Point defect segregation and its role in the detrimental nature of Frank partials in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ thin-film absorbers

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ was studied by aberration-corrected scanning transmission electron microscopy in combination with electron energy-loss spectroscopy and calculations based on density-functional theory. We find that Cu accumulation occurs outside of the dislocation cores bounding the stacking fault due to strain-induced preferential formation of ${\mathrm{Cu}}_{\mathrm{In}}^{-2}$, which can be considered a harmful hole trap in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$. In the core region of the cation-containing $\alpha$-core, Cu is found in excess. The calculations reveal that this is because Cu on In-sites is lowering the energy of this dislocation core. Within the Se-containing $\beta$-core, in contrast, only a small excess of Cu is observed, which is explained by the fact that ${\mathrm{Cu}}_{\mathrm{In}}$ and ${\mathrm{Cu}}_{\mathrm{i}}$ are the preferred defects inside this core, but their formation energies are positive. The decoration of both cores induces deep defect states, which enhance nonradiative recombination. Thus, the annihilation of Frank loops during the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ growth is essential in order to obtain absorbers with high conversion efficiencies.

Figure 1

(a) HAADF image of the positive Frank partial dislocations associated with an extrinsic SF. Two yellow boxes indicate the top and bottom parts of the SF including partial dislocation cores $(\beta$-core in the upper box and $\alpha$-core in the bottom one). Relaxed structure of an extrinsic Frank loop in CIS obtained with DFT. (b) Complete supercell showing the simulated loop, (c) $\beta$-core, and (d) $\alpha$-core.

Figure 2

(a) Two simultaneously acquired HAADF images from the areas indicated by yellow boxes in Fig. 1 show the association of the SF and the Frank loop $(\beta$-core in the upper panels and $\alpha$-core in the bottom ones). Dislocation cores are indicated by orange circles on the image. (b)–(d) Corresponding $\mathrm{Se}{L}_{2,3},\mathrm{Cu}{L}_{2,3}$, and $\mathrm{In}{M}_{4,5}$ elemental distribution maps are shown in red, green, and blue. (e) The red-green-blue composite map is a color-coded superposition of the individual elemental maps.

Figure 3

Simulated structures were superimposed on the HAADF images for the (a) $\alpha$-core and (b) $\beta$-core. Relative formation energies of point defects inside the (c) $\alpha$- and (d) $\beta$-cores of the Frank loop. The chemical potentials for Cu and In were chosen to mimic the experimental Cu-poor conditions, and charged defects are presented as colored bands rather than lines to show also their values when $0\le E_{\mathrm{F}}\le 0.25\mathrm{eV}$. LDOSs of stoichiometric and decorated cores are shown for both (e) $\alpha$-core and (f) $\beta$-core. The band gap of the bulk structure is marked by dotted vertical lines.

Figure 4

The HAADF image shown in Fig. 1 is superimposed with (a) ${\varepsilon }_{xx}$ and (b) ${\varepsilon }_{yy}$ strain components extracted by GPA. Strain fields for an extrinsic Frank loop as predicted from linear elasticity solutions (c) ${\varepsilon }_{xx}$ and (d) ${\varepsilon }_{yy}$ and the $n_d{\sum }_{ij}{\epsilon }_{ij}{G}_{ij}$ term in eV for the (e) ${\mathrm{Cu}}_{\mathrm{In}}^{-2}$ and (f) ${\mathrm{In}}_{\mathrm{Cu}}^{+2}$ antisites.