IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance. The multiple close-to-degenerate (or "convergent") valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure-of-merit in the SnSe single crystal. Up to date, there has been only theoretical calculations but no experimental observation of this particular electronic band structure. In this paper, using Angle-Resolved Photoemission Spectroscopy, we performed a systematic investigation on the electronic structure of SnSe.We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab-initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe for the first time, and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe.Thermoelectric materials could directly convert heat (many times wasted) to electrical power and therefore are of critical importance in energy industry [1][2][3][4][5][6][7]. The conversion efficiency of thermoelectric materials is quantified by the dimensionless figure of merit, = 2 ⁄ ( : Seebeck coefficient, : electrical conductivity, : total thermal conductivity, including contributions from both electrons and phonons, : temperature). Recently, singlecrystalline SnSe, a binary IV-VI semiconductor compound containing non-toxic and earthabundant elements, shows a record high ZT of ~2.6 at 923 K (along the b axis of the roomtemperature orthorhombic unit cell) [8] and the device figure of merit ~1.34 from 300-773 K when hole-doped [9], much higher than that of typical high-performance thermoelectric materials [10-15]. These excellent thermoelectric performances can be attributed to both the relatively low thermal conductivity (~0.7 Wm -1 K -1 at 300 K for the pristine samples) [8] as well as the very high Seebeck coefficient (~160 μVK -1 at 300 K with carrier density of ∼4×10 19 cm -3 ) and power factor ( 2 , ~40 μWcm -1 K -2 at 300K) [9].While the low thermal conductivity is attributed to the giant anharmonic and anisotropic bondings [8,16,17], the high Seebeck coefficient and power factor are deeply rooted in the electronic band structure of SnSe. It has been proposed that SnSe bears an electronic structure with relatively small effective mass (thus high mobility) [8,18,19] and multiple close-to-degenerate ("convergent") valence bands [9,20,21]. As the temperature increases, the carriers are thermally distributed over several convergent bands of similar energy, resulting in the enhanced Seebeck coefficient [22,23]. Besides, the most outstanding electrical conductivity and power factor along the b axis among three axes of SnSe are thought to benefit from particular "pudding-mold-like" band [24][25][26][27][28]. However, although many the...

The SnSe crystal is a promising candidate in the field of thermoelectric materials. In order to elucidate basic physics in the SnSe system, here we report the heavily hole doping SnSe single crystals by the flux method (using alkali halide as solvent). Compared to bad-metal behavior of SnSe grown by the Bridgeman method, the flux-grown SnSe crystals show the metallic conductive behavior consistent with the Landau Fermi liquid (resistivity ρ ∼ T2) with temperatures ranging from 2 to 300 K. Combined angle-resolved photoemission spectroscopy and empirical Landau Fermi liquid theory, screening lengths λ of Coulomb electron–electron interaction U of SnSe grown by the flux method are 6.6 Å and 6.1 eV, which are much higher than those of normal metals. Remarkably, the excellent electrical conductivity (870 S/cm) of the SnSe crystal grown by the flux method at room temperature is attributed to the higher hole concentration (∼3.8 × 1019 cm−3) and large mobility (152.2 cm2 V−1 s−1). Meanwhile, these SnSe crystals still have large Seebeck coefficients (∼190 μV/K). Thus, the SnSe crystals grown by the flux method have an ultrahigh power factor [∼31.5 μW/(cm K2)] at room temperature, which is ten times larger than that of SnSe crystals grown by the Bridgeman method and as best as currently reported results. Our work shows a method for growing heavily hole-doped SnSe crystals, which provides a platform for understanding the electrical properties and improving its thermoelectric performance.