Effects of codoping on tin selenide nanomaterials to enhance the thermoelectric performance above the ambient temperature range

Abstract

An expansive effort has been made to optimize the thermoelectric performance of tin selenide (SnSe) due to its potential for waste heat recovery. p-Type co-doped SnSe with various doping percentages of copper (Cu) and silver (Ag) was synthesized hydrothermally to optimize the thermoelectric parameters. Temperature variation measurements of the thermoelectric parameters viz. Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) were performed for pristine, Cu-doped, and Cu/Ag codoped SnSe samples with a variation of the dopant percentages. Maximum S = 1225 μV K⁻¹ was obtained for pristine SnSe at a temperature of 373 K. The doped samples, comparatively, yielded lower S but higher σ coupled with lower κ. XPS results confirm the incorporation of Ag in the SnSe lattice with 0/+1 valence state, which is evidence for reduced thermal conductivity and enhanced electrical conductivity for Ag codoping in Cu-doped SnSe. DFT calculations have revealed the occurrence of point defects due to lattice distortion at the Ag and Cu dopant sites resulting in lattice anharmonicity-induced improved phonon scattering. Phonon and thermodynamic calculations also demonstrated a drastic decrease of 16.1% of specific heat capacity (C_p) for the Ag and Cu co-doped SnSe compared to pristine SnSe, along with a remarkable decrease in electron effective mass, group velocity at the BZ boundaries and phonon mode softening, thereby unraveling the origin of the improved performance. Maximum ZT = 0.54 was obtained for the SSCA-7-2 (SnSe + 7% Cu + 2% Ag) sample at 383 K.