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.


2 Answers 2


At 300K there are intrinsic free holes and electrons due to thermal excitation across the bandgap. These give rise to absorption. Note the text under the logarithmic graph: " The drop in absorption at the band gap (around 1100 nm) is sharper than might first appear".

Note that there also may be charge carriers due to shallow impurities (doping) and deep impurities, which may induce energy levels in the band gap. This will alter the absorption.

  • $\begingroup$ Intrinsic free carriers exists as you say, but they are not needed to promote below gap absorption. Absorption requires and filled valence states and an empty conduction band state. Free carriers will actually cause a reduction in absorption coefficient. $\endgroup$
    – boyfarrell
    Commented Jun 10, 2020 at 14:03

One can think of several scenarios here:

  • Solar cell is a p-n junction (even more likely - several p-n junctions). This means that actual band structure in the junction is distorted in respect to that of a bulk intrinsic material.
  • The junction is biased, so, in principle, the distance between the valence and conductance band across the junction is smaller than in a bulk material.
  • Doping results in levels within the gap, i.e. one may have excitations from the valence band to empty n-impurity levels or from the filled p-impurity levels to the conduction band.
  • Finally, excitonic states have sub-gap energies - these are distinguishable by their rather sharp lines below the absorption edge.

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