Kinetic condition driven phase and vacancy enhancing thermoelectric performance of low-cost and eco-friendly Cu_2-XS

Low-cost and eco-friendly Cu₂S has attracted wide research interest in recent years. Here, we synthesized Cu_2-xS via a facile solvothermal method. The thermoelectric performance of the as-sintered Cu_2-xS can be easily tuned via kinetic-condition control due to subsequently changed phase contents. Based on detailed characterization, it is found that reducing the NaOH amount can preferentially boost the formation of tetragonal Cu_1.96S and monoclinic Cu_1.94S phases instead of monoclinic Cu₂S phase. This phase content change can promote the formation of Cu vacancies in the high-temperature cubic Cu_2-xS after the phase transition at ∼700 K. The enhanced Cu vacancy levels can effectively enhance both the power factor (S²σ) and figure of merit (zT) due to simultaneously optimized hole concentration (n_H) and reduced phase transition temperature. A peak zT of ∼1.1 is achieved in the Cu_2-xS synthesized with 2 ml NaOH with an average composition of Cu_1.95S. An average zT of as high as ∼0.76 (T = 573-833 K) is obtained due to both optimized n_H leading to enhanced S²σ and reduced phase transition temperature (at ∼700 K).