Improved thermoelectric properties of α-phase Cu₂Se thin films through multiphase nanostructuring

Copper selenide (Cu₂Se) has been extensively studied due to its promising thermoelectric properties in bulk form. However, the miniaturization of thermoelectric devices using thin films is highly desired for smart applications. To date, there are few reports on composite thin films of Cu₂Se for thermoelectric applications, primarily due to their lower conversion efficiency. In the present work, Cu₂Se-based multiphase nanocomposites are presented to demonstrate enhanced conversion efficiency. The detailed structural characterization reveals that thermally evaporated Te-doped Cu₂Se thin films have multiphase compositions. The electrical conductivity decreases after Te-doping, due to enormous scattering of carriers against secondary phases and lattice defects. However, upon further increasing Te-doping concentration, both the electrical conductivity and Seebeck coefficient start increasing simultaneously, due to the formation of Cu₂Te nanoclusters and Te-Se solid solution, in the matrix of Cu₂Se. We emphasize the power factor, with the highest value reaching 234.0 μW mK^-2 at 400 K, as a key indicator of thermoelectric performance. A slightly overestimated value of dimensionless figure-of-merit (ZT) of 0.2 was obtained using the power factor and merely the electronic part of the thermal conductivity. The current synthesis route synergizes the effects of a multiphase system in thin film research to enhance the thermoelectric efficiency of Cu₂Se and related materials classes.