Establishing the Golden Range of Seebeck Coefficient for Maximizing Thermoelectric Performance.

The coupling nature of thermoelectric properties determines that optimizing the Fermi level is the priority to achieve a net increase in thermoelectric performance. Conventionally, the carrier concentration is used as the reflection of the Fermi level in the band structure. However, carrier concentration strongly depends upon the material's effective mass, leading to that the optimal carrier concentration varies over a large scale for different materials. Herein, inspired by the big data survey, we develop a golden Seebeck coefficient range of 202-230 μV K-1 for thermoelectric semiconductors with lattice thermal conductivity of 0.4-1.5 W m-1 K-1. When the measured Seebeck coefficient reaches this range, the corresponding figure of merit is maximized. Using this approach, we exemplarily analyze the characteristics of n-type Pb1-xBixSe thermoelectric materials. With detailed electron microscopy and property characterizations, the high densities of dislocations and pores are found to be responsible for a low lattice thermal conductivity. Moreover, Bi substitution significantly tunes the Seebeck coefficient in a wide range. As a result, the Seebeck coefficient of ∼ -230 μV K-1 in Pb0.98Bi0.02Se is close to the golden range, leading to a figure of merit beyond 1.5. This finding provides an intuitive metric to determine the optimization extent of thermoelectric performance.