Crystal Chemistry, Optical-Electronic Properties, and Electronic Structure of CdInS Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells.

CdInS and InS compounds were both previously studied as buffer layers in CIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdInS-InS system, and a series of CdInS (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 ≈ 0.7 from cubic spinel Fd3̅ m to tetragonal spinel I4/ 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 CdInS to InS while all compounds retain an n-type behavior. In contrast with the Al InS 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 CdInS compounds could be a good alternative to CdInS and InS to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.