Efficient operation of a thermoelectric device requires many variables to optimize. Achieving the maximum zT of a typical thermoelectric semiconductor requires optimization of the carrier concentration. Assuming the carrier concentration can be optimized the maximum zT  is then determined by the quality factor which depends on the material parameters of the semiconductor – effective mass, carrier pocket degeneracy, deformation potential, lattice thermal conductivity which leads to a rational consideration of the trade-offs. For example, many unconventional electronic structures that increase effective mass are being studied or proposed for high efficiency thermoelectric materials.  However high effective mass of the carriers due to flat bands results in low mobility, which leads to lower zT. Instead, high DOS effective mass due to high valley degeneracy leads to high zT.  For example, utilizing the high degeneracy second valence band in PbTe leads to nearly twice the zT than that of the low degeneracy first valence band.  Alloying with point defects is another example where both benefit and detriment occur from the same process. By analyzing the effect on the quality factor a rational criteria can be derived to determine and quantify the benefit of alloying. Finally these parameters are all temperature dependent and by considering the temperature dependence of the quality factor, band gap and optimal carrier concentration further advances in overall efficiency can be engineered.

 

Yanzhong Pei, Heng Wang and G. Jeffrey Snyder “Band Engineering of Thermoelectric Materials” Advanced Materials 24, 6125 (2012)

Heng Wang, Yanzhong Pei, Aaron D. LaLonde and G. Jeffrey Snyder “The Criteria for Beneficial Disorder in Thermoelectric Solid Solutions” Advanced Functional Materials23, 1586 (2013)