In this study, we define a synthetic parameter, optothermal expansivity, as a quantitative guide to evaluating and optimizing PV–T functional materials in both thermal and optical performance. This parameter, ψ AB (or Amlouk–Boubaker expansivity), is calculated using both the thermal diffusivity and the optical effective absorptivity of the material. The values of this parameter, which seems to be a characteristic one, correspond to the total volume that contains a fixed amount of heat per unit time (m3s-1) and can be considered, for future use, as a 3D velocity of the transmitted heat inside the material.
