Preparation And Properties Of Poly Crystalline SnSe-based Thermoelectric Materials

Thermoelectric materials can realize the direct conversion between heat and electric energy, and have received worldwide attention in this era of energy shortage and environmental problems. Layered semiconductor materials SnSe possess simple structure, stable chemical properties, and is nontoxic to environment. As reported, SnSe single crystal exhibits ultralow lattice thermal conductivity （0.23 Wm-1K-1@923 K） and the ZT value can be as high as a record 2.6@923 K, which indicates that SnSe is a very potential new thermoelectric material system. The SnSe single crystal has poor mechanical properties, and require strict growth conditions. SnSe polycrystalline is regarded to overcome these difficulties, but its ZT is only 0.5 owing to the much lower electrical conductivity. Therefore, it is of great significance to improve the ZT value of SnSe polycrystalline by enhancing the electrical transport properties.In this work, We focuse on the SnSe polycrystalline, and aim to enhance the thermoelectric properties of SnSe materials. By Ag/BiCl₃ doping, the p-/n-type SnSe polycrystalline is prepared and their electrical transport properties are siginificantly improved, and the thermoelectric property of high textured Sno.99Ago.o2Se sample the zone-melting method is also investigated. The details are as follows:We first prepared Ag-doped p-type SnSe polycrystalline bulk sample by melting and hot pressing. Our results show that Ag doping can introduce a large amount of hole in the SnSe samples, which significant increases the carrier concentration and electrical conductivity. It is interesting to found that the Ag-doped SnSe keeps higher Seebeck coefficient. Therefore, the power factor of our samples is significantly enhanced by Ag doping. On the other hand, the lattice thermal conductivity of Ag-doped SnSe is lower than that of the undoped SnSe.. This is owing to the increased point defect scattering induced by Ag doping. Overall, the thermoelectric properties of p-type SnSe polycrystalline is significantly improved by Ag doping in a wide temperature range.We then prepared p-type Sn0.99Ag0.02Se sample by zone melting method. Compared with the hot pressing sample, the zone melting one exhibits high textured microstructure. It is found that textured structure make SnSe polycrystalline samples possess higher carrier mobility and higher electrical conductivity. As a consequence, the ZT value of zone-melting samples reaches 1.3@793 K, which is the highest value of reported SnSe polycrystalline by now.We have also investigated the n-type SnSe materials through melting with spark plasma sintering. By BiCl₃ doping, the carrier concentration of n-type SnSe is significantly increased to 1019 cm^-3, leading to a large increase of electrical conductivity. Meanwhile, the lattice thermal conductivity is reduced by the increased defect-phonon scattering. Consequently, a high power factor of 5 μWcm-K^-2 and a ZT of 0.7 have been achieved in SnSeo.95^-4mol% BiCl₃ at 793 K.