Microscopic evidence for the modification of the electronic structure at grain boundaries of Cu(In _1-x,Ga _x)Se ₂ films

We investigated the electronic properties around grain boundaries of polycrystalline Cu(In _1-x,Ga _xSe ₂) films as a function of Ga content, using scanning tunneling spectroscopy. Spectra acquired on samples with low Ga content (x=0 and 0.33) reveal downward band bending with respect to adjacent p-type grains, suggesting type inversion at the surface of grain boundaries. Such a behavior was not observed for samples with high Ga contents. These results are consistent with our atomic force microscopy data and may shed light on the origin of the x-dependent efficiency for polycrystalline Cu(In _1-x,Ga _xSe ₂)-based solar cells.