Direct Insight into Grain Boundary Reconstruction in Polycrystalline Cu(In, Ga)Se2 with Atomic Resolution

This work presents results from high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy on twin boundaries (TBs) and nontwin grain boundaries (GBs) in Cu(In, Ga)Se-2 thin films. It is shown that the atomic reconstruction is different for different symmetries of the grain boundaries. We are able to confirm the model proposed by Persson and Zunger [Phys. Rev. Lett. 91, 266401 (2003)] for Se-Se-terminated Sigma 3 {112} TBs, showing Cu depletion and In enrichment in the two atomic planes closest to the TB. On the contrary, Cu depletion without In enrichment is detected for a cation-Se-terminated TB. At nontwin GBs, always a strong anticorrelation of Cu and In signals is detected suggesting that the formation of In-Cu or Cu-In antisites within a very confined region of smaller than 1 nm is an essential element in the reconstruction of these GBs.