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

Chalcopyrite-based thin-film solar cell absorbers achieve the highest efficiencies with a CdS buffer layer grown by chemical bath deposition (CBD). By shifting the balance from indium to gallium to a specific Ga/(Ga+In)-ratio (abbreviated as GGI-ratio) the band gap can be varied, leading to a theoretical increase in efficiency. However, earlier attempts with the CdS buffer layer have indicated that by increasing the GGI-ratio, the efficiency decreases. In an attempt to overcome this, the option of alternative buffer layers is explored.

Parallel to the search for alternative materials, industry partners are searching for alternative methods which allow for better integration of the buffer layer deposition step in an in-line production process. Alternative dry techniques for buffer layer deposition lack the surface cleaning characteristics of the wet CBD. Here UV-induced ozone and/or low-energy Ar+ ion treatment could provide dry CIGSSe surface cleaning steps. To study the impact of these different treatments, the chemical surface structure of a CIGSSe absorber is investigated.

Diffusion across the interface between buffer layer and absorber has a significant influence on the efficiency of the final solar cell. The CdS/Cu(In,Ga)Se2 interface is known to show selenium diffusion. This work provides an investigation into possible diffusion for the alternative buffer layers In2S3 or ZnS (deposited by CBD) on Cu(In,Ga)Se2 with varying GGI-ratios. For this purpose, a set of surface-sensitive spectroscopic methods, i.e., laboratory based x-ray photoelectron spectroscopy, x-ray-excited Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy, is combined with synchrotron-based soft x-ray emission spectroscopy.