Crystal Chemistry, Optical-Electronic Properties, and Electronic Structure of Cd1-xIn2+2x/3S4 Compounds (0 <= x <= 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells

CdIn2S4 and In2S3 compounds were both previously studied as buffer layers in GIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdIn2S4-In2S3 system, and a series of Cd1-xIn2+2x/3S4 (0 <= x <= 1) materials were prepared and characterized. Our results show that two solid solutions exist. The aliovalent substitution of cadmium(II) by indium(III) induces a structural transition at x approximate to 0.7 from cubic spinel Fd (3) over barm to tetragonal spinel I4(1)/amd that is related to an ordering of cadmium vacancies. Despite this transition, the variation of optical gap is continuous and decreases from 2.34 to 2.11 eV going from CdIn2S4 to In2S3 while all compounds retain an n-type behavior. In contrast with the AlxIn2S3 solid solution, no saturation of the gap is observed. Moreover, XPS analyses indicate a difference between surface and volume composition of the grains for Cd-poor compounds. The use of Cd1-xIn2+2x/3S4 compounds could be a good alternative to CdIn2S4 and In2S3 to improve GIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.