Thermoelectric power factor of composites

To improve the performance of thermoelectric materials, a highly effective and widely implemented approach is to create multiphase composites. These composites are designed to impede phononic heat transport, which accounts for the majority of thermal conductivity in conventional thermoelectric semiconductors. In 1999, Bergman and Fel [J. Appl. Phys. 85(12), 8205-8216 (1999)] reported that also the electronic properties, specifically the power factor S2 sigma, can be significantly enhanced in two-phase composites consisting of a highly conducting, simple metal anda high-performance thermoelectric arranged in an optimal manner, sparking great experimental interest. In this work, we challenge the theoretical results of Bergman and Fel and the conclusions drawn therein by utilizing a simple serial model. We show that, while the improvement of the power factor is indeed extraordinary, the results lead to a misleading interpretation of the overall thermoelectric performance of the material. As a result, we argue that the power factor is not a suitable metric for evaluating multiphase materials and composites, and that the figure of merit zT must be used instead. Nonetheless, we demonstrate that the best thermoelectric composite consists of a highly conductive metal and a high-performance thermoelectric.