Structures And Thermoelectric Properties Of Cu-Sn-S-based Ternary Compounds

Thermoelectric(TE)materials,which enables a direct conversion between heat and electricity by carriers transporting,have good applications in many areas,such as power generation?solid-state refrigeration and so on.The efficiency of TE divices is closely related to their TE performance.Therefore,it is of significance to enhance TE performance.Because of the earth-abundance of the constituent elements and thermal stability in air,ternary chalcogenide Cu-Sn-S-based compounds have attracted much attention in recent years.But their low electrical conductivities restrict their applications in thermoelectrics,due to their wide bandgap and the inharmonious relationship of the Seebeck coefficient and carrier concentration.Therefore,it is crucial to research new Cu-Sn-S-based compounds and improve their TE performance.In this work,we foucuse on the investigations of the structures and TE performance of Cu3 Sn S4-based ternary compounds.In order to better understand the structures and transport properties of Cu3Sn1+x S4,we have calculated the electronic band structures by first-principles.Based on the calculation and analysis,we coordinate the Seebeck coefficient with carrier concentration,thereby enhancing the TE performance.The main results are summarized below:(1)According to first-principles calculation,excess Sn in Cu3Sn1+x S4 resides in the interstitial or Cu sites,which makes the Fermi level(Ef)unpin and gradually move toward the conduction band(CB),although the calculated bandgap(Eg)remains almost constant(~1.25 e V).(2)Lattice thermal conductivity reduces significantly,due to an increased phonon scattering by carriers.Besides,excess Sn reduces the carrier concentrations and increases the Seebeck coefficient.(3)Based on the estimation of the quality factor B,it's confirmed that x=0.2 is the critical Sn content in Cu3Sn1+x S4.The ZT value of Cu3Sn1.2S4 reaches 0.75 at 790 K,about 2.8 times that of the stoichiometric Cu3 Sn S4.(4)Doping of Ag in Cu3 Sn S4 reduces lattice thermal conductivity and increases the ZT value to 0.49.This ZT value is about ~2 times that of pristine Cu3 Sn S4.(5)Through different elements(Se,Te and Zn)dopings in Cu3 Sn S4 and Cu3Sn1.2S4,the materials bring about different lattice distortion and scatterings of phonon.However,the improvement in TE performance of Cu3 Sn S4 is limited,with the ZT value being about ~7% higher than that of pristine Cu3 Sn S4.