Effects of heavy alkali metals (Rb, Cs) post-deposition treatments on CuInGaSe₂ using a spin-coating technique

Abstract This study used a controlled environment to explore the post-deposition treatment (PDT) effects on CuInGaSe 2 (CIGSe 2 ) semiconducting thin films using a non-vacuum spin-coating technique for doping the CIGSe layers with Cs and Rb. The structural characterization confirmed the successful deposition of chalcopyrite structures with no phases belonging to any alkali metals after the PDT, with crystallite sizes in the range between 40–67 nm, and with a slight change in the x-ray Diffraction peak positions indicating a change from copper-rich to copper-poor phases. The morphological studies confirmed the increase in grain sizes after the PDT. The energy-dispersive x-ray spectroscopy chemical studies showed that there is a reduction in the copper content after PDT. The topographical studies showed a change in the surface morphology with modifications of the grain parameters. In addition, the electrical characterization showed a significant increase of the effective carrier mobility after the treatments, consistent with the grain size increase observed by both microscopic (scanning electron microscope and AFM) studies. Raman characterization of the CIGSe 2 films showed the A1 optical phonon mode of CIGS chalcopyrite structures and peaks at lower frequencies belonging to ordered vacancy compounds (OVCs). The deconvolution of the Raman spectroscopy broad peaks for the CIGSe 2 films after their PDT confirmed the formation of alk-InSe 2 OVC phases on top of the absorber layer.