My understanding is that although we are taught that solar cells only absorb photons of energy higher than the bandgap of the material, some of the sub-bandgap photons still gets absorbed, which is evident when looking at the absorption coefficient spectra (it is not zero where it is below the bandgap).
First, am I correct on this? Second, if so, what happens to the sub-bandgap photons that are absorbed?
For example, you can see the absorption spectra of Silicon here: pveducation.org/pvcdrom/materials/optical-properties-of-silicon
The band gap of Silicon is 1.14 eV at 300 K, which corresponds to a wavelength of 1087 nm. You can see that the absorption coefficient is non zero for wavelength greater than 1087 nm, which means given enough thickness, the sub-bandgap photon will be absorbed.