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

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)(1.16)NbSe2 for the first time and tried to optimize its high hole concentration (p similar to 9.6 x 10(21) 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 similar to 4.0 x 10(21) cm(-3)) and similar to 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 similar to 120 mu W 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 k(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 k(lat), this Se misfit shows a poor maximum ZT (similar to 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.