Dear Omar,

Your question would more clear if you added the whole layer structure of the multijunction solar cells.

However i will try to answer your question from the conceptual point of view. The functions of the substrates are:

- Mechanical support of the active material, which may be thin films deposited on the substrate. So, they have to have good mechanical properties,with high durability.

- They must have specific electrical properties. Metal layers are used to be part of the multijunction structure eg as a metallic electrode to access the cell electrically. Metal layers are required for electrical conduction to serve as metallic electrode.

-The substrate must has also specific thermal properties such as thermal expanial sion matching and good thermal conductivity to dispatch the heat in the solar cells.

- Material cost and availability

- Material process ability.

- compatibility with each other

- resistant against environmental variations.

I think ceramics combine with metal are very suitable as a substrate for the multjunction solar cells.

wish you success 

Dear Dr. Zekry,

Thank you for your response.

I've attached a layered schematic of the configuration I am referring to in my question. The thermal expansion coefficient of an Al2O3-rich ceramic does agree with that of the MJPV cell. However, I am particularly wondering about the role of the ceramic layer in heat dissipation from the MJPV cell, especially under concentrated irradiance, since ceramics have low thermal conductivities compared to metals.

I am also not sure why the second metallic layer exists if the first layer functions as a back electrode.

You have to have electrically insulating material as a core substrate, it may be glass , plastic or ceramics such alumina. The alumina may have the best thermal conduction among the three materials. It is also strong and durable. The alumina here is in form of a foil with small thickness such that its thermal resistance is reduced. The alumina sheet and the underlying metal  layer forms a combined bi layer structure  to improve the mechanical and thermal properties of the substrate such as the ability of folding and rolling.

wish you success.