CuAlSe Inclusions Trigger Dynamic Cu Ion Depletion from the CuSe Matrix Enabling High Thermoelectric Performance.

Atomic-scale incorporation of CuAlSe inclusions within the CuSe matrix, achieved through a solid-state transformation of CuSe template precursor using elemental Cu and Al, enables a unique temperature-dependent dynamic doping of the CuSe matrix. The CuAlSe inclusions, due to their ability to accommodate a large fraction of excess metal atoms within their crystal lattice, serve as a "reservoir" for Cu ions diffusing away from the CuSe matrix. Such unidirectional diffusion of Cu ions from the CuSe matrix to the CuAlSe inclusion leads to the formation, near the CuAlSe/CuSe interface, of a high density of Cu-deficient β-CuSe nanoparticles within the α-CuSe matrix and the formation of Cu-rich CuAlSe nanoparticles with the CuAlSe inclusions. This gives rise to a large enhancement in carrier concentration and electrical conductivity at elevated temperatures. Furthermore, the nanostructuring near the CuAlSe/CuSe interface, as well as the extensive atomic disorder in the CuSe and CuAlSe phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity. Consequently, a significant improvement in ZT is observed for selected CuSe/CuAlSe composites. This work demonstrates the use of in situ-formed interactive secondary phases in a semiconducting matrix as an elegant alternative approach for further improvement of the performance of leading thermoelectric materials.