Advancing Thermoelectric Materials: A CALPHAD Approach to Charged Defects in Semiconductors

Thermoelectric (TE) devices that offer a route for energy recovery/production often need to be optimized by doping with other elements to improve the thermoelectric figure of merit (zT) which qualifies the efficiency of the device. The solubility of these elements in the bulk system is usually in such dilute concentrations that they can best be described as 0-D defects in an otherwise essentially uniform crystal. However, these defects/dopants are essential to the efficient performance of thermoelectric devices. As a result, an accurate understanding of the solubility limits of these dopants is necessary for the manufacture of high-performing Thermoelectrics. Phase diagrams, which are lines of equilibrium or phase boundaries at specific thermodynamic conditions, can be useful for visualizing the solubility limit. To that end, the method called Computer Coupling of Phase Diagram and Thermochemistry, or alternately Calculation of Phase Diagrams (CALPHAD) is an established technique for visualizing the phase diagram of material systems.<br/>In our previous work, we showed that CALPHAD is a viable tool to both visualize the effects of the dominant defects on the shapes of the phase diagram as well as provide an estimate of the defect concentration. We further extend those principles into describing charged defects in semiconductors. The CALPHAD technique has been used extensively as a thermodynamic framework to optimize the manufacture of alloys by modeling the Processing, structure, properties, and performance relationship. By developing a formalism for describing these charged defects in a CALPHAD framework, we set the stage for a more complete integration of CALPHAD with thermoelectric materials and semiconductors in general