  Taking into account the temperature gradients in solar cells, it is shown
that their efficiency can be increased beyond the Shockley-Queisser limit (J.
Appl. Phys. 32 (1961) 510). The driving force for this gain is the temperature
gradient between this region and its surroundings.
  A quantitative theory is given. Though the effect is found to be weak in
conventional solar cells, it is argued that it can be substantially increased
by proper choice of materials and design of the device. In particular, it is
shown that the insertion of a quantum well can enhance the efficiency beyond
one of the single gap cell, due to the presence of temperature jumps at the
heterojunctions.
