Physics of large thermoelectric power factors in SnSe nanoflakes in mid-temperature range.

We have theoretically investigated the underlying physics of observed high electrical conductivity (σ), simultaneous increase of σ and Seebeck coefficient (S) with temperature, and large power factors (PFs) in nominally undoped SnSe nanoflakes sintered at different temperatures, reported recently in Mandavaet al(2022Nanotechnology33155710). Given the fact that S and σ show unusual temperature trends and that the undoped SnSe samples are highly porous and disordered, the conventional Boltzmann theory does not appear to be an appropriate model to describe their transport properties. We have, instead, used a strong disorder model based on percolation theory where charge and energy transport take place through hopping between localized states to understand these observations. Our model is able to explain the observed temperature dependence of σ and S with temperature. Large σ can be explained by a high density of localized states and a large hopping rate. The sample sintered at a higher temperature has lower disorder (σDOS) and higher hopping rate (1/τ0). We findσDOS= 0.151 eV and 1/τ0= 0.143 × 1015s-1for sample sintered at 673 K andσDOS= 0.044 eV and 1/τ0= 2.023 × 1015s-1for sample sintered at 703 K. These values are comparable to the reported values of transition frequencies, confirming that the dominant charge transport mechanism in these SnSe nanoflakes is hopping transport. Finally, we suggest that hopping transport via localized states can result in enhanced thermoelectric properties in disordered polycrystalline materials.

Investigating the key role of carrier transport mechanism in SnSe nanoflakes with enhanced thermoelectric power factor.

A novel SnSe nanoflake system is explored for its thermoelectric properties from both experiments andab initiostudy. The nanoflakes of the low temperature phase of SnSe (Pnma) are synthesized employing a fast and efficient refluxing method followed by spark plasma sintering at two different temperatures. We report an enhanced power factor (12-67µW mK-2in the temperature range 300-600 K) in our p-type samples. We find that the prime reason for a high PF in our samples is a significantly improved electrical conductivity (1050-2180 S m-1in the temperature range 300-600 K). From ourab initioband structure calculations accompanied with the models of temperature and surface dependent carrier scattering mechanisms, we reveal that an enhanced electrical conductivity is due to the reduced carrier-phonon scattering in our samples. The transport calculations are performed using the Boltzmann transport equation within relaxation time approximation. With our combined experimental and theoretical study, we demonstrate that the thermoelectric properties of p-type Pnma-SnSe could be improved by tuning the carrier scattering mechanisms with a control over the spark plasma sintering temperature.