Estudo Ab-Initio das propriedades estruturais, eletrônicas, ópticas e fotovoltaicas dos compostos NaBiS2 e NaBiSe2

Abstract

Ferroelectric semiconductor compounds with photovoltaic properties (photoferroics) represent promising alternatives to conventional semiconductors used in solar cells, as they are not subject to the energy conversion efficiency limitation imposed by the ShockleyQueisser limit. In this context, this thesis investigates the potential of the compounds NaBiS2 and NaBiSe2, both with orthorhombic and ferroelectric structures, for applications in photoferroic devices. Density Functional Theory (DFT)-based calculations were conducted to explore their structural, electronic, and optical properties, employing the modified Becke-Johnson exchange potential for a more accurate description of the electronic structure and optical properties. Additionally, energy conversion efficiencies were estimated using the spectroscopic limited maximum efficiency method to assess the potential performance of these materials in photovoltaic cells. The results indicate that the direct band gap energies of NaBiS2 and NaBiSe2 are approximately 1.2 eV and 0.75 eV, respectively. NaBiS2 exhibits a band gap value considered ideal for photovoltaic applications under solar illumination, while NaBiSe2 shows a lower-than-expected value from a theoretical standpoint, yet still relevant for absorption in the near-infrared region. Both compounds display absorption coefficients in the visible spectrum range with intensities on the order of 105 cm−1, a favorable optical characteristic that allows the use of relatively thin lms to efficiently absorb sunlightan essential factor for high-performance devices. A pronounced optical anisotropy was also observed, associated with the electronic structure of both compounds. Visible light absorption mainly arises from transitions between the 3px,y states of S to the 6x,y states of Bi, and from the 4x,y states of Se to the 6x,y states of Bi. Thus, the local BiS(Se)6 structure of the studied compounds is crucial for their optical properties. The spectroscopic limited maximum efficiencies obtained were 32.2% for NaBiS2 and 23.3% for NaBiSe2. The value for NaBiS2 surpasses that of the perovskite MAPbI2 (29.5%), which is currently one of the most efficient photoferroics, with an experimental efficiency around 20.3%. Combining these results with previously published estimates for ferroelectric polarization and carrier mobility, this thesis reveals the unexplored potential of the orthorhombic compounds NaBiS2 and NaBiSe2 for application in photoferroic devices and next-generation solar cells.