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

Abstract

CdIn₂S₄ and In₂S₃ 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 CdIn₂S₄-In₂S₃ system, and a series of Cd_{1- x}In_{2+2 x/3}S₄ (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 I4₁/ 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 Al _xIn_2-xS₃ 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 Cd_{1- x}In_{2+2 x/3}S₄ compounds could be a good alternative to CdIn₂S₄ and In₂S₃ to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.