Photothermal spectroscopy by atomic force microscopy on Cu(In, Ga)Se2 solar cells

We performed photothermal [PT] measurements by atomic force microscopy on Cu(In, Ga)Se₂ [CIGS] solar cells to examine non-radiative recombination properties very locally under various kinds of incident light with photon energies above or below the bandgap of CIGS. The results indicate that the intensity and spatial distribution of the PT signal strongly depended on the photo-excitation condition, from which we have found the possibilities that the photo-generated free electrons could accumulate around the grain boundary owing to the built-in electric field and that the sub-gap states with discrete energy levels are present in the CIGS materials.