Synergistically Optimizing Carrier Concentration and Decreasing Sound Velocity in n-type AgInSe₂ Thermoelectrics

The ternary chalcopyrite AgInSe₂ has emerged as a promising n-type thermoelectric material with ultralow lattice thermal conductivity. We report the thermoelectric transport properties of sodium-doped Ag_1-xIn_1+xSe₂ using an effective mass model. Temperature-dependent Hall coefficient measurements pave the way to understand the thermal activation behavior of carriers and the corresponding carrier scattering mechanisms. A Pisarenko plot shows that a single parabolic band model can accurately describe the transport behavior witnessed in the Seebeck coefficient and Hall carrier concentration in most samples. Furthermore, the lattice thermal conductivity of the doped samples approaches the theoretical minimum at high temperature, which is mainly due to the decrease of sound velocity. The intrinsically ultralow lattice thermal conductivity of AgInSe₂ can be attributed in part to several low-frequency optical phonon modes and strong acoustic-optical phonon hybridization due to an avoided crossing feature in our calculated phonon dispersions. With enhanced power factor arising from the optimization of carrier concentration and depressed lattice thermal conductivity, a maximum figure-of-merit zT of 0.74 was achieved for x = 0.01 with 1 mol % Na sample at 800 K, which doubles that of the pristine sample. The quality factor analysis implies that there is still much room for further improving their thermoelectric performance.