Defect levels in Cu(In,Ga)Se2 polycrystalline layers by sub-bandgap photo-induced current transient spectroscopy

Photoinduced current transient spectroscopy (PICTS) employing sub-bandgap excitation pulses is used for studying defect levels in polycrystalline thin films of Cu(In,Ga)Se2 (CIGS). It is shown that the persistent photoconductivity effect accompanying photocurrent measurements distorts and often totally obscures PICTS spectra in the case of copper-poor polycrystalline layers. In order to overcome this difficulty, the use of sub-bandgap light in PICTS measurements is proposed. The results for both types of excitation - sub-bandgap (wavelength 1300 nm) and above bandgap (wavelength 975 nm) - are compared. We show that sub-bandgap light provides better-resolved PICTS spectra than in the case of standard measurements when contribution of photocurrent decay due to persistent photoconductivity is significant. The results for a set of CIGS polycrystalline layers fabricated using various preparation protocols are shown and discussed. Two most pronounced peaks are identified with transitions observed previously in the defect level spectra of the CuInSe2 and CuGaSe2 epitaxial layers.