Intermediate-level doping strategy to simultaneously optimize power factor and phonon thermal conductivity for improving thermoelectric figure of merit

The conventional doping strategy for thermoelectric materials generally focuses on a shallow donor/acceptor model with the energy level close to the band edge as for electronic devices. However, thermoelectric devices operate over a large temperature difference, and the optimal carrier concentration increases with increasing temperature. A shallow level cannot meet the requirement over a large temperature range. Here, an innovative strategy of introducing an intermediate level is proposed. Such an intermediate level introduces more carriers with increasing temperature, consistent with the trend of increasing optimal doping concentration with temperature, enabling larger power factor over a broader temperature range. Furthermore, the intermediate level typically requires more impurities, leading to increased phonon scattering. This strategy allows simultaneous optimization of carrier concentration over a wide temperature range and suppression of thermal conductivity via stronger point-defect phonon scattering. Experimental results from heavily-doped ZrCoSb employing shallow, intermediate, and deep levels successfully corroborate this strategy, where simultaneously improved power factor and figure of merit are obtained by introducing an intermediate level. Our work indicates that the performance of known thermoelectric materials should be reevaluated by introducing an intermediate level to unleash their full potential.