Point defect segregation and its role in the detrimental nature of Frank partials in Cu In,Ga Se2 thin film absorbers

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material Cu In,Ga Se2 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 Cu amp; 8722;2In, which can be considered a harmful hole trap in Cu In,Ga Se2. In the core region of the cation containing amp; 945; 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 amp; 946; core, in contrast, only a small excess of Cu is observed, which is explained by the fact that CuIn and Cui 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 Cu In,Ga Se2 growth is essential in order to obtain absorbers with high conversion efficiencie