Highly enhanced thermoelectric performance of halogen element modified SnSe2 materials via point defect engineering

Abstract

Lead-free SnSe₂ materials have intrinsic low thermal conductivities and are promising thermoelectrics. However, their relatively low electrical conductivity limits the improvement of thermoelectric performance. This study successfully introduced halogen elements (X = F, Cl, Br, and I) into SnSe₂ to increase carrier concentration for better thermoelectric performance. The defect formation energy calculation results showed the lowest value for Se_Cl, indicating that incorporating Cl into the SnSe₂ matrix could achieve higher thermoelectric performance. Experimental results further verified this conclusion. At 323 K, the electrical conductivity of the SnSe_1.94Cl_0.06 sample is 86 S cm⁻¹, 61 times higher than that of pristine SnSe₂. Combined with the DFT calculation results, the decreased bandgap of the Cl-doped sample benefitted the transport of carriers. Due to the increased electrical conductivity, a peak power factor value of 457 μW m⁻¹ K⁻² is achieved at 473 K. Meanwhile, the formed point defects decreased the lattice thermal conductivity to 0.38 W m⁻¹ K⁻¹ at 773 K, 14% lower than that of the pristine SnSe₂ sample. Consequently, the SnSe_1.97Cl_0.03 sample obtains a peak ZT value of 0.63 at 773 K, five times higher than that of pristine SnSe₂. This strategy of thermoelectric performance optimization through the introduction of halogen elements can be applied to other thermoelectric material systems.