| Thermoelectric materials can directly convert the heat into electricity and have a promising applications in the fields of thermoelectric power generation and refrigeration as a type of important green energy materials.Single-crystal SnSe has attracted extensive attention as an emerging thermoelectric material since 2014 due to its outstanding thermoelectric properties.The growth methods and thermoelectric properties of single-crystal SnSe have become a focus of research in thermoelectric materials.It is a great challenge for the researchers how to effectively balance the competition between impurity removing and impurity doping during single crystal growth process.In this dissertation,n-type SnSe single crystals with Cl doping were synthesized using Bridgman method and the effect of Cl doping on the thermoelectric properties of the SnSe single crystal were investigated.The surface oxidation behavior of Cl doped SnSe single crystal was studied.Furthermore,the thermoelectric properties of Cl and Te co-doped polycrystalline SnSe synthesized by melting and hot pressing were investigated.The main contents and conclusions are as follows:1.Single crystals of Cl doped n-type SnSe were grown by Bridgman method and the effect of Cl doing on thermoelectric properties of SnSe single crystal samples was investigated.The results showed that the transition of p type to n type in SnSe single crystal was observed with the increase Cl content.The electron concentration was raised to 5.1×1018 cm-3 in SnSe0.97Cl0.03 sample,which processed high electric conductivity and mobility.The sample with x = 0.03 obtained a high power factor of17.8 μW cm-1 K-2 at 323 K.In the temperature range of 323 K to 773 K,the average ZT value was reached 0.72,which was 3.8 times that of undoped SnSe.The results indicated that Cl doping could effectively enhance thermoelectric performance of SnSe single crystal in the low temperature region.2.The surface oxidation of Cl doped n-type SnSe single crystal was studied.It was unveiled that Sn（OH）Cl oxide layer was formed on the cleavage surface when exposed to oxygen-containing atmospheres,forming a heterostructure with a Schottky junction type（p-n junction）.The presence of Sn（OH）Cl oxide layer can be detrimental to the electrical properties of Cl doped n-type SnSe single crystal.The heterojunction of Sn（OH）Cl/SnSe existed steadily at up to 200 ℃ and disappeared at high temperature.Our results elucidated that protecting the surface from oxidation should be taken into consideration when optimizing the thermoelectric properties of Cl doped n-type SnSe single crystal.3.A series of SnSe0.85-xTexCl0.1（x = 0,0.02,0.04,0.06,0.08,0.1）samples have been synthesized by melting and hot-pressing methods and the effects of Cl and Te co-doping on thermoelectric properties was investigated in the temperature range of323773 K.It is revealed that the incorporation of Te in SnSe0.85Cl0.1 generates a large number of impurity defects that accounts for an increase of carrier concentration and a reduction of electrical conductivity due to enhancement of the carrier scattering.As a result,the power factor has not been significantly improved by Cl and Te co-doping.Meanwhile,Te doping contributes to reducing the thermal conductivity through scattering phonons induced by the point defects.A maximum ZT value of 0.81 at 773 K is obtained in SnSe0.81Te0.04Cl0.1. |
