n‐Type SnSe₂ Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

Abstract

convert waste heat into electricity. [2] The thermoelectric performance is evaluated by the dimensionless figure of merit ZT of the materials, as ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity, respectively. [3] Over the past decades, substantial progress has been made in the development of high performance TE materials by adopting advanced processing techniques, [4] new material design concepts, [5] and band engineering [6] on traditional TE materials. In addition, pursuing new materials with unique micro/nanostructures [7] and/ or special crystal structures [8] also has proven successful in yielding high TE performance. [9] For practical applications, developing earth-abundant, lowcost TE materials with high ZT values is highly desired. In this regard, Pb-free Sn-based compounds such as SnSe, [10] SnTe, [11] Mg 2 Sn, [12] and TiNiSn [13] -based compounds are considered to be promising for medium temperature TE applications. Recently, the CdI 2 -type dichalcogenide semiconductor SnSe 2 has drawn a lot of interest due to its unique layered structure (Figure S1, Supporting Information), in which the intralayer Sn and Se atoms are covalently bonded in the ab planes and the layers are held together by interlayer van der Waals interactions along the c-axis. Thus SnSe 2 shows anisotropic electronic and optoelectronic properties. [14] More importantly, theoretical studies [15] have predicted that SnSe 2 could be a promising n-type TE material with a predicted high ZT value of 2.95 along the a-axis, while a lower ZT of 0.68 along the c-axis at 800 K with a carrier concertation of 10 20 cm −3 due to the anisotropic electrical and thermal transport properties. [16] This indicates that the orientation and carrier concentration are the key factors for high TE performance. However, only a few experimental studies on SnSe 2 have been reported, and the low carrier concentration (≈10 17 cm −3 ) and random-oriented grains still result in a low power factor (≈150 µW m −1 K −2 ) and ZT (≈0.2). [16b] Herein, we employ a defect chemistry approach [17] by simultaneously introducing a selenium (Se) deficiency and chlorine (Cl) doping in SnSe 2 nanoplate-based pellets, in which the nano plates show a preferable orientation of the (001) planes along the primary surface of the pellet (in-plane). This yields a sharp increase in the in-plane electrical conductivity and It is reported that electron doped n-type SnSe 2 nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe 2 shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all-scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice the...

Cu Intercalation and Br Doping to Thermoelectric SnSe2 Lead to Ultrahigh Electron Mobility and Temperature‐Independent Power Factor

“…For example, the polycrystalline SnSe 2 sample with carrier concentration of 4.3 × 10 18 cm −3 shows 80% lower electron mobility of 7.4 cm 2 V −1 s −1 [40] than the single crystal sample with the similar carrier concentration. The achieved PF is record high to date for both polycrystalline SnSe 2 [42,43,45,46] and SnSe. The examples are densifying the nanoparticles prepared by colloidal synthesis, [42] packing the nanoplatelets delicately, [43] and embedding dense nanoscale precipitates into SnSe 2 nanopowders.…”

“…The theoretical Pisarenko relation between S and n H is plotted for pristine SnSe 2 at 300 (green line) and 773 K (red line) ( Figure S6, Supporting Information). 2020, 30,1908405 nanoplate-based pellet [43] and SnSe 2 -1.9 at% Cu nanocomposite [42] as well as the state-of-the-art bulk polycrystalline p-type Na 0.01 (Sn 0.95 Pb 0.05 ) 0.99 Se [33] and n-type SnSe 0.985 Br 0.015 single crystal [47] from the previous reports. [59] The S values at 300 and 773 K for SnCu x Se 2−y Br y (x = 0, 0.005 and y = 0, 0.02) in this work and SnSe 2−z Br z (z = 0.001-0.05) from the previous report [46] fit well on the lines, indicating that Cu and Br doping does not reduce the effective mass in the conduction band near the Fermi level.…”

“…[24,36,43] The observed temperature-independent and high power factor has been a sought-after goal for TE materials because TE devices comprising them can generate stable and high output power density regardless of the given temperature gradient, which is of pivotal importance in commercialization of TE technology. [24,36,43] The observed temperature-independent and high power factor has been a sought-after goal for TE materials because TE devices comprising them can generate stable and high output power density regardless of the given temperature gradient, which is of pivotal importance in commercialization of TE technology.…”

“…[24] Second, optimizing carrier concentration to enhance power factor of the TE materials sets an upper limit because of the inverse relationship between carrier concentration and Seebeck coefficient. The examples are densifying the nanoparticles prepared by colloidal synthesis, [42] packing the nanoplatelets delicately, [43] and embedding dense nanoscale precipitates into SnSe 2 nanopowders. However, this pathway for high power factor is generally infeasible in polycrystalline samples because their mobility is restricted by a high degree of grain boundaries.…”

“…However, this pathway for high power factor is generally infeasible in polycrystalline samples because their mobility is restricted by a high degree of grain boundaries. [43] Here, we report a new strategy that concurrently introducing the oppositely charged Br dopant into the SnSe 2 lattice and Cu intercalants between the SnSe 2 layers simultaneously increase the charge-carrier mobility and concentration. [41] As a consequence, the means of improving TE performance for SnSe 2 or materials with similar crystal structure has been limited to suppressing lattice thermal conductivity via nanostructuring.…”