High thermoelectric performance of polycrystalline In4Se3−δ(CuI)x: synergistic effects of the Se-deficiency and CuI-doping
Polycrystalline n-type In4Se3-based materials are established and promising mid-temperature range thermoelectric materials. Herein, we report that synergistic effects of a Se-deficiency and CuI-doping significantly improve the thermoelectric properties of CuI-doped In4Se3−δ materials via increasing the power factor (PF), an increase of up to 105%, and enhancing the figure-of-merit (ZT), by about 80%, compared to the undoped ones. The CuI-doping (x) significantly lowers the electrical resistivity and enhances the power factor (PF); and the PF increases with increase of x. But the maximum solubility of the CuI-dopant (xm) is restricted by the amount of the Se-deficiency (δ), and it closely approaches the δ in the form of xm < δ. Thus, the greater the δ, the greater the PF. On the other hand, the greater the Se-deficiency, the more Cu that will occupy the intercalation sites, which increases the atom packing density and thus indispensably increases the thermal conductivity (κ); and consequently the lower the δ, the lower the κ. Such a synergistic effect ensures a balance between improving the PF and lowering the κ, so as to give rise to a considerable ZT enhancement. For example, In4Se2.70(CuI)0.2 and In4Se2.50(CuI)0.3 provide a ZTmax of 1.20 and 1.08, respectively, which are about 80 and 66% enhanced in comparison with the un-doped ones. Especially, a ZTmax = 1.34 at 723 K is obtained for In4Se2.95(CuI)0.01, representing the highest value obtained for Pb-free polycrystalline In4Se3-based materials to date. This insight offers guidance for further studies on these and related TE materials.