Improving Thermoelectric Preformance Of P-type Single Crystal SnSe_1-xS_x（0≤x≤0.5） Via Band Engineering

Thermoelectric conversion technology can directly convert heat into electric power,which provides an effective route for harvesting waste heat.In the past two decades,the ‘phonon-glass,electron-crystal’ concept has inspired many achievements in the TE materials research,leading to a great improvement for TE performance.However,almost of all study focus on polycrystalline materials rather than single crystal.In this study,we systematically investigate SnSe-based single crystal materials with layered structure and ultralow lattice thermal conductivity.Single crystals are grown by the modified Bridgman-Stockbarge method.Combing with electronic band theory,lattice dynamics and First-principle calculations,we are capable of making in-depth insight into the relation among the electrical and thermal transport,electronic structure and lattice structure in this material,which provides an experimental and theoretical fundament for follow-up study.The derails are summarized briefly as follows:（1）The electronic structure of SnSe calculated by Density Functional Theory shows it has a band gap of 0.62 e V with the top of valance band mainly composed of Se 4pz orbitals and the bottom of conduction band composed of Sn 5py and 5px orbitals.The distribution of charge density indicates the appearance of lone-pair electrons that lead to a strong anharmonicity along the interlayer.The relative potential energy as a function of atomic off-center displacement also indicates a strong anharmonicity along the a-and c-axis.All above factors lead to the ultralow lattice thermal conductivity of SnSe.,The analysis of Cp data combined with the phonon density of states（DOS）indicates the slight local vibration for SnSe.Utilizing the lattice dynamics approach,the lattice thermal conductivity dominated by the three phonon interaction is calculated and the results are consistant with our experimental data.A number of optical modes with high group velocity are discovered in phonon dispersion curve and are estimated to make 50% contribution to the lattice thermal conductivity,even up to 85%.（2）Combining experimental with computational results,we confirm that Na-and Ag-doping can shift the Fermi level to the position with multi-band feature and modify the band edge in SnSe system,leading to a two orders of magnitude improvement on carrier density and electrical conductivity.As a result,the room temperature value of power factor for the doped sample is 2.8 m Wm-1K-2,which is an order of magnitude higher than that of the undoped crystal in the same direction.Finally,the average z T of 1.17 is attained from 300 K to 800 K along the b-axis of 3 at% Na-doped SnSe,with the maximum z T reaching 2 at 800 K.（3）The electronic structure of SnSe1-x Sx solid solutions is investigated via the First-principle calculations.With the increase of S element,the third and fourth valence band（GZ2 and GA2）will converge to the first band（GZ1）in SnSe1-x Sx solid solutions and the energy degeneration occurs as x = 0.4,while the energy offset between first band（GZ1）and second band（GA1）is maintained.The temperature-dependent Cp below 300 K indicates the existence of slight local vibration for SnSe1-x Sx materials.Cp increases around 300 K but decreases aroud 3 K with the increase of S content in that the S atoms incline to excite phonons with high frequency as suggested by the lattice dynamics analysis.Meanwhile,the results of experiments show that the increasing S content can postpone the phase transition and thus maintain the low lattice thermal conductivity.Hence,a maximum z T of 1.1 at 823 K is obtained along the b-axis for the SnSe0.7S0.3 sample without doping in constrat to the maximum z T value of 0.82 at 773 K is obtained for pure SnSe.（4）In Sn（1-x）Na_xSe_0.9S_0.1 single crystal,the results from calculations and experiments indicate that the electrical properties of SnSe_0.9S_0.1 are characterized by a two band model transport behavior with carrier density up to 2×1019 cm-3 and a four band model transport behavior as carrier density up to 8×1019 cm-3.Comparing to the power factor of doped SnSe,the power factor of Sn0.97Na0.03Se_0.9S_0.1 reaches 4.2 m Wm-1K-2 at room temperature due to the participatation of multiple bands in electrical transport,with a 40% improvement at least below 600 K.The experimental results show the great decrease in the lattice thermal conductivity for the samples with Na-and S-doping.Based on the analysis of the charge density difference,the point defects produced by isoelectronic substitution（S/Se）cannot effectively scatter phonons,whereas the generation of “pseudo point defects” induced by anisoelectronic substitution（Na/Sn）via affecting other atoms around Na and in the adjacent Van der Waals layer can greatly scatter phonons to reduce lattice thermal conductivity.Finally,through synergistic band engineering strategy for single crystalline SnSe,a high z Tave over a wide temperature range approaching 1.53 in the range from 300 K to 773 K and 1.60 in the range from 300 K to 923 K in Na-doped SnSe_0.9S_0.1 solid solution single crystals,and the maximum z T of 2.3 at 773 K are obtained.