Optimizing Band Structure And Thermoelectric Properties Of SnSe-based Single Crystals

The emerging global energy crisis and concern for the environment have intensified interest in more effective means of power generation with sustainable and clean energy sources.More than 60%of the energy produced is wasted as heat.Thermoelectric materials have sparked considerable interest due to their effective solid-state mutual energy conversion between waste heat and useful electrical energy.Thermoelectric technology has received a lot of attention as a next-generation,environmentally-friendly energy source.The low performance of the thermoelectric materials is now the most challenging for commercialization and practical application of the technology.The thermoelectric（TE）materials that have been studied the most in the past,such as bismuth telluride（Bi₂Te₃）and lead telluride（PbTe）have high thermoelectric performance through various strategies.Still,they have been a bottleneck for more comprehensive practical applications by the presence of toxic lead（Pb）or very expensive tellurium（Te）.Tin selenide（SnSe）has received a lot of attention due to its enormous potential for realizing thermoelectric devices that are high-performing,non-toxic,and cost-effective.SnSe is a layered orthorhombic-structured material with strong anisotropy.SnSe single crystals have emerged in the thermoelectric material systems for their excellent thermoelectric performance and have attracted many researchers to study them in depth.This doctoral dissertation will investigate the optimizing the band structure and thermoelectric properties of SnSe,an eco-friendly material among various thermoelectric materials.1.In this context,SnSe-based single crystals with high thermoelectric performance are successfully fabricated using the modified vertical Bridgman method.Pb and Zn codoping converges the energy offset between multiple valence bands by significantly modifying the band structure,enhancing the Seebeck coefficient.The carrier concentration and electrical conductivity can be optimized,leading to an enhanced power factor.The Dual-atom point-defect effect created by the substitution of Pb and Zn in the SnSe lattice introduces strong phonon scattering,significantly reducing lattice thermal conductivity as low as 0.284 W m^-1K^-1.Excellent thermoelectric properties in Sn_0.99-xPb_xZn_0.01Se crystals have achieved via valence band convergence and point defect engineering strategies.As a result,a maximum ZT of 1.9 at 773 K is achieved in Sn_0.93Pb_0.06Zn_0.01Se crystals along the bc-plane direction.This study highlights the crucial role of manipulating multiple electronic valence bands to further improve the SnSe thermoelectrics.2.A high ZT value of～1.01 at 773 K is realized in Sn_0.95Se single crystal through the compelling strategy of introducing cation vacancies(V_Sn).Vacancy engineering is an effective approach to tune the electrical performance through optimizing the carrier concentration and along this,vacancies can also serve as phonon scattering centers to reduce the thermal conductivity.The electrical and thermal transport properties of Sn_0.95Se crystal are improved compared to pristine SnSe.Sn_0.95Se sample shows a high power factor value of～6.57μW cm^-1 K^-2 and low lattice thermal conductivity with～0.470 W m^-1 K^-1 at773 K along the bc-plane,contributing to improve the ZT value.3.The effect of Pb alloying combined with Sn vacancies on the thermoelectric properties of SnSe single crystals synthesized by the modified Bridgman method is demonstrated.The study found that enhanced carrier concentration induced by Pb doping and introducing Sn vacancies increase the electrical conductivity.Enhanced Seebeck coefficient followed by the density of states enhancement contributes to optimizing the power factor of Sn_0.95-xPb_xSe crystals.As Pb alloying has a significant effect on modifying the multiple valence bands.Modulated Sn vacancies along with Pb substitutional doping significantly enhance the lattice anharmonicity of SnSe crystals and substantially increase the phonon-defect scattering by suddenly changing the local bond environment in its lattice structure.Pb substitution and Sn vacancies induce strong phonon scattering centers.Lattice anharmonic properties of SnSe and point-defect have resulted in significantly low lattice thermal conductivity.Crystal with composition Sn_0.92Pb_0.03Se has exhibited the lowest K_latof 0.297 W m^-1K^-1in all the synthesized samples.Combining the above two favorable factors in terms of electrical and thermal transport performance,Sn_0.92Pb_0.03Se crystal achieved a maximum ZT of 1.74 at 773 K.Introducing cation vacancies and band engineering through Pb alloying provide a perspective to realize high thermoelectric performance in thermoelectric materials.