Effect Of Multi-element Co-doping On Thermoeletric Properties Of N-type Polycrystalline SnSe

Thermoelectric materia is a kind of functional material that can directly convert heat into electricity,which is one of important ways to solve the energy crisis and environmental problems.In recent years,SnSe single crystal has attracted much attention due to its extremely low thermal conductivity and high thermoelectric performance.However,SnSe single crystal suffers from the rigid crystal growth conditions,poor mechanical properties,and high cost of their production,which limits its large scaled application.To overcome this challenge,polycrystalline SnSe has become a research focus and has been widely concerned and studied.Although high ZT values have been achieved in p type polycrystalline SnSe,it is still difficult to improve the thermoelectric performance for n type due to a substantial amount of inherent Sn vacancies.In this work,we try to enhance the thermoelectric performance of n type polycrystalline SnSe by co-doping multi-element.The main content and conclusions are as follows:1.The effect of Bi and Pb co-doping on thermoelectric properties of n-type polycrystalline SnSe is investigated.The Sn0.96-x Bi0.04 Pbx Se?x = 0.02,0.1,0.2,0.3?samples have been synthesized by rapid induction melting and rapid induction hot pressing.Bi doping can transfer the SnSe from p-type to n-type and have a low ZT value of 0.05.Then,we introduced the Pb to enhance the electrical transport performance in Bi-doped SnSe.As a result,the electrical conductivity increases from9.3 Scm-1 to 42.7 Scm-1 and ZT value reaches 0.21 at 774 K in Sn0.66Bi0.04Pb0.3Se,which is 77 % higher than that of Bi single doped sample.2.The effect of MoCl₅ doping on thermoelectric properties is investigated.n-Type polycrystalline SnSe0.95-x % MoCl₅?x = 0.5,1.0,1.5,2?samples have been synthesized by melting and hot-pressing.The multipoint defects of Clse and Mosn increased the carrier concentration up to 1.76 × 1019 cm3 in SnSe0.95-1.5 % MoCl₅ sample.The doping of MoCl₅ enhanced the electrical conductivity due to the increased carrier concentration and reduced the thermal conductivity by introducing the multipoint defects.As a result,the maximum ZT value of 0.66 at 773 K is achieved in SnSe0.95-1 % MoCl₅.3.We studied the effect of FeCl₃ doping on the thermoelectric properties of n-type polycrystalline SnSe.A series of SnSe0.95-x % FeCl₃?x = 0,0.5,1,2,2.5,3?samples have been synthesized by melting and hot-pressing.The electron concentration tuned to 4.78 × 1018 cm-3 with the FeCl₃ content of x = 0.5.With further increasing FeCl₃-doping content,the electron concentration decreased,indicating the substitution of Fe in Sn sites generating holes.Meanwhile,the doped FeCl₃ results in massive defects,which contribute to a low thermal conductivity and a ZT of 0.67 at773 K.4.The effect of co-doped metal cations?Ti ? Sb ? Ni?with Cl on the thermoelectric properties of n-type polycrystalline SnSe is investigated.A series of Sn0.97 M0.03 Se0.86 Cl0.09?M = Ti ? Sb ? Ni?samples have been by melting and hot-pressing.We found that the incorporation of cations?Ti?Sb?Ni?decreased the concentration of carriers and resulted in the inferior electrical conductivities in the phase transition region.Meanwhile,Ni co doped with Cl element contributes greatly to reducing the lattice thermal conductivity through scattering phonons.The maximum ZT value of 0.7 at 773 K is achieved in Sn0.97 Ni0.03 Se0.86 Cl0.09 sample.