Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe₂Heterojunctions

Abstract

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 nearsurface 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 wzCdS/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.

The role of oxygen doping on elemental intermixing at the PVD‐CdS/Cu (InGa)Se2 heterojunction

“…This could have significant implications for the possibility of collection of photo‐generated carriers from the buffer layer, which until now has not been observed, improving electron transport through the buffer to the transparent contact, as well as potentially reducing interface recombination. Furthermore, we also showed the presence of Cu rich domains in the CdS and the concurrent Cd doping layers on the CIGS surface, suggesting the formation of a p‐n homojunction at the CdS/CIGS interface …”

“…© [2016] IEEE. Reprinted with permission from He et al [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…The high‐resolution TEM image shown in inset is the enlarged version of the yellowed square region, suggesting the single‐crystalline Cu 2 S. © [2016] IEEE. Reprinted with permission from He et al [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…To reduce the Ga ion surface damage and implantation, a small current and voltage (8 pA, 1 kV) were used for the final polishing step. More detailed focused ion beam lift‐out TEM sample preparation parameters can be found in He et al To protect the fresh samples from being oxidized gradually in air, the samples were stored in a special TEM sample preservation capsule that could be evacuated and back filled with dry nitrogen. Immediately before analysis, all of the TEM samples were cleaned using a Fischione Model 1040 NanoMill specimen preparation system to further minimize the Ga‐induced amorphous layer and damage.…”

“…Furthermore, we also showed the presence of Cu rich domains in the CdS and the concurrent Cd doping layers on the CIGS surface, suggesting the formation of a p-n homojunction at the CdS/CIGS interface. 15,16 Absorbed light loss in the CdS window layer is a known limiting factor for further improving the quantum efficiency in the blue light region. An oxygenated CdS thin film deposited via reactive sputtering was proven to be effective in increasing the bandgap of the window layer and thus benefiting the performance of CdS/CdTe solar cells 17,18 and CdS/CIGS solar cells.…”