Giant Seebeck effect in Ge-doped SnSe

Abstract

Thermoelectric materials may contribute in the near future as new alternative sources of sustainable energy. Unprecedented thermoelectric properties in p-type SnSe single crystals have been recently reported, accompanied by extremely low thermal conductivity in polycrystalline samples. In order to enhance thermoelectric efficiency through proper tuning of this material we report a full structural characterization and evaluation of the thermoelectric properties of novel Ge-doped SnSe prepared by a straightforward arc-melting method, which yields nanostructured polycrystalline samples. Ge does not dope the system in the sense of donating carriers, yet the electrical properties show a semiconductor behavior with resistivity values higher than that of the parent compound, as a consequence of nanostructuration, whereas the Seebeck coefficient is higher and thermal conductivity lower, favorable to a better ZT figure of merit.

Figure 1. XRD patterns for as-grown Sn_1−xGe_xSe (x = 0, 0.1, 0.2, 0.3, 0.4) intermetallic compounds.

Figure 2

Rietveld plots for Sn_0.8Ge_0.2Se from (a) XRD data, defined in the space group Pnma. A strong preferred orientation is observed, enhancing [h00] reflections (b) NPD data at RT. Observed (crosses), calculated (full line) and difference (at the bottom).

Figure 3

Evolution of the unit-cell parameters of as-grown Sn_1−xGe_xSe (x = 0, 0.1, 0.2, 0.3) (a) a parameter, (b) b parameter, (c) c parameter, (d) unit cell volume. The lines are guides for the eye.

Figure 4. Crystal structure of Sn_0.8Ge_0.2Se including 95% probability displacement ellipsoids.

The coordination polyhedra (trigonal pyramids) (Sn,Ge)Se₃ are enhanced.

Figure 5

SEM images of as-grown Sn_0.8Ge_0.2Se, exhibiting a nanostructure consisting of platelets (perpendicular to [100] direction) for (a) x40000 and b) x50000 magnification, showing typical platelet thickness between 20 and 40 nm.

Figure 6. Calculated electronic density of states (DOS) of SnSe (black solid line), Sn_0.88Ge_0.12Se (red dashed line) and Sn_0.81Ge_0.19Se (blue dotted line).

Figure 7

(a) Seebeck coefficient vs temperature for Sn_1−xGe_xSe (x = 0.1, 0.2, 0.3) compounds (b) Thermal variation of the electrical resistivity, exhibiting characteristic semiconducting behavior. Both panels include the data for SnSe taken from ref. . Two independent measurements are shown for x = 0.1 and 0.3. Below around 150–200 K the resistance of the Ge-doped SnSe samples increases beyond the limits (few MΩ) of the electronics used and this influences the Seebeck voltage, too.

Figure 8. Thermal conductivity vs temperature, for Sn_1−xGe_xSe (x = 0, 0.1, 0.2, 0.3).