Structure-transformation-induced abnormal thermoelectric properties in semiconductor copper selenide

Thermoelectric effects and related technologies have attracted a great interest due to world-wide energy harvesting. Thermoelectricity has usually been considered in the context of stable material phases. Here we report that the fluctuation of structures during the second-order phase transition in Cu ₂Se semiconductor breaks the conventional trends of thermoelectric transports in normal phases, leading to a critically phase-transition-enhanced thermoelectric figure of merit zT above unity at 400 K, a three times larger value than for the normal phases. Dynamic structural transformations introduce intensive fluctuations and extreme complexity, which enhance carrier entropy and thus the thermopower, and strongly scatter carriers and phonons as well to make their transports behave critically.