Cu_2−xS and Cu_2−xSe Alloys: Investigating the Influence of Ag, Zn, and Ni Doping on Structure and Transport Behavior

Cu2−xS and Cu2−xSe (0 ≤ x ≤ 0.2) alloys stand out as highly promising materials for thermoelectric applications, owing to the phonon–liquid electron–crystal (PLEC) convention. In this study, we undertake a comprehensive investigation to reassess the synthesis conditions, with a focus on achieving pure-phased systems through a direct reaction between elements at elevated temperatures. Simultaneously, we present experimental evidence showcasing the feasibility of doping these systems with Ag, Ni, and Zn. The study demonstrates that obtaining single-phased systems requires multi-step processes, and the dissolution of chosen impurities appears doubtful, as evidenced by numerous foreign phase segregations. Additionally, it is revealed that the partial dissolution of individual impurities deteriorates the operational parameters of these chalcogenides. For the optimal Cu1.97S composition, it reduces the thermoelectric figure-of-merit ZT from 1.5 to approximately 1.0, 0.65, and 0.85 for Ag-, Ni-, and Zn-doped systems, respectively, while marginally improving their stability. For metal-like Cu1.8Se, the ZT parameter remains at a low level, ranging between 0.09 and 0.15, showing slight destabilization during subsequent operating cycles. The article concludes with an in-depth analysis of the basic thermoelectric performance exhibited by these doped systems, contributing valuable insights into the potential enhancements and applications of Cu2−xS and Cu2−xSe alloys in the field of thermoelectric materials.