Study On Influence Of Metal Halide Doping On Thermoelectric Properties Of N-type SnSe

SnSe is a class of thermoelectric materials with great application potential.Both p-type and n-type single crystals have shown good thermoelectric properties in the mid-temperature region.However,for polycrystalline SnSe,which is easy to synthesize and low in cost,the development of p-type materials and n-type materials is extremely unbalanced.So far,the thermoelectric properties of n-type polycrystalline SnSe are far inferior to that of p-type ones.Therefore,it is meaningful to search for suitable dopants to enhance the thermoelectric properties of n-type polycrystalline SnSe.At present,the more commonly used n-type dopants are halogens and various halides.Compared with single halogen doping,halide doping can introduce a variety of point defects to strongly scatter phonons and achieve simultaneous optimization of electrical and thermal transport performance.However,the effects of doping with different cations are quite different,and the underlying mechanism remains to be explained.Based on this,we selected three different transition metal halides（Nd Cl₃,Ce Cl₃,and Mn Cl₂）and Cl element to dope n-type polycrystalline SnSe,and studied its influence on the thermoelectric properties of SnSe and its internal mechanism.The main findings of this paper are as follows:（1）A series of Cl doped n type polycrystalline SnSe were successfully prepared by melting and SPS sintering,and the effect of Cl doping on the thermoelectric properties of n-type polycrystalline SnSe was investigated.The results show that Cl doping can significantly improve the carrier concentration and electrical conductivity of the materials.The high carrier concentration suppresses the deterioration of the Seebeck coefficient due to intrinsic excitation,and significantly improves the high temperature Seebeck coefficient of the samples.The power factor of SnSe_0.92Cl_0.03sample reaches the maximum value of 4.49μW·cm^-1·K^-2at 773 K.When the doping concentration is low,Cl doping can effectively enhance phonon scattering and reduce thermal conductivity.When the doping concentration is high,a large amount of Sn phase with high thermal conductivity will be precipitated in the samples,which can increase the thermal conductivity.As a result,the SnSe_0.92Cl_0.03sample obtains the maximum ZT value of 1.17 at 773 K.（2）A series of Nd Cl₃and Ce Cl₃doped n type polycrystalline SnSe were prepared by melting and SPS sintering,and the effects of these two rare earth halide doping on the thermoelectric properties of n-type polycrystalline SnSe were investigated.The results show that these two halide doping can not only effectively inhibit the precipitation of Sn and affect the room temperature carrier concentration and electrical conductivity of the samples,but also effectively improve the electrical conductivity and Seebeck coefficient after thermal excitation,thereby improving the power factor.In addition,excessive doping will introduce a large number of point defects and chloride precipitates,which can reduce the thermal conductivity.As a result,the SnSe_0.95-1.5mol%Nd Cl₃sample obtains the highest ZT value of about1.30 along the parallel direction at 773 K,which is about 30 times higher than that of the undoped sample.The SnSe_0.95-0.5mol%Ce Cl₃sample obtained the highest ZT value of about 1.17 in the parallel direction at 773 K,which was about 26 times higher than that of the undoped sample.（3）A series of Mn Cl₂doped n type polycrystalline SnSe were prepared by melting and SPS sintering,and the effect of Mn Cl₂doping on the thermoelectric properties of n-type polycrystalline SnSe was investigated.The study found that excessive Mn Cl₂doping can not only lead to the precipitation of Sn Cl₂phase in the samples,which affects the carrier concentration and mobility of the material,but also improve the electrical conductivity and Seebeck coefficient of the samples after thermal excitation.In addition,minor doping of Mn Cl₂can simultaneously introduce a large number of point defects and Sn Cl₂precipitates,which can significantly enhance the phonon scattering and reduce the thermal conductivity of the material.However,with the gradual increase of Mn Cl₂doping amount,the precipitation of Mn phase will lead to a slow increase in the thermal conductivity of the material.As a result,the SnSe_0.95-0.5mol%Mn Cl₂sample obtained a maximum ZT value of 1.01 at773 K,which was nearly 22 times that of the undoped samples.