Enhancing the thermoelectric performance of BiGa2X4 (X=S, Se) P-type semiconductors by optimizing charge carrier concentration or chemical potentials

Abstract

We present an extensive analysis of the structural, electronic, optical, elastic, and thermoelectric properties of BiGa2X4 compounds, where X represents either sulfur (S) or selenium (Se). Our approach employed the all-electron full potential linearized augmented plane wave (FP-LAPW) technique, offering a comprehensive understanding of these materials' characteristics. The calculated lattice constants (a), the unit cell height (c), and the c/a ratio closely match experimental data, affirming the accuracy of our calculations. A pivotal focus of our study was on the electronic properties, including the indirect bandgaps (A→M−Γ) and (M→A). We found that BiGa2S4 exhibited an indirect bandgap (Eg) of 2.504 eV, while BiGa2Se4 possessed a slightly lower value of 1.878 eV. This variation was primarily attributed to the intricate interactions among bismuth, sulfur, and selenium atoms, particularly involving p−p orbital interactions. Additionally, we explored the optical characteristics of these compounds, determining their maximum absorption wavelengths. BiGa2S4 exhibited an absorption peak at 4.476 eV, whereas BiGa2Se4 displayed a slightly lower maximum absorption at 3.741 eV. Moreover, BiGa2Se4 showcases a higher dielectric constant, which augments its potential for optoelectronic applications. A critical aspect of our research is the assessment of the elastic properties, elucidating that both compounds exhibited fragility and anisotropy. We observed that at 300 K, the lattice thermal conductivity (kL) for BiGa2S4 and BiGa2Se4 was measured at 1.57W/mK and 1.14W/mK, respectively, indicating low thermal conductivity. At 1000 K, both BiGa2S4, and BiGa2Se4 exhibit significant ZT values of 0.8389 and 0.8722, respectively. The ZT values of the p-type semiconductors are notably higher than those of the n-type. At T = 900 K, the optimized ZT values for BiGa2S4, and BiGa2Se4 are found to be 0.82909 and 0.90548, respectively. Achieving these values requires either increasing the concentration of charge carriers to n = 0.11715 x 1022 cm−3 for BiGa2S4 and n = 0.0812 x 1022 cm−3 for BiGa2Se4, or reducing the chemical potentials by 0.40151 Ryd and 0.38001 Ryd, respectively.