First Principles Study On Intrinsic Defects Of Te Based Thermoelectric Materials

At present,in the double test of environmental degradation and energy crisis,thermoelectric materials as a new type of green energy conversion materials have been widely concerned.However,the low conversion efficiency of thermoelectric materials has become the bottleneck of large-scale commercial application.Defect control and external doping are the main means to control thermoelectric properties.Due to the low defect concentration in experiments,it is difficult to study the defects systematically.First principles calculation combined with thermodynamics method is widely used in the theoretical study of semiconductor defects.Te based compounds are a kind of thermoelectric materials with great development potential.It is very important to study their intrinsic defects for controlling carrier concentration and optimizing thermoelectric properties.There are many kinds of Te based thermoelectric materials.In this paper,three typical Te based thermoelectric materials XTe(X=Ge?Sn?Pb)?X2Te3(X=Sb?Bi)and Ge2Sb2Te5 are selected as the research objects to study their intrinsic defects and formation mechanism by first principles and thermodynamics.The results show that the cations vacancy defects(VGe2-,VSn2-)of GeTe(R3m&Fm3m)and SnTe in XTe system compounds are found has a lower formation energy,which leads to a strong p-type conductivity.The formation energies of intrinsic defects in PbTe depends on the chemical potential of Pb,and they are the VTe2+ and TePb2+ in Pb-rich and Pb-poor conditions respectively,which lead the intrinsic n-type conductivity.The results of Crystal orbital Hamilton populations(COHPs)of XTe system show that the antibonding state of X-Te atoms in GeTe and SnTe at valence band maxima(VBM)is stronger,so that the instability can be reduced and easy to form vacancy defects.Furthermore,the band alignment results show that in GeTe and SnTe,the energy generated by the main defect is lower due to the higher energy level position,which leads to the p-type conductivity.The type of conductivity in PbTe depends on the chemical potential of Pb.The results of defects formation energy of Sb2Te3 system show that SbTel and VSb have the lowest intrinsic defects formation energy in Sb-rich and Sb-poor condition respectively,which lead to the p-type conductivity of Sb2Te3;At the same time,the most stable defects in Bi-rich and Bi-poor chemical potential of Bi2Te3 are the accepter defect BiTe1 and donor defect TeBi,leads to the conductivity of p-type and n-type,which is consistent with the experimental results.The results of COHPs of X2Te3 system compounds show that the interaction between Sb and Te atom at the valence band maxima are stronger,which is conducive to the formation of the counter position defect.The results of band alignment show that the lone pair effect in Bi2Te3 compounds leads to the lower valence band maxima of Bi2Te3 than Sb2Te3 by 0.45eV just like in PbTe,which causes the difference of intrinsic defects and conducting types.The study of ternary Ge2Sb2Te5 compounds shows that the formation energies of intrinsic defects can be greatly influenced by their chemical potential.In the case of Tepoor,SbTe is the lowest defect.In the case of Te-rich,the VGe is the most stable one,which indicates that the defect situation in Ge2Sb2Te5 has certain correlation with the formation compounds GeTe and Sb2Te3.In conclusion,the intrinsic defects and formation mechanism of three typical Te based thermoelectric materials XTe(X=Ge?Sn?Pb)?X2Te3(X=Sb?Bi)and Ge2Sb2Te5 are investigated by first principles calculation and thermodynamic analysis.The calculated results are consistent with the experimental results.By comparing the relative positions of energy levels,a more general rule is summarized.It provides more perspectives and possibilities for understanding the intrinsic defects of materials and optimizing the properties of thermoelectric materials through defect control.