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

• Photothermal (PT) signals on CIGS solar cells were acquired by AFM.
• The dependence of PT signal on the excitation condition was observed.
• The PT signals also were depended on the Ga content in CIGS.
• The possibility of the electron accumulation near a grain boundary was discussed.
• The existence of sub-gap states as the electron trap in the CIGS was suggested.

The non-radiative recombination properties of Cu(In,Ga)Se2 [CIGS] solar cells were investigated locally through photothermal [PT] measurements by atomic force microscopy under various kinds of incident light with photon energies above or below the bandgap of CIGS. We found that the intensity and spatial distribution of the PT signal strongly depended on the photon energy of the incident light and on the Ga content of the CIGS layer. These results suggest the possibilities that photo-generated free electrons could accumulate near the grain boundary because of the built-in electric field and that sub-gap states with discrete energy levels are present in the CIGS material.