Phase Tuning for Enhancing the Thermoelectric Performance of Solution-Synthesized Cu_2-S

Cu₂S, consisting of Earth-abundant and eco-friendly elements, is a promising candidate for thermoelectric energy conversion. The stoichiometric Cu₂S has intrinsically low thermal conductivity together with meagre electrical conductivity. Herein, in order to improve the electrical properties, we develop a facile solvothermal method for synthesizing a series of Cu_2-S with tunable phase compositions. With reduction in the CuCl/Na₂S molar ratio in the solution precursors from 2:1 to 1.75:1, the solvothermal products transform from single-phase Cu₂S into Cu₂S-Cu_1.96S composites, which are preserved in the hot-pressed pellets. The decreased CuCl/Na₂S molar ratio results in increased room-temperature Hall carrier concentration, leading to enhanced electrical conductivity. Specifically, the pellet corresponding to the 1.75:1 CuCl/Na₂S precursor ratio obtains a power factor of 8.04 μW cm^-1 K^-2 at 815 K, which is ∼735% of the value obtained by stoichiometric Cu₂S. Along with the moderate thermal conductivity, this sample achieves an enhanced zT value of 0.87 at 815 K, representing a ∼250% increase when compared to stoichiometric Cu₂S. This study demonstrates an effective strategy in enhancing the thermoelectric performance of solution-synthesized Cu₂S-based materials by phase tuning.