I am working within the heat transfer module in COMSOL 5.4. I need to determine the temperature variation in the vertical direction within a silicon solar cell, which has a radiative cooler (RC) placed on top of the cell. What this means is that:

1. RC fully transmits light in the PV band (0.3-1.1 um) so that the cell receives all light and can generate electricity.

2. RC reflects light in the Vain band (1.1-4 um) so that the cell does not receive low bandgap energy which heats up the cell.

3. RC emits thermal radiation in the atmospheric band (4-25 um) so that all excess heat from the cell gets radiated to the outer space and the cell cools.

I am using the heat transfer with the surface to surface radiation multiphysics node. The sun has been modeled using external radiation source. I'm a bit confused about the boundary conditions that need to be set. I need to set a value for the transmittance of the RC but I can't do that since the option is not available in COMSOL if I use external radiation source as the sun. Also, how can I set the transmittance of the sky so that heat can be radiated out into space?

Any help will be appreciated. Thanks.