Secondary-Phase-Assisted Grain Boundary Migration in ${\mathrm{CuInSe}}_2$

Significant structural evolution occurs during the deposition of ${\mathrm{CuInSe}}_2$ solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the ${\mathrm{CuInSe}}_2$ grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

Figure 1

Structure (a) before and (b) after in situ heating in STEM (extracted from movie 1 in the Supplemental Material [8]) shows that the CIS grains with high-density planar defects (marked by orange dashed ovals) tend to be consumed by the grains without planar defects. GBs migration directions are marked by orange arrows.

Figure 2

Simultaneous STEM imaging and EELS elemental mapping during in situ heating of a $\mathrm{CS}/\mathrm{CIS}$ specimen (extracted from movies 3–6). In the contrast-enhanced images, the white dashed lines mark the original GB position, while the red dashed lines mark its current position. The grain with high-density planar defects became smaller during heating, while the grain on its right side without planar defects grew larger. Cu accumulated and In depleted at the migrating GB between these two grains.

Figure 3

(a) EELS mapping shows Cu enrichment and In depletion at all GBs. (b)–(d) Atomic-resolution $Z$-contrast images show a gradual phase transition from (d) a chalcopyrite phase ${\mathrm{CuInSe}}_2$ inside the grain towards (c) an antifluorite ($Fm\text{−}3m$) phase ${\mathrm{Cu}}_{2-x}\mathrm{Se}$ near the GB. (e) EELS elemental maps confirm this gradual phase change: Se atoms stay in their lattice, while Cu and In interdiffuse.

Figure 4

(a) DFT calculations show that in the presence of $V_{\mathrm{Cu}}$, the diffusion barrier for an In atom (blue ball) to move from one interstitial to another interstitial position is only $\sim 0.38\mathrm{eV}$. The red balls show the Cu sublattice. (b) STEM $Z$-contrast imaging shows bright contrast at dislocations along a small-angle GB at the intermediate CIS phase. (c) Columns with bright contrast appear between the normal $\mathrm{Cu}/\mathrm{In}$ and Se sites, both at and near the dislocation. (d) EELS mapping shows the interstitial columns are In, and Cu depletion occurs in the area, supporting the DFT results of easy formation of $V_{\mathrm{Cu}}+{\mathrm{In}}_i+V_{\mathrm{Cu}}$ clusters.

Figure 5

Sketch showing the GB migration mechanism.