Solar cells are obviously the most important part of the Solar Panel. Any combination of cells(Any tier) can be used on any Panel, meaning you could potentially use Nanorite and Xerothium cells on a Litherite Solar Panel, or Litherite and Aethium cells on a Xerothium Solar Panel. You are not limited to what cells you want to use in your Solar Panel.
The main thing that is different between each tier of cell is the efficiency. The higher the tier of cell, the higher the efficiency. Keep in mind that the Solar Panels themselves also have a max efficiency that they will operate at.