In Situ Characterization of the 2-Stage Cu(In,Ga)Se₂ Formation Process

Cu(In,Ga)Se₂ (CIGSe) solar cells offer benefits such as cost-effectiveness, efficient manufacturing processes, application versatility, and a lower carbon footprint compared to Si photovoltaics. The best-performing CIGSe solar cells and industrial-scale modules are usually deposited through multi-stage processes where the material's composition and temperature are adjusted based on empirical optimization methods. In-depth understanding of the formation process of CIGSe during deposition could enable further power conversion efficiency improvements. Most available studies on the CIGSe formation process have used in-situ X-ray diffraction experiments to follow phase changes during the deposition process. To improve the understanding of the materials processing and ultimately their performance, we conducted a combination of various in-situ experiments during the selenization stage of a sequential deposition process, namely Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Special attention is given to phase transformations during the material growth, particularly formation of secondary phases, segregation of elements between the different components, and how this affects the crystallinity. Samples were prepared on glass/Mo substrates by sputtering of a Cu-In-Ga alloy at room temperature with simultaneous supply of Se by evaporation, forming an amorphous CIGSe layer. A Se capping layer was evaporated on top of the CIGSe to provide sufficient Se for the diffusion reaction. In-situ Raman spectroscopy performed during crystallization shows that the A₁ vibrational mode of the α-phase of CIGSe starts to appear at 175 °C with a significant increase in intensity above 350 °C. The FWHM of the A₁ mode peak decreases until 350 °C indicating an increase in structural ordering in the material. When cooling down, the FWHM further decreases due to a reduction of the phonon-phonon interactions. With increasing temperature, the spectrum shifts to lower wavenumbers as result of the thermal expansion and changes in phonon occupation numbers. After 6 minutes at 600 °C, a decrease in the peak intensity is followed by broadening. In a sample without the Se capping layer, the intensity of the A₁ mode peak is lower and only becomes noticeable at a higher temperature of 425 °C. In-situ XRD indicates that CIGSe starts to crystallize above 150 °C with preferential orientation along [112] and that Cu₂Se starts to form at 400 °C. Between 200 °C and 400 °C, the crystallite size of (112) CIGSe increases from 7 to 52 nm. For the sample without the Se capping layer, the crystallite size first increases at 200 °C and then again above 350 °C, reaching a maximum size of 50 nm at 600 °C.