Point defect segregation and its role in the detrimental nature of Frank partials inCu(In,Ga)Se2thin-film absorbers

Abstract

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 ${\mathrm{Cu}}_{\mathrm{In}}^{\ensuremath{-}2}$, 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 $\ensuremath{\alpha}$-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 $\ensuremath{\beta}$-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.