Effect of sulfur substitution on the thermoelectric properties of (SnSe)1.16NbSe2: charge transfer in a misfit layered structure

Abstract

Misfit layered compounds have gained tremendous interest in the past couple of years due to the unique opportunities they offer for ZT enhancement arising from their phonon glass-electron crystal (PGEC) behavior. In this study we investigated the thermoelectric properties of polycrystalline layered misfit selenide (SnSe)_1.16NbSe₂ for the first time and tried to optimize its high hole concentration (p ∼ 9.6 × 10²¹ cm⁻³) through sulfur (S) doping. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured in both directions, perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction over the temperature range of 300 K to 760 K. S doping resulted in the decrease of hole concentration (p ∼ 4.0 × 10²¹ cm⁻³) and ∼50% increase in hole mobility by increasing the charge transfer from SnSe to NbSe₂ layer. This provided a 15% enhancement in power factor over the entire temperature range with a value of ∼120 μW K⁻² m⁻¹ at 760 K in the out-of-plane direction. These power factor values are still quite low for this misfit to be considered as a potential thermoelectric material. Further p optimization needs to be performed through charge transfer tuning in order to increase the power factor. For all the samples, the κ_lat is quite low and lies in the range from 0.9 to 2.3 W K⁻¹ m⁻¹ in the out-of-plane direction at 760 K. Despite such low κ_lat, this Se misfit shows a poor maximum ZT (∼0.03 at 760 K) owing to high carrier concentration (low power factor). Nonetheless, this work provides further insights into the interdependence of electrical properties and charge transfer in misfit systems.