Theoretical Study On Thermoelectric Properties Of Two-dimensional Janus M₂P₂S₃Se₃（M=Zn,Cd） Monolayer

The shortage of fossil energy and the emission of greenhouse gas all over the world arouse the upsurge of exploring new energy.Thermoelectric technology can directly convert waste heat into electric energy for human utilize,which provides a simple and effective strategy for alleviating energy crisis and environmental pollution.Thermoelectric devices take the internal carrier as the working medium,which have the advantages such as stability,reliability,ease of operation,and environmental protection.However,their conversion efficiency is limited by the inherent properties of materials.The thermoelectric properties of various materials can be quantified using the dimensionless thermoelectric figure of merit（=/）and the thermoelectric power factor（=）.High performance thermoelectric materials should have high thermoelectric conversion power and efficiency,which requires materials with large Seebeck coefficient（）,high electrical conductivity（）and low thermal conductivity（）,is the sum of the electronic thermal conductivity（）and the lattice thermal conductivity（）.Whereas,,andare often unfavorably entangled with each other via concentration of charge carriers,band structure,and scattering of charge carrier.Thus,significant efforts have been devoted to exploration of suitable thermoelectric materials or optimization of the currently available thermoelectric materials to achieve highly efficient thermoelectric performance.In recent years,the study of two-dimensional Janus transition metal dichalcogenides（TMDs）has been stimulated with the synthesis of monolayer Mo SSe consisting of mirror asymmetric（Janus）triatomic layers.On account of the breaking of mirror symmetry,the Janus structures has a built-in electric field along the out-of-plane direction,and exhibit strong piezoelectric polarization and significantly large Rashba spin splitting,which is favored to the thermoelectric performance.The family of van der Waals layered transition-metal phosphorus trichalcogenides（TMPTs）is predicted to have excellent thermoelectric properties.The TMPTs possess most of the specific properties of TMDs,and their wider band gaps enable them to operate in a higher temperature environment,making two-dimensional Janus TMPTs highly attractive in thermoelectric applications.In this work,combining density functional theory with Boltzmann transport theory,the electronic structure,carrier and phonon transport properties,and thermoelectric properties have been systematically studied for two-dimensional MPSe₃ and Janus M₂P₂S₃Se₃（M=Zn,Cd）monolayers.The main contents and results in this paper are listed as follow:The two-dimensional MPSe₃ and Janus M₂P₂S₃Se₃（M=Zn,Cd）monolayers are indirect bandgap semiconductors.The high polarizability of M-Se/S bonds in the distorted octahedron MSe₆ and MS₃Se₃ inside the structure lead to anharmonic phonon behavior,which produce an intrinsic lattice thermal conductivityas low as 1.75,1.48,1.06 and 1.99 W m^-1 K^-1 at 300 K for Zn PSe₃,Cd PSe₃,Zn₂P₂S₃Se₃ and Cd₂P₂S₃Se₃monolayers,respectively,when the temperature rises to 1200 K,decreases to 0.44,0.38,0.27 and 0.50 W m^-1 K^-1,respectively.Theof Zn₂P₂S₃Se₃ monolayer is about40%lower than that of Zn PSe₃ monolayer,which is mainly due to the greater anharmonic scattering rate of the former.The lowerof Zn₂P₂S₃Se₃ is largely attributed to more pronounced flat modes of the phonon dispersion in the frequency range of 1–1.7 THz caused by the softer Zn-Se/S bonds.Moreover,the energy differences?of the multi-valley valence band for Zn₂P₂S₃Se₃ and Cd₂P₂S₃Se₃ is reduced from 0.17 and 0.22 e V of Zn PSe₃ and Cd PSe₃ monolayers to 0.04 and 0.17 e V,respectively,indicating that Janus M₂P₂S₃Se₃ monolayer structure has a trend of multi-valley valence band convergence,which is conducive to improving.Surprisingly,the polar optical phonon scattering（POP）plays a dominant role in carrier transport in the MPSe₃ and M₂P₂S₃Se₃ monolayers,which dramatically reduced carrier lifetime and conductivity by about an order of magnitude.Finally,Janus Zn₂P₂S₃Se₃ monolayer has potential application prospects as an n-type thermoelectric material.At 1200 K,thevalue of Zn₂P₂S₃Se₃ monolayer is0.86 at the optimum electron concentration of 1.5×10¹³ cm^-2 and 0.67 at the optimum hole concentration of 3×10¹³ cm^-2.The results show that the asymmetric Janus Zn₂P₂S₃Se₃ monolayer is expected to become an excellent n-type high-temperature thermoelectric material.