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A2: Medium temperature materials
Measurements and modeling of electronic transport properties of p-type Cu2Se and n-type Ag2Se suggest that these materials could be useful thermoelectrics with figure of merit zT greater than 1 at 300 K and above. The exceptional performance can be traced to the extremely low thermal conductivity associated with the atomic disorder and liquid-like behavior of the copper and silver atoms. In addition the conduction band appears to have very high mobility leading to a high quality factor and therefore high potential zT if precisely doped, but it also makes the optimum dopant concentration small and therefore difficult to tune.
These materials also show a dramatic enhancement in Seebeck coefficient and zT when coupled with the second order phase transition to the liquid-like state. We interpret this as due to the coupling between the structural and charge carrier entropy which enhances the thermopower.
Chemical derivatives and related sulfides and tellurides show similar behavior providing new opportunities for earth abundant thermoelectric materials. These new mechanisms for achieving high zT provide new opportunities to study the complex, coupled transport physics. The added challenges for long term use due to the mobile ions also lead to new opportunities to engineer materials and systems to address these issues.
