The distribution of impurities in the interfaces and window layers of thin-film solar cells

We report a systematic multielement study of impurities in CdS window layers by dynamic and quantitative secondary-ion-mass spectrometry (SIMS) with high depth resolution. The study was carried out on CdTe∕CdS solar cell structures, with the glass substrate removed. The analysis proceeded from the transparent conductive oxide free surface to the CdTe absorbing layer with a view to examining the influence of the ${\mathrm{CdCl}}_2$ heat treatment on the distribution and concentration of impurities in the structures. Special attention was paid to the impurities present in the CdS window layer that may be electrically active, and therefore affect the characteristics of the CdTe∕CdS device. It was shown that Cl, Na, and Sb impurities had higher concentrations in CdS following cadmium chloride $\left({\mathrm{CdCl}}_2\right)$ heat treatment while Pb, O, Sn, and Cu conserved the same concentration. Furthermore, Zn, Si, and In showed slightly lower concentrations on ${\mathrm{CdCl}}_2$ treatment. Possible explanations of these changes are discussed and the results compared with previous SIMS measurements from the “back wall” (i.e., from the CdTe free surface through the glass substrate) obtained from the same structures.