Grain-Boundary Physics in Polycrystalline ${\mathrm{CuInSe}}_2$ Revisited: Experiment and Theory

Current studies have attributed the remarkable performance of polycrystalline ${\mathrm{CuInSe}}_2$ (CIS) to anomalous grain-boundary (GB) physics in CIS. The recent theory predicts that GBs in CIS are hole barriers, which prevent GB electrons from recombining. We examine the atomic structure and chemical composition of (112) GBs in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS) using high-resolution $Z$-contrast imaging and nanoprobe x-ray energy-dispersive spectroscopy. We show that the theoretically predicted Cu-vacancy rows are not observed in (112) GBs in CIGS. Our first-principles modeling further reveals that the (112) GBs in CIS do not act as hole barriers. Our results suggest that the superior performance of polycrystalline CIS should not be explained solely by the GB behaviors.

Figure 1

Crystal structure of CIS with a (a) perspective and (b) $[1\overline{1}0]$ zone axis-projection view. The large (green and blue online) balls are In and Cu atoms, and the small (red online) balls are Se atoms. (c) High-resolution $Z$-contrast image of a (112) GB in CIGS thin film.

Figure 2

(a) High-resolution $Z$-contrast image showing nanodomains with chemical compositional fluctuation in GIs around a (112) GB in CIGS thin film. (b) Measured intensity line profile from the dotted (green online) box shown in (a).

Figure 3

Atomic structures of the (a) (112) GB CIS determined directly from $Z$-contrast image, (b) basal-plane stacking faults, and (c) perfect CIS single crystal.

Figure 4

Plane-averaged charge densities of CBM states and VBM states at the $\Gamma$ point for the supercells contain (a) a perfect CIS single crystal, (b) (112) GB, and (c) stacking faults. The thick and thin bar lines indicate the positions of the mixed In-Cu and the Se planes, respectively.