Band profile around grain boundary of Cu(InGa)Se2 solar cell material characterized by scanning probe microscopy

We performed photo-assisted Kelvin probe force microscopy (P-KFM) measurements on Cu(InGa)Se₂ (CIGS) solar cells and scanning tunneling spectroscopy (STS) measurements on as-grown CIGS thin films. From those measurements, we estimated the band profile around grain boundaries (GBs). The results indicate that the conduction band edge bends downwards near GB and that the bandgap near GB is broadened. Therefore we can conclude that the photo-generated electrons and holes are easily separated by the built-in field, and consequently their recombination at GBs should be suppressed. On the other hand, the built-in potential at GB was apparently lowered as an increase of the Ga content. Hence we can deduce that the photo-carrier separation effect may be reduced, which causes the slight degradation of some performance in the CIGS solar cell with high Ga content.