Kinetic condition driven phase and vacancy enhancing thermoelectric performance of low-cost and eco-friendly Cu2−xS

Low-cost and eco-friendly Cu S has attracted wide research interest in recent years. Here, we synthesized Cu S via a facile solvothermal method. The thermoelectric performance of the as-sintered Cu S 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 S and monoclinic Cu S phases instead of monoclinic Cu S phase. This phase content change can promote the formation of Cu vacancies in the high-temperature cubic Cu S after the phase transition at ∼700 K. The enhanced Cu vacancy levels can effectively enhance both the power factor (S σ) and figure of merit (zT) due to simultaneously optimized hole concentration (n ) and reduced phase transition temperature. A peak zT of ∼1.1 is achieved in the Cu S synthesized with 2 ml NaOH with an average composition of Cu S. An average zT of as high as ∼0.76 (T = 573-833 K) is obtained due to both optimized n leading to enhanced S σ and reduced phase transition temperature (at ∼700 K)