Optoelectronic Properties of TiS2: A Never Ended Story Tackled by Density Functional Theory and Many-Body Methods.

Herein is reported a thorough computational investigation on the bulk TiS2 material with the CdI2 structure type and the ideal 1:2 Ti:S stoichiometry. Computations were performed using some of the most refined models, e.g., a hybrid functional together with dispersion effects (Grimme's), the GW ansatz, and the Bethe-Salpether equation for the optical properties. We showed that switching from Perdew-Berke-Enzerhof (PBE) to PBE0 leads to a gap opening. Moreover, our results demonstrate unambiguously that van der Waals interactions must be properly treated with dispersion effects in order to retrieve the experimental crystal structure and the appropriate c/ a ratio. Indeed, the calculations prove that when one uses a highly accurate computational protocol, the bulk hexagonal TiS2 is a semiconductor with a small gap, whereas using the generalized gradient approximation (GGA) PBE functional leads to a semimetal. Furthermore, the band structure is significantly modified when dispersion parameters are taken into account. Pressure effects were also investigated, and they fully describe the previously simulated electronic transition behavior of the material, e.g., TiS2 becomes semimetallic under strain.

PyDEF 2.0: An Easy to Use Post-treatment Software for Publishable Charts Featuring a Graphical User Interface.

Herein we present an open-source program automating the post-treatment of solid-state ab initio calculations performed with VASP, the most used solid-state simulation package. The program plots Density of States (DOS) and Band Diagrams, enabling the user to conduct efficiently a detailed study of electronic properties of a material. Our tool includes a complete module dedicated to point defects studies, proposing various corrections which can be activated at will and innovative property calculations such as defect concentrations. This is the first program of its kind to offer a direct plot of the stability domain of the studied matrix with respect to its components' chemical potentials. We also implemented the retrieval of optical indices ɛ1 and ɛ2 , the computation of the refractive and extinction coefficients (n, k) and the reflexivity R of the material. All features of the piece of software are available through a thoroughly designed user-friendly, elegant and efficient Graphical User Interface (GUI) to be accessible to material scientists with various expertises, from both the experimental and theoretical sides. All figures are of publishable quality and can be customized as desired. © 2018 Wiley Periodicals, Inc.

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.

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 Cd1- xIn2+2 x/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 ≈ 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 xIn2-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 Cd1- xIn2+2 x/3S4 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.