[ars]

The way a solar cell works is it absorbs only at one single frequency.

Photons below that frequency are lost and not used at all. If the photon is above the frequency then the energy up to that cutoff is used and the rest is wasted (either emitted as a new photon or as heat).

Meaning if you set the target frequency at infrared then you loose all the additional energy UV has over infrared.

By making multiple layers (junctions) you waste less energy, the more the better (in theory anyway).