Device-Level ZT Correlation for Validating Single-Leg Efficiency in GeTe Thermoelectrics

Although numerous high-zT GeTe systems have been reported, the correlation between zT and conversion efficiency η remains poorly defined. Reporting peak or even average zT often leads to an overestimation of η as these metrics assume temperature-independent transport properties-an unrealistic simplification. In 2017, the thermoelectric device ZT for a single material (making either n- or p-type leg) was introduced to evaluate the maximum efficiency of a thermoelectric leg with temperature-dependent properties. This work provides an experimental validation of Device ZT as a practical metric for predicting thermoelectric conversion efficiency. By combining calibrated single-leg efficiency measurements, heat-flow analysis, and finite-element simulations, we establish a direct correlation between material transport properties, Device zT, and experimentally measured device performance. The CuSb-GeTe leg achieves an efficiency of 5.4% at ΔT = 400 K, while conventional zT_avg-based estimates overpredict performance by nearly twofold. In contrast, Device ZT accurately reproduces the measured efficiency. These findings provide an experimentally validated framework for translating material-level thermoelectric properties into realistic device performance and establish Device ZT as a practical metric for evaluating thermoelectric devices.