Nanoscale Electronic Structure Characterization in CIGS with Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) was used to provide electronic structure characterization of CuIn
_1-x
Ga
_x
Se
₂
(CIGS) solar cells. Two applications within this context were explored: spatially-resolved bandgap profiling using a newly-developed and simplified analysis approach, and detection of sub-gap defect states with high energy and spatial resolution. The simplified STEM-EELS bandgap profiling method was found to closely track nominal bandgap trends calculated from compositional data, with only a small, fixed offset (0.35 eV). Sub-gap energy levels in two different CIGS samples were identified at EV + 0.43 eV and EV + 0.56 eV, 0.56 eV, exactly consistent (i.e.<; 0.01 eV discrepancy) with trap states measured via conventional deep level transient spectroscopy (DLTS) in both cases. Furthermore, direct correlation between STEM-EELS and scanning DLTS (S-DLTS) measurements made at the same sample site providing strong confirmation that the energy levels detected via STEM-EELS are indeed the same trap levels detected via (S-)DLTS.