This paper proves, using thermodynamic arguments, that ideal multiple quant
um well (MQW) solar cells cannot exceed the efficiency of ideal ordinary so
lar cells unless carriers in the well are pumped by the absorption of a sec
ond 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 thermodynami
cs and, in this way, the theoretical potential of this cell for achieving e
fficiencies of 63.2% as compared to the 40.7% of ordinary cells, is confirm
ed.