Enhancement of the Seebeck Coefficient by Energy Filtering in Mixed-Phase Cu2−xS Films at Room Temperature

In this work, we present and report on the evolution of thermoelectric properties altered through changes in the energy barrier height in thermally evaporated mixed-phase copper sulfide thin films. The physical interpretations depend on the conception of degenerate energy levels near the top of the valence band. The energy barrier at grain boundaries was highlighted and assumed to be the origin of the rapid evolution of the conductivity and Seebeck coefficient of the film annealed at 723 K. The position of the energy levels of the active carriers with respect to the Fermi energy reinforces the effect of annealing temperature on the Seebeck coefficient and electrical conductivity and was observed to transform the system from a system with fully ionized impurities to a system with impurities that are not fully ionized, which enhances the barrier height. The evolution of the Seebeck coefficient is explained in terms of thermal activation. The sample annealed at 623 K exhibited the lowest barrier height of 32 meV, with an activation energy of 111 meV. The sample annealed at 673 K had a barrier height of 46 meV with an activation energy of 136 meV. Finally, the sample annealed at 723 K exhibited a barrier height of 103 meV, which explains its relatively high room-temperature Seebeck coefficient, with a pronounced effect of temperature.