Experimental study of graded bandgap Cu(InGa)(SeS)2 thin films grown on glass/molybdenum substrates by selenization and sulphidation

High-performance Cu(InGa)(SeS)₂ (CIGSS) thin film absorbers with an intentionally graded bandgap structure grown by a two-stage method have been studied. Materials obtained from Showa Shell Sekiyu K.K., Japan have been grown using selenization and sulphidation of the Mo/Cu–Ga/In stacked precursors. Full characterizations have been carried out using X-ray diffraction, Raman, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence, inductively coupled plasma mass spectroscopy, glow discharge optical emission spectroscopy (GDOES) and photoelectrochemical (PEC) techniques to study various properties. The material layers were found to be polycrystalline with the (1 1 2) preferred orientation, and the largest grains were about 2 μm. Raman measurements show the presence of at least five different phases within the material. XPS confirmed the copper depletion and the richness of sulphur at the top surface region. Although the PEC studies indicate the overall electrical conductivity of the layer as p-type, GDOES profiling reveals the segregation of different phases at different depths suggesting the possibility of having buried junctions within the material itself. The results are presented together with suggestions for further improvements of CIGSS solar cell material.