Layered Materials With Excellent Thermoelectric Performance At Mid-temperature Studied By First-principles Calculation

In 2020,the Chinese government made a solemn promise to the international community of"carbon peak and carbon neutrality".The key to achieving the ambitious goal lies on vigorously promoting clean energy and improving energy efficiency.In the industrial field,the low-quality waste heat whose temperature is located in the medium temperature zone（60-225°C）accounts for about 15%of the total energy consumption of the whole society.A large amount of industrial waste heat resources are not effectively used and are directly discharged into the environment with the medium,causing serious environmental pollution and energy waste.Thermoelectric materials are environmentally friendly new energy materials that can directly convert heat and electric energy.Their thermoelectric power generation effect has great advantages and application prospects in the recovery and utilization of low-quality industrial waste heat.However,the conversion efficiency of traditional thermoelectric materials in the mid-temperature region is not high enough,which severely restricts their large-scale application.Therefore,it is of great scientific significance to find materials with excellent thermoelectric properties in the mid-temperature region.In recent years,due to the development of nanotechnology,new thermoelectric materials have been discovered,among which layered materials have attracted widespread attention due to their special crystal structure and transport properties.Based on the first principles,this paper systematically studies the band structure and phonon spectrum of single-layer Ge XS（X=P,As）and single-layer Bi In X（X=P,As）,the transport properties of electrons and phonons,and the thermoelectric figure of merit.The electronic energy band structure obtained by first-principles calculations shows that single-layer Ge XS（X=P,As）are semiconductors with band gaps of 1.85 e V and 1.87 e V,and their valence bands exhibit obvious double degeneracy,so they have a large Seebeck coefficient,and the single-layer Ge As S reaches 2070μVK^-1 in the Armchair direction.Through the phonon Boltzmann transport equation,it is found that Ge XS（X=P,As）exhibits very low lattice thermal conductivity at room temperature,which is0.86 and 0.69 Wm^-1K^-1 in the Armchair direction,respectively.The Zigzag directions are 0.96and 0.82 Wm^-1K^-1,respectively,which are attributed to their small phonon group velocity,large Greenisen parameter,and low phonon relaxation time.Using the calculated electron transport parameters and phonon transport parameters,we found that Ge XS（X=P,As）has a higher thermoelectric figure of merit at 500K when it is at the best n-type doping.The maximum values of the two directions are 3.06（Armchair direction）and 3.51（Zigzag direction）,as well as 3.21（Armchair direction）and 2.54（Zigzag direction）.Using the same method,we also studied the thermoelectric properties of another layered compound Bi In X（X=P,As）.The calculation results show that the small phonon group velocity,large Green Eisen parameter,and low phonon relaxation time of these two materials effectively suppress their phonon transport ability.The thermal conductivity is only 0.64 and0.35 Wm^-1K^-1 along the Armchair direction at room temperature,and 0.81 and 0.27 Wm^-1K^-1along the Zigzag direction.These two materials belong to direct band-gap semiconductors,with band gap values of 1.28 and 1.04 e V,respectively.The double degeneracy of the valence band at theГpoint and the steep electronic density of states near the Fermi level effectively increase the Seebeck coefficient.At 500K temperature,optimized n-type doping concentration,the maximum thermoelectric figure of merit of single-layer Bi In P and Bi In As are 3.36（Armchair direction）and 3.50（Zigzag direction）,respectively.The two types of layered compounds studied in this paper both show excellent thermoelectric properties in the mid-temperature region,indicating that they have potential applications in waste heat power generation.In addition,some of their commonalities（for example,the larger atomic weight can lead to lower phonon group velocity and lower thermal conductivity,the fold structure can suppress the transport of phonons,and the degeneracy of the energy band can lead to the improvement of electron transport performance）are useful for the searchment of"electronic crystal-phonon glass"thermoelectric materials..Our theoretical calculation research will also stimulate experimental scientists’interest in the preparation,characterization,and thermoelectric performance tuning of such materials.