Spectroscopic characterization of surfaces and interfaces of gallium-rich chalcopyrite solar cell absorbers with wet-chemical buffer layers

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Chalcopyrite-based thin-film solar cell absorbers achieve high efficiencies using a CdS buffer layer grown by chemical bath deposition (CBD). By adjusting the Ga/(Ga+In) ratio (GGI-ratio), the band gap can be varied, potentially increasing efficiency. However, previous attempts showed that increasing the GGI-ratio led to decreased efficiency. To address this, alternative buffer layers are explored alongside methods for better integration of the buffer layer deposition in an in-line production process. Dry techniques for buffer layer deposition lack the surface cleaning capabilities of wet CBD. UV-induced ozone and low-energy Ar+ ion treatments may offer effective dry surface cleaning steps for CIGSSe. The study investigates the chemical surface structure of a CIGSSe absorber and the diffusion across the interface between the buffer layer and absorber, which significantly impacts solar cell efficiency. The CdS/Cu(In, Ga)Se2 interface is known for selenium diffusion. This research examines potential diffusion for alternative buffer layers like In2S3 or ZnS (deposited by CBD) on Cu(In, Ga)Se2 with varying GGI-ratios. A combination of surface-sensitive spectroscopic methods, including laboratory-based x-ray photoelectron spectroscopy, x-ray-excited Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy, along with synchrotron-based soft x-ray emission spectroscopy, is employed for this investigation.