Spatial variations in the chemical composition of the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface were studied by photoemission electron microscopy (PEEM). Energy filtered PEEM images indicate significant differences in the magnesium and zinc distribution. Local photoemission measurements reveal a relative difference in the derived Mg/(Zn+Mg) composition of the (Zn,Mg)O material of up to (11.4 ± 0.7)%, which can be expected to induce band gap fluctuations of (60 ± 30) meV. Furthermore, local areas with significant accumulations of sodium could be observed.
In order to estimate the impact of the Mg/(Zn+Mg) fluctuations on the (Zn,Mg)O band gap (Eg), we fitted the z = Mg/(Zn+Mg) dependent band gap values reported in Refs. 30 and 31 with a polynomial 2nd order in the given 0 ≤ z ≤ 0.45 range: Eg(z) [eV] = 3.31 +3.11 × z – 2.13 × z2. Assuming that the computed Eg(z) relation is also valid for higher Mg contents resulting in single-phase (Zn,Mg)O, the observed differences in the local XPS derived Mg/(Zn+Mg) compositions have been translated into a band gap variation.