We report on direct evidence of Cd doping of the CuInGaSe2 (CIGS) surface in physical vapor deposited (PVD) CdS/CIGS heterojunctions by scanning transmission electron microscopy (STEM) and related techniques. We find Cd doping of the CIGS near-surface region regardless of the presence or absence of Cu rich domains in the CdS for both zinc-blende (zb) and wurtzite (wz) CdS. However, we find that the Cd penetrates much farther into the CIGS when Cu-rich domains are present in the CdS. This suggests that Cu exchanges with Cd, increasing the concentration gradient for Cd in the CIGS and thus driving Cd into the CIGS surface. The Cd doping is clearly resolved at atomic resolution in aberration-corrected STEM-high angle annular dark field images. In zb-CdS/CIGS heterojunctions, Cd is shown to substitute for both Cu and Ga atoms, while in wz-CdS/CIGS heterojunctions Cd seems to predominantly occupy Cu sites. Cd doping in the CIGS surface layer suggests the formation of a p-n homojunction in the CIGS, which may account for the high device efficiencies, comparable to CBD-CdS/CIGS processed structures.

Elemental intermixing at the CdS/CuIn 1−x Ga x Se 2 (CIGS) heterojunction in thin-film photovoltaic devices plays a crucial role in carrier separation and thus device efficiency. Using scanning transmission electron microcopy in combination with energy dispersive X-ray mapping, we find that by controlling the oxygen in the sputtering gas during physical vapor deposition (PVD) of the CdS, we can tailor the degree of elemental intermixing. More oxygen suppresses Cu migration from the CIGS into the CdS, while facilitating Zn doping in the CdS from the ZnO transparent contact. Very high oxygen levels induce nanocrystallinity in the CdS, while moderate or no oxygen content can promote complete CdS epitaxy on the CIGS grains. Regions of cubic Cu 2 S phase were observed in the Cu-rich CdCuS when no oxygen is included in the CdS deposition process. In the process-of-record sample (moderate O 2 ) that exhibits the highest solar conversion efficiency, we observe a ~26-nm-thick Cu-deficient CIGS surface counter-doped with the highest Cd concentration among all of the samples. Cd movement into the CIGS was found to be less than 10 nm deep for samples with either high or zero O 2 . The results are consistent with the expectation that Cd doping of the CIGS surface and lack of Zn diffusion into the buffer both enhance device performance.

The Cu migration behavior in PVD-CdS/PVD-Cu(In,Ga)Se2 (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 Cu2S, 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.