Thermodynamic consistency of sub-bandgap absorbing solar cell proposals

This paper proves, using thermodynamic arguments, that ideal multiple quantum well (MQW) solar cells cannot exceed the efficiency of ideal ordinary solar cells unless carriers in the well are pumped by the absorption of a second photon. In this case, the cells behave identically to the intermediate band (IB) solar cell that allows for sub-band photon absorption. It is also proven that the IB solar cell complies with the second law of thermodynamics and, in this way, the theoretical potential of this cell for achieving efficiencies of 63.2% as compared to the 40.7% of ordinary cells, is confirmed