Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe2 Heterojunctions

The Cu migration behavior in PVD-CdS/PVD-Cu(In,Ga)Se
₂
(CIGS) heterojunctions is investigated by high-resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS). Incorporation of Cu into the CdS forms Cu-rich domains but has no effect on epitaxy of the CdS. Epitaxy is commonly observed in the CdS studied. Secondary ion mass spectroscopy depth profiles confirm the presence of Cu in the CdS. In some cases, Cd is completely replaced by Cu, resulting in a Cu-S binary compound epitaxially grown on the CIGS and fully coherent with the surrounding CdS. This is most likely to be cubic Cu
₂
S, based on lattice spacing measurements from HREM images and EDS elemental quantification. In addition, we find that the buffer layer crystal structure influences the extent of Ga depletion at the CIGS surface, which is more pronounced adjacent to zinc-blende CdS than wurtzite CdS. Density functional theory calculations reveal that Cu clustering and different Ga depletion widths can be attributed to the inherent anisotropy of wurtzite CdS and differences in CIGS point-defect migration barriers. Understanding the influence of these effects on device properties is a critical step in developing more efficient CdS/CIGS-based photovoltaics.