 Consider the solar spectrum; only a small portion of the light reaching the ground is blue, but those photons have three times the energy of red light.Silicon's bandgap is 1.1 eV, about that of red light, so in this case blue light's energy is lost in a silicon cell.If the bandgap is tuned higher, say to blue, that energy is now captured, but only at the cost of rejecting lower energy photons.