The rational design and development of economical, high-performance, and stable counter electrodes (CE) are critical to bringing the quantum dot-sensitized solar cell (QDSSCs) from the laboratory to a practical application. In this respect, we used a two-step approach to fabricate ternary copper chalcogenide (Cu2−xSySe1−y) alloyed semiconductors onto fluorine-doped tin oxide (FTO). In the first step, the binary copper chalcogenides CuS nanostructures that are synthesized using the microwave-irradiation technique are screen-printed onto the FTO substrate and annealed in a nitrogen atmosphere to obtain Cu2−xS CE. In the second step, ternary Cu2−xSySe1−y alloyed electrocatalyst is obtained through a composition engineering approach in which the elemental Se was incorporated on the surface of as-synthesized Cu2−xS nanostructures using the drop-casting method. Compared to the pristine Cu2−xS CE, the as-synthesized Cu2−xSySe1−y CE has exhibited tunable crystal structures, compositions, morphologies. The electrochemical analysis revealed that the optimized Cu2−xSySe1−y CE has exhibited low charge transfer resistance (Rct), and excellent reduction activity to Sn2− species of the polysulfide electrolyte. Accordingly, QDSSCs assembled with Cu2−xSySe1−y CE have delivered conversion efficiencies of 8.02%, which are higher than those of pristine Cu2−xS CE (7.24%). Noticeably, Cu2−xSySe1−y CE has demonstrated outstanding electrochemical stability in polysulfide redox couple, exhibiting no substantial fluctuations in either the current density or shape of the curve even after 200 continuous cyclic voltammetry (CV) cycles. Moreover, the best cell devices constructed using Cu2−xSySe1−y CE validated remarkable stability under open-air conditions, retaining <60% of the original performance after 120 h of illumination. Overall, the ease of synthesis, low cost, time efficiency, and excellent electrocatalytic characteristics of the Cu2−xSySe1−y alloyed semiconductors film demonstrated in this work make it an encouraging applicant for use as a CE material in photovoltaic applications.