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-Cu_2-δSe nanoparticles within the α-Cu₂Se matrix and the formation of Cu-rich Cu₁₊AlSe₂ 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.