Theoretical Study On The Thermoelectric Properties Of α-In₂Se₃

The global energy crisis becomes increasingly serious as the traditional fossil fuel cannot meet people’s demand and also brings environmental pollution.It is thus of urgent necessary to develop renewable and clean energy.Thermoelectric materials can realize the conversion of electricity and heat,which has attracted extensive attention from the science community.The performance of thermoelectric materials is usually determined by a dimensionless figure of merit:ZT（28）S ²sT/k,where T,S,σandκare respectively the absolute temperature,the Seebeck coefficient,the electrical conductivity and the thermal conductivity（including electronic and phonon contributions）.As these transport coefficients are usually coupled with each other and related to the carrier concentration,it is quite difficult to obtain a high ZT value for a given system.Recently,it has been found that In₂Se₃is a semiconductor with unique layered structure and moderate band gap,which suggests lower lattice thermal conductivity and potential thermoelectric applications.In this thesis,we use first-principles calculations to study the structural,electronic,phonon and thermoelectric transport properties of several bulk and two-dimensionalα-In₂Se₃.We first focus on the so-called 2H phase ofα-In₂Se₃.It is found that the system has an indirect band gap of 1.12 e V with a large degeneracy and strong dispersion near the conduction band minimum,which suggests higher n-type power factor.On the other hand,the existence of several different chemical bonds enhances the scattering of phonons and causes small lattice thermal conductivity.Meanwhile,the inter-layer van der Waals interaction leads to even smaller thermal conductivity along the z-direction.As a result,the ZT value of n-type system can reach 0.9（x-direction）and1.8（z-direction）at 550 K.We then investigate the crystal structures and thermoelectric properties of 3R phase ofα-In₂Se₃.The calculated result shows that the system is semiconducting with an indirect gap of 1.07 e V.Similar to the 2H phase,the 3R-α-In₂Se₃also has layered structure with exactly the same building block.However,the tighter stacking of 3R phase decreases the inter-layer distance and further reduces the lattice thermal conductivity along the z direction.Combined with its higher power factor,the ZT value（z-direction）of n-and p-type systems can be respectively enhanced to as high as3.0 and 2.0 at 550 K.In addition,we find that both 2H and 3R phases has the same building block of quintuple layer,which exhibits a larger band gap of 1.40 e V.Although the layer exhibits a higher Seebeck coefficient,its electrical conductivity and the power factor remains lower.As a consequence,the ZT value of the quintuple layer cannot be optimized significantly,which is found to be even lower than that of the bulk counterpart.However,it is worth noting that the lattice thermal conductivity of the system remains small,and it is expected that by doping or strain we can realize band convergence so that the thermoelectric performance of the quintuple layer can be enhanced.