In this study, we define a synthetic parameter: optothermal expansivity as a quantitative guide to evaluating and optimizing both the thermal and the optical performance of PV–T functional materials. The definition of this parameter, ψ
AB (Amlouk–Boubaker parameter), takes into account 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 (m
3 s
-1) and can be considered as a 3D velocity of the transmitted heat inside the material.
As the PV–T combined devices need to have simultaneous optical and thermal efficiency, we try to investigate some recently proposed materials (β-SnS
2, In
2S
3, ZnS
1-xSe
x|
0 ≤x<0.5 and Zn-doped thioindate compounds) using the newly established ψ
AB/E
g abacus.
