Research On Enhancing The Thermoelectric Figure-of-Merit Of Pb(Te,Se) Based Materials By Doping In A Wide Temperature Range

Thermoelectric material is a kind of functional material that can directly convert heat energy into electricity.Thermoelectric materials have irreplaceable applications in aerospace,biomedical and many other fields.Average ZT is an important physical parameter that can characterize the thermoelectric conversion efficiency.Therefore,it is crucial to have a high average ZT in a wide temperature range.The main purpose of this paper is to improve the average ZT of Pb Se and Pb Te based materials in a wide temperature range.We suggested to improve the average ZT by doping with transition metals(Ti,V,Cr,Zr,Nb,Mo,Hf,Ta or W).A systematically experimental research,together with the theoretical calculations was conducted.All the transition metals-doped Pb Se are n-type conductors.The optimum ZT values of nine doped samples were obtained by adjusting the doping concentration.Peak ZT~1.1,~1.15 and ~1.2 at about 873 K was achieved in Ti0.015Pb0.985 Se,Zr0.005Pb0.995 Se and Nb0.01Pb0.99 Se,respectively,with increased room temperature Hall carrier concentration to ~1019-1020 cm-3.However,the lower temperature ZT(< 600 K)is not favorable compared with other transition metal(V,Cr,Mo,or W)-doped Pb Se with lower doping concentration.A higher room temperature thermoelectric figure of merit makes V,Cr,Mo,and W-doped Pb Se have a higher average ZT,which may be due to the lower doping limit and lower carrier concentration(~1018-1019 cm-3)of these four doped Pb Se.First-principles calculation found resonant states created in n-type Pb Se doped by Ti,V,Zr,Nb,Mo,Hf,Ta or W like in Cr-doped Pb Se,while Pisarenko plots show absence of resonant states due to the deep residence of the states and the limited doping concentration confirmed by the calculations.We realized the purpose of increasing the average ZT by co-doping with deep and shallow level dopants in Pb Te based materials to optimize the carrier concentration in a wide temperature range.Carrier concentration is an important factor that can affect the thermoelectric properties.The optimal carrier concentration at different temperature is not the same.However,the traditional doping method can only lead to a constant carrier concentration,which benefits to the thermoelectric performance in a limited temperature range.In this work,we demonstrated that a temperature-dependent carrier concentration can be realized by simultaneously introducing shallow and deep defect levels.In which acts as the deep donor level and Ga or Na which acts as the shallow level are co-doped in Pb Te.Ga or Na supplies carrier concentration and In can build up a half-filled deep defect state in the band gap which can trap electrons or holes at room temperature and release them with increasing temperature.As a result,the carrier concentration changed with the changing temperature.The optimized ZT value can be obtained by balancing the carrier concentration over the whole temperature range.A room temperature ZT of ~ 0.35 and a peak ZT of ~ 1.2 at 873 K for In0.005Pb0.995Te0.9925Ga0.0075 can be achieved.