General Efficiency Theory of Thermoelectric Conversion

In this Letter, we show thermoelectric conversion efficiency is \emph{exactly} determined by \emph{three} independent material parameters $Z_{\rm gen}$, $\tau$, and $\beta$. Each parameter is a figure of merit hence improving $\tau$ or $\beta$ is an additional way to increase the efficiency. The $Z_{\rm gen}$ generalizes the traditional figure of merit \emph{zT}. Two degrees of freedom $\tau$ and $\beta$ reflect the temperature gradients of the material properties and are crucial to evaluate the heat current altered by non-zero Thomson heat and asymmetric Joule heat escape. Physical insights on high $\tau$ or $\beta$ materials explain why the single parameter approaches can be inaccurate for efficiency prediction.