Electronic Structure Study of the CdS Buffer Layer in CIGS Solar Cells by X-Ray Absorption Spectroscopy: Experiment and Theory

A systematic investigation of the electronic structure of the CdS buffer layer of CIGS solar cells has been undertaken using S K-edge X-ray absorption spectroscopy (XAS), both experimentally and theoretically. We found from XAS that growing CdS films by chemical bath deposition (CBD) exhibits more long-range disorder when compared to single crystal CdS, CdS grown by atomic layer deposition (ALD) and theory. We investigated the significance of a variety of point defects and potential atomic substitutions in first-principles estimates of the differential S K-edge XAS sensitivity. We find that substituting some sulfur atoms with e.g. oxygen or selenium does not introduce significant changes to the partial S(3p) density of states of the conduction band in CdS and we infer that the electronic structure modifications of these defects are spatially localized and do not hybridize strongly with bands with significant sulfur p character. Moreover, by comparison with experimental XAS should be sensitive to CdSO₄; however this is not in the CdS layers of CIGS devices studied here. We also find that the CBD of CdS on FTO-glass and separately on CIGS does not produce any significant changes in the local atomic structure of CdS indicating no CIGS-influenced growth.