Preparation And Performance Optimization Of SnSe And AgBiSe₂ Thermoelectric Materials With Low Thermal Conductivity

The energy crisis and environmental pollution impel the development of sustainable and clean energy technology.Thermoelectric materials can realize reversible conversion between heat and electricity,providing solutions to address the issue for sustainable development of energy.In recent years,Seeking intrinsically low lattice thermal conductivity materials is a way to develop new thermoelectric materials,but facing the difficulty of improving electrical transport properties.SnSe and AgBiSe₂ are typical low thermal conductivity semiconductors.Single crystal SnSe has attracted much attention for its excellent thermoelectric performance.However,polycrystals SnSe exhibit unsatisfactory figure-of-merit,especially for n-type SnSe.AgBiSe₂ has the ambipolar doping bottleneck but with strong n-type doping.P-type AgBiSe₂ has been predicted as a promising room-temperature thermoelectric material,but the origin of its p-type conduction is not yet fully understood and high-performance p-type AgBiSe₂ has not yet been experimentally achieved.Additionally,n-type AgBiSe₂ exhibits excellent thermoelectric performance in the high-temperature cubic phase,but its performance around room temperature is poor.In this paper,the method of doping,alloying,or defect engineering is used to improve the thermoelectric performance of n-type SnSe,p-type and n-type AgBiSe₂.The main results are as follows:The thermoelectric performance of n-type polycrystalline SnSe is improved by heavy Br doping at the Se site.The results show that a high concentration of Br doping effectively increases the carrier concentration from 1.6×1017 cm-3（p-type）in undoped SnSe to 1.3 × 1019 cm-3（n-type）in Br-doped SnSe0.88Br0.12,which enhances the electrical conductivity and power factor.Simultaneously,the composition fluctuation and dislocations induced by the substitution of Br on the Se site effectively enhance the phonon scattering and reduced the lattice thermal conductivity.As a result,a peak ZT of 1.3 at 773 K is obtained in SnSe0.9Br0.1 along the hot-pressing direction,which is 330%higher than that of undoped SnSe.This work impels the application process of n-type polycrystalline SnSe-based thermoelectric devices.A new way for preparing polycrystalline SnSe through high-energy ball milling and hot forging is proposed in this work and the thermoelectric performance of n-type polycrystalline SnSe is optimized by doping lanthanides at the cation site.The ball-milling process can strengthen the formation of donor-like defects in SnSe,leading to a conversion from p-to n-type with increasing ball-milling time.Further doping Pr at the Se site increases the carrier concentration,thereby ehancing the electrical conductivity.As a result,a peak ZT of 0.7 at 773 K is obtained in Sn0.97Pr0.03Se sample.A higher degree of orientation is achieved by hot forging,leading to a lowered thermal conductivity parallel to the hot-pressing direction.Finally,the maximum ZT value of the hot-forged Sn0.97Pr0.03Se sample increases to 0.9 at 773 K.Doping with rare earth elements Y,La,Ce,or Dy can also increase the electrical conductivity of n-type SnSe,thereby improving the thermoelectric performance.Among them,Ce has the best doping effects,and the peak ZT value of Sn0.97Ce0.03Se sample reaches 0.9 at 773 K.Additionally,Pr or Ce doping increases the phase transition temperature of SnSe,which is beneficial to expand the working temperature range.This work provides a new method of electronic doping for SnSe-based materials.The defect chemistry of AgBiSe₂ is revealed in this work.The intrinsic Se vacancies are the main defect in AgBiSe₂ and lead to the intrinsically n-type semiconductor.The stable and repeatable p-type AgBiSe₂+x based materials are successfully prepared by adding the excess amount of Se.On this basis,Pb doping at the Bi site is used to optimize the carrier concentration for further improving the thermoelectric performance of AgBi1-xPbxSe2.12.It is demonstrated that the hole concentration increases from 8.6×1017 cm-3 to 2.1×1019 cm-3 with increasing Pb content,thereby effectively increasing the power factor.First-principles calculations show that the valence band has much much higher band degeneracy than that of the conduction band,which explains the internal mechanism of high electrical properties of p-type AgBiSe₂.Combined with the intrinsic lower thermal conductivity,a decent ZT of 0.2 at room temperature and a peak ZT of 0.5 at 450 K are achieved in AgBi1-xPbxSe2.12.The optimized carrier concentration has been predicated to be as high as 2×1020 cm-3 using the theoretical model,indicating that further noticeable enhancement in the thermoelectric performance of p-type AgBiSe₂ would be very likely if the hole concentration can be further improved.In addition,doping with Na,Mg or Ca also increase the hole concentration of AgBiSe₂ and improve the thermoelectric performance.The maximum ZT value～0.7 at 473 K is obtained in AgBi0.98Ca0.02Se2,which is the highest value for the p-type AgBiSe₂-based materials.This work indicates that defect engineering is highly promising for overcoming the doping bottleneck,and lays the foundation for the subsequent optimization of the thermoelectric performance for p-type AgBiSe₂ materials.The room-temperature cubic structure of（AgBiSe₂）1-x（SnSe）x alloys are designed and the thermoelectric performance of the alloys is optimized by introducing the Se vacancy.The results show that the alloy is the rhombohedral phase when 0<x<0.12,and changes from a rhombohedral to a cubic phase with increasing temperature.When 0.12≤x<0.18,the alloy is a mixture phase of cubic and rhombohedral phase,but the alloy changes into the cubic phase when x≥0.18.With increasing SnSe content,the carrier concentration,power factor,and thermal conductivity of the alloys first decrease and then increase.When x=0.06,the maximum ZT value reaches 0.66 at 723 K,which is 46%higher than that of AgBiSe₂.On this basis,the carrier concentration of rhombohedral phase Ag0.94Bi0.91Sn0.06Se1.94-y alloys is optimized by adjusting the Se vacancies.With the Se content decreasing,the carrier concentration,mobility,and power factor first increase and then decrease.At the same time,the lattice thermal conductivity first decreases and then increases.When y=0.02,the average ZT between 300 and 723 K reaches 0.44,which is 100%higher than that of AgBiSe₂,and is the highest value among that of previous reported n-type AgBiSe₂-based materials.By studying the thermoelectric properties of cubic phase Ag0.82Bi0.82Sn0.18Se1.82-x alloys,it is found that the carrier concentration and power factor increase after reducing the Se content.However,the contribution of the electronic thermal conductivity to total thermal conductivity increases at high temperature range simultaneously.Finally,when z=0.01,the peak ZT of the alloy at 723 K is 0.52.