Effect of Te doping on thermoelectric properties in vast off-stoichiometric tin-rich SnSe crystals

As a promising candidate for next generation thermoelectric material, SnSe has attracted extensive attentions due to its intrinsic ultralow thermal conductivity and adjustable electrical transport properties. However, unavailability of Sn-rich crystal leaves a gap in the Sn-deficient, stoichiometric, and Sn-rich series to study the effects of point defects and bonding nature evolution on thermal transport properties in SnSe compound. Also, historic references recorded contradicting electrical transport properties in polycrystalline Te-doped SnSe samples. This study shifted the research object from polycrystalline sample to single crystal to avoid the disturbance of grain boundary scattering and texture on the effects of Te doping. Vast off-stoichiometric Sn-rich Te-doped SnSe crystals were obtained by a self-flux method. The evolution of phase structure and Sn/Se atomic ratio were recorded and correlated. Our results confirmed that the Te atoms are incorporated in the Se sub-lattice, and have the potential to change the cation-anion atomic ratio in Sn-rich crystals. Crystal Sn1.17Se0.97Te0.027 with low concentration of Te shows rather robust thermodynamic properties, while crystal Sn(1.10)Se(0)(.)(88)Te(0)(.115 )is unstable and prone to decompose above 297 degrees C. Te incorporation and aroused defects are capable of decreasing the hole concentration as other donor dopants, at least at low doping level. Te doping has the potential to change both the band gap and the band structure, and may lead to convergence of multiple valence bands. An increase of thermal conductivity with increase of cation-anion atomic ratio Sn/(Se + Te) was recorded in Te-doped crystals, along with much lower thermal conductivity compared with the pristine one. The results indicates that the excess Sn in Sn-rich samples may cause a change in the bonding nature between cross-plane Sn-Se slabs and makes the material behaves less layer-like and shows more three dimensional thermal transport characterization as in Br-doped SnSe crystals. (C) 2020 Elsevier B.V. All rights reserved.