Question about Si, GaN (solar cells, diodes and photons)

Hi,

I have a question which I'm not 100 % sure how to express myself.
The question is why Gallium nitride (GaN) is a bad material to choose when constructing solar cells, but good for light-emitting diodes and vice versa for Silicon (Si).

I'm pretty sure it must have something to do with the different band gaps, where GaN has a band gap at 3.4 eV and Si has 1.12 eV, and that each material lets through specific wave lengths of light...

Anyone who think they can help me out?