NaInX2 (X = S, Se) layered materials for energy harvesting applications: first-principles insights into optoelectronic and thermoelectric properties

In recent times, layered chalcogenide semiconductors have attracted great interest in energy harvesting device applications. In the present study, the structural, electronic, optical and thermoelectric properties of two isostructural chalcogenide materials, NaInS2 and NaInSe2 with hexagonal symmetry (R-3m) have been studied using the first principles method. A very good agreement has been found between our results with the available experimental and theoretical ones. The studied materials are semiconducting in nature as confirmed from the electronic band structure and optical properties. The strong hybridizations among s orbitals of Na, In and Se atoms push the bottom of the conduction band downward resulting in a narrower band gap of NaInSe2 compared to that of NaInS2 compound. Different optical (dielectric function, photoconductivity, absorption coefficient, reflectivity, refractive index and loss function) and thermoelectric (Seebeck coefficient, electrical conductivity, power factor and thermal conductivity) properties of NaInX2 (X = S, Se) have been studied in detail for the first time. It is found that all these properties are significantly anisotropic due to the strongly layered structure of NaInX2 (X = S, Se). Strong optical absorption with sharp peaks is found in the far visible to mid ultraviolet (UV) regions while the reflectivity is low in the UV region for both the compounds. Such features indicate feasibility of applications in optoelectronic sector. The calculated thermoelectric power factors at 1000 K for NaInS2 and NaInSe2 along a-axis are found to be 151.5 μW/cmK2 and 154 μW/cmK2, respectively and the corresponding ZT values are ~ 0.70. The obtained thermal conductivity along a-axis for both compounds is high (~ 22 W/mK).This suggests that the reduction of such high thermal conductivity is important to achieve higher ZT values of the NaInX2-(X = S, Se) compounds.