- High thermoelectric conversion efficiencies can be achieved by making use of materials with, as high as possible, figure of merit, ZT, values. Moreover, even higher performance is possible with appropriate geometrical optimization including the use of functionally graded materials (FGM) technology. Here, an advanced n-type functionally graded thermoelectric material based on a phase-separated (PbSn0.05Te)0.92(PbS)0.08 matrix is reported. For assessment of the thermoelectric potential of this material, combined with the previously reported p-type Ge0.87Pb0.13Te showing a remarkable dimensionless figure of merit of 2.2, a finite-element thermoelectric model is developed. The results predict, for the investigated thermoelectric couple, a very impressive thermoelectric efficiency of 14%, which is more than 20% higher than previously reported values for operating under cold and hot junction temperatures of 50 °C and 500 °C, respectively. Validation of the model prediction is done by a thermoelectric couple fabricated according to the model's geometrical optimization conditions, showing a good agreement to the theoretically calculated results, hence approaching a higher technology readiness level.