Quantum Wells in Photovoltaic Cells
  The fundamental efficiency limit of a single bandgap solar cell is about 31% at one sun with a bandgap of about Eg = 1.35 eV (1), determined by the trade-off of maximising current with a smaller bandgap and voltage with a larger bandgap. Multiple bandgaps can be introduced to absorb the broad solar spectrum more efficiently. This can be realised in multi- junction cells, for example, where two or more cells are stacked on top of each other either mechanically or monolithically connected by a tunnel junction. An alternative or complementary (see section 1.4) approach is the quantum well cell (QWC). 
C Rohr, P Abbott, I M Ballard, D B Bushnell, J P Connolly, N J Ekins-   Daukes, K W J Barnham
cond-mat.mes-hall physics.optics