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[en] The halide perovskite-based solar cells are attractive due to their excellent power conversion efficiency and low cost. The electronic structure, optical, and thermoelectric properties of Ge-based halide perovskite KGeIBr materials (x = 0; 1; 2; 3) are investigated based on the density functional theory (DFT). In addition, all these materials are stable in the cubic phase. These materials exhibit semiconducting behavior at normal pressure with a bandgap value lying between 0.611 and 1.126 eV when substitution of iodine atom to bromine one. The physical properties of Ge atom-based halide perovskite compounds indicate that the KGeIBr material can be a good candidate for solar cell applications. As far as our knowledge, this is the first theoretical prediction of the optical properties for these compounds, which still wait for experimental confirmation.
[en] The perovskite type oxide SrHfO had a huge scientist interest for the past few years thanks to its properties, which allowed it to be applied in different area, in our case we focused on the photovoltaic field application and it is known that this technology has been based on the use of semiconductors with a specific gap value since its birth, which indicates that the gap value is an important element who influences on the efficiency of panels. The aim of our work is based on reducing the gap value by applying different percentage of doping SrHfOS (x = 0%, 8% and 16%) and the determination of electronic and optical properties of all percentage of S using density functional theory (DFT). As a result we reduced the gap value from 5.60 eV corresponding to 0% of S to 2.09 eV corresponding to 16% of S and the band gap is changed from an indirect band gap equivalent to 0% of S to a direct band gap for 8% and 16% of S.
[en] The structural, electronic and magnetic properties of Fe7S8 material have been studied within the framework of the ab initio calculations, the mean field approximation (MFA) and Monte Carlo simulation (MCS). Our study shows that two forms of the iron atoms, Fe2+ with spin S = 2, and Fe3+ with spin = 5/2 are the most probable configurations. A mixed Ising model with ferromagnetic spin coupling between Fe2+ and Fe3+ ions and between Fe3+ and Fe3+ ions, and with antiferromagnetic spin coupling between Fe2+ ions of adjacent layers has been used to study the magnetic properties of this compound. We demonstrated that the magnetic phase transition can be either of the first or of the second order, depending on the value of the exchange interaction and crystal field. The presence of vacancies in every second iron layer leads to incomplete cancellation of magnetic moments, hence to the emergence of the ferrimagnetism. Anomalies in the magnetization behavior have been found and compared with the experimental results. (paper)