Nanofluids are identified as advanced working fluids in the solar energy conversion field with superior heat transfer characteristics. This research work introduces carbon-based diamond nanomaterial and Therminol®55 oil-based nanofluids for implementation in a concentrated photovoltaic/thermal (CPV/T) solar collector. This study focuses on the experimental formulation, characterization of properties, and performance evaluation of the nanofluid-based CPV/T system. Thermo-physical (thermal conductivity, viscosity, and rheology), optical (UV-vis and FT-IR), and stability (Zeta potential) properties of the formulated nanofluids are characterized at 0.001�0.1 wt. concentrations of dispersed particles using experimental assessment. The maximum photo-thermal energy conversion efficiency of the base fluid is improved by 120.80 at 0.1 wt.. The thermal conductivity of pure oil is increased by adding the nanomaterial. The highest enhancement of 73.39 is observed for the TH-55/DP nanofluid. Furthermore, dynamic viscosity decreased dramatically across the temperature range studied (20�100 °C), and the nanofluid exhibited dominant Newtonian flow behavior, with viscosity remaining nearly constant up to a shear rate of 100 s�1. Numerical simulations of the nanofluid-operated CPV/T collector have disclosed substantial improvements. At a concentrated solar irradiance of 5000 W/m2 and an optimal flow rate of 3 L/min, the highest thermal and electrical energy conversion efficiency enhancements are found to be 11 and 1.8, respectively. © 2022 by the authors.