A systematic investigation of the electronic structure of copper indium gallium selenide (CIGS) solar cells has been been undertaken using both experiment and theory.  CIGS is normally coated with a thin layer of CdS. Using X-ray absorption spectroscopy (XAS), we found that growing CdS films by atomic layer deposition (ALD) results in less disorder than films produced by chemical bath deposition. Partial substitution of S with O has minimal impact on the sulfur-derived electronic structure, as observed in XAS. With respect to bulk CdS, these thin films present some spectral differences which we have attempted to assign to particular point defects based on theoretical XAS calculations, which also provide implications for device performance. In the CdS coating layer, we find that sub 10% sulfur substitution with, e.g., oxygen or selenium, does not produce significant changes in the S 3p projected density of states in the CdS conduction band, as measured by XAS.   In the bulk CIGS layer we find evidence for oxygen contamination in many working solar cells, as well as sensitivity of the cadmium, gallium and selenium environment to defects to XAS measurements.  We show, both experimentally and theoretically, that defects in CIGS often cause the metals to oxidize and that the chemical bath deposition used to deposit CdS can often rectify this damaged state.

Electronic structure study of CIGS solar cells by X-ray absorption spectroscopy: Experiment and theory

249th National Meeting (2015)

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