Recently, applying spectral beam splitting technologies into concentrated photovoltaic/thermal (CPV/T) systems have received unprecedented attention owing to their abilities to take full advantage of solar broadband. Such hybridized systems require liquid absorptive-filters capable of enhancing the combined conversion efficiency achievable by controlling their ‘thermal’ and ‘optical’ characteristics. Although numerous studies have strived to develop the optical performance of thermal fluids for peaking the spectral window of the PV, maximizing their heat-transfer capabilities have not gained widespread attention. Therefore, this study aims to fill the gap and provides the appropriate thermal-optical selective criteria of liquid spectral-filters under various solar concentrations (CRs) using a precisive 3D optical-thermal coupled numerical model. The results revealed that using liquid filters with high thermal conductivity (k) and heat capacity (Cp) jointly has the potential to improve the total solar energy conversion in all CRs. Whereas low k and Cp thermal fluids have the preference under low CRs and vice versa in high CRs from the exergy viewpoint. Besides, the presence of high thermal absorption filters above the undesired PV-wavebands promotes co-conversion efficiency cross all CRs, while the high permeability filters over the spectral window of PVs are recommended from the exergy viewpoint. Since several liquid filters can meet various user needs (whether it is thermal or electrical outputs), the ideality of optical characteristics of used fluid-filters primarily depends on the desired energy form, so researchers should carefully select the appropriate thermal fluid for the desired purpose.