Thermoelectric degrees of freedom determining thermoelectric efficiency

For over half a century, the development of thermoelectric materials has based on the dimensionless figure of merit zT, assuming that the efficiency is mainly determined by this single parameter. Here, we show that the thermoelectric conversion efficiency is determined by three independent parameters, Z(gen), tau, and beta, which we call the three thermoelectric degrees of freedom (DoFs). Z(gen) is the well-defined mean of the traditional zT under nonzero temperature differences. The two additional parameters tau and beta are gradients of material properties and crucial to evaluating the heat current altered by nonzero Thomson heat and asymmetric Joule heat escape. Each parameter is a figure of merit. Therefore, increasing one of the three DoFs leads to higher efficiency. Our finding explains why the single-parameter theory is inaccurate. Further, it suggests an alternative direction in material discovery and device design in thermoelectrics, such as high tau and beta, beyond zT.

Automated summary

The traditional theory of thermoelectric materials based on a single parameter zT is inaccurate. The thermoelectric conversion efficiency is actually determined by three independent parameters: Z(gen), tau, and beta. Increasing any of these three degrees of freedom can lead to higher efficiency, suggesting a new direction in material discovery and device design beyond zT.