Thermoelectric device performance beyond average ZT: Holistic consideration of materials and design

In the current era where many renewable energy technologies are commercially mature and optimized, thermoelectrics remain relatively under-utilized. This can be partly attributed to their limited device efficiencies despite their high materials performances. On the materials level, the performance of thermoelectrics can be defined by zT or average zT, which is directly correlated to device power conversion efficiency. Due to this direct correlation, it is a longstanding assumption that maximizing average zT will result in higher efficiency. Although this practice holds true for many cases, it does not provide a complete picture due to the complex interplay between electronic and thermal transport properties. In this work, we surveyed the literature to find that high average zT does not always translate to high efficiency. Instead, it was found that compatibility factor, which dictates the optimal ratio of electrical and heat current passing through the material, is equally important in determining the device efficiency. Other factors such as electrical and thermal impedance matching, as well as contact resistance are also discussed. This review concludes with food for thought on future thermoelectric research beyond average zT to realize the untapped potential in this special class of materials.