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

Abstract

A novel SnSe nanoake system is explored for its thermoelectric properties from both experiments and ab initio study. The nanoakes 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 W/mK2 - 67 W/mK2 in the temperature range 300 K-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 S/m - 2180 S/m in the temperature range 300 K-600 K). From our ab initio band 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 trans- port 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.

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

“…Next, we discuss the possible origin of the localized states in the nanoflakes. As described in [27], the density of sample S1 is 5.83 g cm −3 and S2 is 6.04 g cm −3 as compared to the bulk SnSe value of 6.18 g cm −3 , suggesting the presence of large fraction of voids. The electronic states of nanoflakes of dimensions 20-40 nm can be visualized as states of Quantum dots of similar dimensions.…”

“…Before presenting our numerical results we briefly summarize the motivation behind our calculations. The earlier theoretical attempt to explain the experimental results used Boltzmann transport theory (band theory) for Bloch electrons [27]. In that work, the temperature dependence of the relaxation time was approximated by a particular form ∼T s , where the parameter s depends on the size of the nanoflakes and was obtained by fitting it to the experimental results.…”

“…Recently, Mandava et al [27] reported enhanced TE properties of compacted undoped SnSe nanoflakes assemblies; their measurements showed two interesting features; (1) the measured electrical conductivity (σ) values at 541 K for two samples sintered at different temperatures (673 K, sample 1, S1 and at 703 K, sample 2, S2) are 1667 S m −1 and 2180 S m −1 respectively, which are substantially high for undoped SnSe systems, and (2) both σ and Seebeck coefficient S showed increasing trend with temperature which is unlike the trends reported in most of the work on bulk SnSe. The measured values of σ and S give rise to large PFs.…”

“…Mandava et al carried out theoretical calculations of σ using a standard model for treating transport in bulk single crystals (Boltzmann transport theory using temperatureindependent bulk electronic structure) [27]. They included energy and temperature-dependent relaxation time (associated with carrier-phonon scattering) and a size-dependent boundary scattering.…”