High Thermoelectric Figure of Merit via Tunable Valley Convergence Coupled Low Thermal Conductivity in AIIBIVC2V Chalcopyrites

Development of efficient thermoelectric materials requires a designing approach that leads to excellent electronic and phononic transport properties. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we report unprecedented enhancement in electronic transport properties of A(II)B(IV)C(2)(V) (group II = Be, Mg, Zn, and Cd; group IV = Si, Ge, and Sn; and group V = P and As) chalcopyrites via isoelectronic substitution. Multiple valleys in conduction bands, present in these compounds, are tuned to converge by substitution of group IV dopant. Additionally, this substitution improves the convergence of valence bands, which is found to have a direct correlation with the tetragonal distortion of these chalcopyrites. Furthermore, several chalcopyrite compounds with heavy elements such as Zn, Cd, and As possess low phonon group velocities and large Gruneisen parameters that lead to low lattice thermal conductivity. Combination of optimized electronic transport properties and low thermal conductivity results in maximum ZT of 1.67 in CdGeAs2 at moderate n-type doping. The approach developed here to enhance the thermoelectric efficiency can be generalized to other class of materials.