<p>Nanofluids, with their tunable optical properties, have emerged as promising spectral beam-splitting (SBS) media for improving the overall performance of photovoltaic/thermal (PV/T) systems. However, widespread application has been hindered by the high cost, complex fabrication processes, and limited stability of many existing nanofluids. This study proposes a hybrid nanofluid composed of antimony-doped tin oxide (ATO) and zinc oxide (ZnO) as a low-cost and adaptable SBS medium. The system’s optical-thermal-electrical performance is systematically evaluated in SBS-PV/T configurations employing both silicon (Si) and gallium arsenide (GaAs) solar cells. ATO and ZnO provide complementary spectral absorption in the near-infrared and ultraviolet regions, respectively, while maintaining high transmittance in the visible spectrum. By varying the mass ratio and total concentration of the nanofluid, system-level performance is optimized for different PV cell types. Results indicate that the optimal ATO-ZnO composition is cell-dependent: for a thermal-to-electrical weighting factor of 3, and the Si-based PV/T system achieves the highest merit function (MF) of 1.80 at 1.0×10⁻⁴ in weight concentration with an ATO/ZnO mass ratio of 1:3; while the GaAs-based system shows optimal performance at a 3:1 ratio of ATO to ZnO with an MF of 1.36. These findings demonstrate the practical viability and adaptability of ATO-ZnO hybrid nanofluids for enhancing the energy efficiency of SBS-PV/T systems, offering a scalable solution for solar thermal-electrical cogeneration.</p>