High thermoelectric efficiency realized in SnSe crystals via structural modulation

Abstract

Crystalline thermoelectrics have been developed to be potential candidates for power generation and electronic cooling, among which SnSe crystals are becoming the most representative. Herein, we realize high-performance SnSe crystals with promising efficiency through a structural modulation strategy. By alloying strontium at Sn sites, we modify the crystal structure and facilitate the multiband synglisis in p-type SnSe, favoring the optimization of interactive parameters μ and m^*. Resultantly, we obtain a significantly enhanced PF ~85 μW cm^-1 K^-2, with an ultrahigh ZT ~1.4 at 300 K and ZT_ave ~2.0 among 300-673 K. Moreover, the excellent properties lead to single-leg device efficiency of ~8.9% under a temperature difference ΔT ~300 K, showing superiority among the current low- to mid-temperature thermoelectrics, with an enhanced cooling ΔT_max of ~50.4 K in the 7-pair thermoelectric device. Our study further advances p-type SnSe crystals for practical waste heat recovery and electronic cooling.

Fig. 1. Theoretical simulations on the electrical parameters with dynamic carrier concentration n from Pnma phase to Cmcm phase at 300 K.

a Carrier mobility μ. b Electrical conductivity σ. c Seebeck coefficient S. d Power factor PF.

Fig. 2. Temperature-dependent electrical performance for SnSe-9%Pb-x%Sr crystals.

a Electrical conductivity σ. b Seebeck coefficient S. c Power factor PF. and d Weighted mobility μ_W. Experimental data and multiband simulations with n on the e Seebeck coefficient S. And f Carrier mobility μ.

Fig. 3. SR-XRD measurements and Rietveld refinements of p-type SnSe-9%Pb−1.2%Sr crystal.

a Diffraction patterns from 300 to 873 K. b Temperature-dependent lattice parameters for hole-doped SnSe, SnSe-9%Pb, and SnSe-9%Pb−1.2%Sr. The arrows show that T_pt declines from ~800 K in SnSe to ~748 K in SnSe-9%Pb and ~723 K for SnSe-9%Pb−1.2%Sr, indicating that Sr alloying further promotes the phase transition of SnSe.

Fig. 4. Structural modulation by alloying Sr in SnSe.

a The crystal structure of Pnma- and Cmcm-SnSe, with the angle 1 marked, indicating the increased crystal symmetry. b The values of angle 1 in hole-doped SnSe, SnSe-9%Pb, and SnSe-9%Pb-1.2%Sr with temperature, showing that Sr alloying declines angle 1 and enhances the crystal symmetry over the whole temperature range. c The characteristic temperatures for p-type SnSe, SnSe-9%Pb, and SnSe-9%Pb-1.2%Sr crystals, including the bands merging temperature (T_m) from VBM 1 and 2 to VBM (1 + 2), bands alignment temperature (T_a) between VBM 3 and VBM (1 + 2), and completing phase transition temperature (T_pt). The reduction of the characteristic temperatures implies that Sr alloying further promotes the multiband synglisis in p-type SnSe.

Fig. 5. Microstructure characterization on the p-type SnSe-9%Pb−1.2%Sr crystal.

a The ABF-STEM image exhibits the microstructure morphology of the sample. b The ADF-STEM image indicates the Sr-alloyed area. c1–c4 Its corresponding elemental analysis. d The HAADF-STEM image of one Sr-alloyed area displays the expanded arrangement of atoms. e1–e2 The corresponding GPA mappings along the horizontal (ε_xx) and vertical (ε_yy) axis. f Another Sr-alloyed region which reveals the contraction of atomic arrangements. g1–g2 Its GPA maps. h The undoped area of zigzag structure with Pnma phase. i, j The enlarged atomic-resolved HAADF image demonstrates the high-symmetry phase around the Sr-alloyed area coming from d and f, respectively.

Fig. 6. Thermal transports, ZT, and device efficiencies for SnSe-9%Pb-x%Sr crystals.

a Total thermal conductivity κ_tot and lattice thermal conductivity κ_lat. b ZT and ZT_ave at 300–673 K. c Comparisons on the experimental conversion efficiencies η between the devices in this study (fabricated by using the SnSe-9%Pb−1.2%Sr crystals) and the reported results among comparable temperature differences ΔT (SKD skutterudites, BST Bi_2-xSb_xTe₃, HH half-Heusler, TAGS-85 (GeTe)₈₅(AgSbTe₂)₁₅, S-L single-leg)^,,–. d Maximum cooling temperature difference ΔT_max for the fabricated seven-pair thermoelectric devices using the SnSe-9%Pb−1.2%Sr crystals, and the ΔT_max for the SnSe-9%Pb crystals was plotted for comparison.