Grain boundary investigations on sulfurized Cu(In,Ga)(S,Se)2 solar cells using atom probe tomography

In this study grain boundaries (GBs) and grain interiors in a sulfurized Cu(In,Ga)(S,Se)2 (CIGSSe) photovoltaic thin film have been investigated by atom probe tomography. Grain boundaries could be clearly localized by the strong agglomeration of sodium, which was additionally observed in tube-shaped clusters. These GBs were proven to contain no oxygen or alkali metals which confirms the blocking function of the used diffusion barrier sputtered on the soda lime glass substrate. Further, the concentrations of the CIGSSe matrix atoms across the GBs were studied. Here, copper deficiency and enrichment appear to be correlated with the distance from the back contact (BC). Agglomeration of sulfur in all grain boundaries near to the BC indicates interface diffusion of sulfur. Moreover, our measurements reveal the existence of a thin layer upon the back contact in which the sulfur, copper and gallium contents are significantly increased. The corresponding band-gap widening may establish the function of minority carrier repulsion from the back contact.