Title: Realizing High Thermoelectric Performance of Ag/Al Co-Doped Polycrystalline SnSe through Band Structure Modification and Hydrogen Reduction

Abstract: Single crystal tin selenide (SnSe) has a recorded high thermoelectric figure of merit (ZT) value of 2.6 at 923 K, but it is easy to form mechanical cracks and difficult to apply in thermoelectric conversion devices. Polycrystalline SnSe has better mechanical properties but inferior ZT values, which needs further optimization for applications. This work aims at enhancing the thermoelectric performance of polycrystalline SnSe through synergistic optimization of sliver (Ag) and aluminum (Al) co-doping with the hydrogen reduction. The effects of Ag and Al doping on electronic transport properties are systematically investigated by density functional theory calculation and experimental measurement. Compared with pristine SnSe, Ag doping can effectively increase the hole concentration to 1.58 × 10¹⁹ cm⁻³ and improve the conductivity. Results also indicate that using Al dopant may slightly decrease the hole concentration but reduce the thermal excitation temperature, and introduce point defects reducing the lattice thermal conductivity through scattering phonons. In addition, the hydrogen reduction of sample powders before synthesis can effectively remove Sn oxides and reduce lattice thermal conductivity. At last, a state-of-the-art maximum ZT value of 1.69 at 823 K is obtained in Ag_0.01Al_0.01Sn_0.98Se. This study provides a theoretical basis and technical guidance for designing high-performance polycrystalline SnSe.