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

TL;DR

This study uses first-principles calculations to analyze the optoelectronic and thermoelectric properties of layered NaInS2 and NaInSe2 materials, revealing their potential for energy harvesting applications.

Contribution

It provides the first detailed theoretical analysis of the optical and thermoelectric properties of NaInS2 and NaInSe2, highlighting their anisotropic behavior and application potential.

Findings

NaInSe2 has a narrower band gap than NaInS2.

Both materials show strong optical absorption in the visible to UV range.

The thermoelectric power factor is high at 1000 K, with ZT around 0.70.

Abstract

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 micro Watt /cmK2 and 154 micro Watt/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.