Synergic Regulation Of The Electrical And Thermal Transport Properties In Lead Chalcogenides

Thermoelectric(TE)materials,which are widely used in the waste heat recovery and solid-sate cooling device,are capable of directly converting heat into electricity and vice versa via Seebeck and Peltier effects.The performance of a thermoelectric material is jointly determined by its electrical and thermal transport properties.The energy conversion efficiency of a thermoelectric device is regulated by the materials dimensionless thermoelectric figure of merit z T(z T=S~2s(14)k),which involves electron and phonon transports in crystalline solid.Therefore,in order to obtain a high TE performance,both the electron and phonon transport must be synergistically regulated.However,traditional strategies focused on optimizing one transport coefficient usually deteriorate the others due to the strong coupling effect between the thermoelectric transport coefficients(S,s,k).For example,optimizing electrical conductivity though increasing carrier concentraion also leads to decreased Seebeck coefficient and increased thermal conductivity.Lead chalcogenides have a simple crystal structure but may show complex microstructure.Thus,they are ideal compunds to investigate novel physical phenomena in bulk thermoelectric materials.Herein,we propose a novel strategy involving dynamic doping effect to synergetically regulate the electron and phonon transports of n-type lead chalcogenides.We have found that the dynamic doping effect of the Cu ions can simultaneously regulate the electrical and thermal transport properties of the n-type lead chalcogenides and thus leads to a high TE performance over a wide temperature range.The main achievements are summarized as below:We discover the dynamic doping effect of the interstitial Cu in n-type PbSe,which can regulate both electrical and thermal transport properties.For the electrical transport properties,the electron concentration of the PbSe-Cu system can be optimized over a wide temperature range due to the fast diffusion of Cu ions into the interstitial sites of PbSe,which is superior to other conventional n-type doping methods.For the thermal transport properties,unexpected low thermal conductivities for the PbSe system are also demonstrated due to the hierarchical phonon scattering.Consequently,both the peak z T and the device ZT of the Cu-doepd n-type PbSe system are substantially boosted.The maximum material z T and device ZT of the sample with x=0.00375 reach 1.45 and unity at T=813 K,respectively.Moreover,the dynamic doping effect has also been identified to boost the thermoelectric performance of the Cu intercalated PbS system,suggesting it maybe a generally applicable route to improve the TE performance of other IV–VI compounds.In the PbTe-Cu system,we propose an effective strategy involving the dynamic doping effect of interstitial Cu atoms to fully optimize the electrical-transport properties of n-type PbTe over a wide temperature range.For the electrical transport properties,by intercalating Cu,the temperature-dependent carrier concentration of PbTe is found to well match the theoretically optimal profile.Furthermore,high carrier mobility is largely maintained because the dynamic behavior of the interstitial Cu does not alter the band structure or change the effective mass.For the thermal transport properties,we found the negative effects of the dynamic doping on the thermal transport properties.The strong dynamic doping effect will lead to a dramatic increase of the heat capacity,which will lead to increased total thermal conductivity and decreased peak z T.Based on our findings,we further propose a concept of‘interstitial engineering'to reinforce the dynamic doping effect,which is of fundamental importance for optimizing the thermoelectric properties.Furthermore,we also propose a set of criteria to exhibit the dynamic doping effect:1)the extrinsic dopant shall have a proper atomic/ionic size because the temperature-dependent solubility of the extrinsic dopant must meet the requirement for carrier-concentration optimization for a given material;and 2)the extrinsic dopant and its dynamic behavior shall have little influence on both the electronic band structure and the carrier-scattering mechanism,which enables higher carrier mobility at a given carrier concentration.Consequently,a peak z T of?1.33 and a calculated leg efficiency of 12%have been achieved for the sample with 0.2at%Cu.Moreover,the reproductivity of the thermoelectric transport properties for all Cu doped PbTe samples has been examined.Finally,we have also tried the co-doping on PbTe-Cu systems.In the PbSe-PbS-Cu system,we have successfully controlled the degree of dynamic doping phenomenon at arbitrary temperature range through the“interstitial engineering”.For the electrical transport properties,the enlarged interstitial space of PbSe manipulated by alloying with PbS results in a prominent dynamic doping effect at high temperatures.Furthermore,the band gap and the density-sate-of effective mass for the conduction band of PbSe also increase through S alloying,leading to the increase in optimal carrier concentration at high temperatures.Thus,the power factor of Cu doped PbSe-PbS system at high temperatures is greatly boosted in comparison with the PbSe-Cu system.For the thermal transport properties,alloying PbS with PbSe has led to dramatically decreased lattice thermal conductivity due to the alloying scattering.Consequently,a peak z T of?1.6 are achieved for the 1.8 at%Cu doped(PbSe)0.6-(PbS)0.4 sample,which is superior to most Te-free lead chalcogenides.