Semi-quantitative and non-destructive analysis of impurities at a buried interface: Na and the CdS/Cu(In,Ga)Se-2 heterojunction

We demonstrate how a combination of photoelectron spectroscopy and x-ray em ission spectroscopy can be utilized to derive semi-quantitative information about the localization of impurities at buried interfaces. In the case of the CdS/Cu(In,Ga)Se-2 (CIGS) thin-film solar cell heterojunction, segregate d Na, which stems from the soda-lime glass substrate or is deliberately add ed, plays an important role. We find that almost all Na atoms are located a t the external CIGS surface or at the CdS/CIGS interface, and that the Na c oncentration in the bulk of the CIGS film is <1 ppm, Moreover, we show that the Na surface coverage at internal CIGS surfaces is significantly lower t han at the external CIGS surface or CdS/CIGS interface, which demonstrates that the internal surfaces may not be regarded merely as a special case of the external surface. Copyright (C) 2000 John Wiley & Sons, Ltd.