Cu rich domains and secondary phases in PVD-CdS / PVD-CuIn1−xGaxSe2 heterojunctions

The Cu migration behavior in PVD-CdS/PVD-Cu(In,Ga)Se2 (CIGS) heterojunctions is investigated by high resolution electron microscopy and energy dispersive X-ray spectroscopy (EDS) mapping. The incorporation of Cu into CdS forms Cu-rich domains in the CdS across the heterojunction and has no effect on epitaxy of the CdS film, which is commonly observed in the materials studied. 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, which is most likely cubic Cu2S by lattice spacing measurement from HREM images and EDS elemental quantification. The presence of a binary Cu-S phase as a heterojunction partner material may have significant impact on the resulting device performance although only modest loss of Voc occurs in the devices studied.