger peaks from the same surface confirmed this finding. The increased Cu/In ratio and the chemical shift were reversed after KCN etch. These findings are discussed in the context of the model of a Cu-depleted, wide-band gap surface region in CIGSSe solar cells as a prerequisite for high efficiency