CuAlSe2 Inclusions Trigger Dynamic Cu+ Ion Depletion from the Cu2Se Matrix Enabling High Thermoelectric Performance.

Atomic-scale incorporation of CuAlSe2 inclusions within the Cu2Se matrix, achieved through a solid-state transformation of CuSe2 template precursor using elemental Cu and Al, enables a unique temperature-dependent dynamic doping of the Cu2Se matrix. The CuAlSe2 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 Cu2Se matrix. Such unidirectional diffusion of Cu ions from the Cu2Se matrix to the CuAlSe2 inclusion leads to the formation, near the CuAlSe2/Cu2Se interface, of a high density of Cu-deficient β-Cu2-δSe nanoparticles within the α-Cu2Se matrix and the formation of Cu-rich Cu1+yAlSe2 nanoparticles with the CuAlSe2 inclusions. This gives rise to a large enhancement in carrier concentration and electrical conductivity at elevated temperatures. Furthermore, the nanostructuring near the CuAlSe2/Cu2Se interface, as well as the extensive atomic disorder in the Cu2Se and CuAlSe2 phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity. Consequently, a significant improvement in ZT is observed for selected Cu2Se/CuAlSe2 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.