Why doesn't silicon emit light? Why doesn't silicon re-emit light when light falls on it? Or if it emits light then why we can't see it?Hence it has valence electrons.
 A: There is an effect that allows silicon to emit light, but this applies to very small structures.  If your question is in regards to "bulk" silicon, then the reason it doesn't emit light is because every electron that has the requisite energy to be in the conduction band needs to have a momentum shift, via collisions with the lattice, in order to lose its energy and end up back in the valence band.  Silicon is what's known as an in-direct bandgap semiconductor for this reason.  If you're looking for a semiconductor material that emits light you should look for a direct bandgap material.
A: Of course silicon emits light. Every light, that is not reflected, heat by his energy the exposed body. And any body, which has a higher temperature than the surrounding environment, loses more EM radiation than the body gets from the EM source (in this case from the sun). One has to consider, that infrared radiation (heat radiation) as well as visible light is a part of the EM spectra. BTW, every body with temperature greater 0 Kelvin radiates (means, all bodies radiate).
Last not least, doped silicon as used in solar cells radiates in the IR spectra when a electric current flows through the solar cell.
