TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Component: The solar cell has a p region and an n region, the p region and the n region having band gap greater than a photon energy corresponding to an absorption edge of the solar cell to suppress Shockley recombination of electrons and holes. The solar cell has one of a multiple-layer reflector or a Bragg stack beneath the solar cell to form a reflector operative to reflect radiation with an energy between an absorption edge of the quantum well stack and an absorption edge of the bulk semiconductor regions back to the quantum well stack. The solar cell is a tandem solar cell having a further absorption region beneath the quantum well stack and with a band gap such that the re-radiated photons are absorbed with high probability. The further absorption region is a further strain balanced multiple quantum well stack having greater than thirty quantum wells. The further absorption region is an active germanium substrate. The solar cell has a multiple-layer reflector or Bragg stack grown beneath the solar cell which forms a reflector operative to reflect the radiation with energy between the absorption edge of the quantum well stack and an absorption edge of the bulk semiconductor regions back to the quantum well stack with high reflectivity over a large distribution of incidence angles.Preferred Parameter: The quantum well stack has an absorption edge above 0.9 microns.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The quantum well stack comprises GaAs1-xPx/InyGa1-yAs layers, GaxIn1-xP/InyGa1-yAs or GaAsxP1-x/InyGa~yAsNz layers, where x, y and z are chosen so that an equilibrium lattice parameter of the quantum well stack as a free standing structure is equal to a lattice parameter of a substrate of the solar cell for a given absorption edge and produce strain-balanced quantum well layers and quantum well barriers, and z represents the addition of a small proportion of nitrogen atoms. The tandem solar cell contains a quantum well stack comprising GaxIn1-xP/GayIn1-yP layers, where x and y are chosen to minimize stress and the further strain balanced multiple quantum well stack comprises GaAs1-xPx/InyGa1-yAs layers where x and y are chosen so that a equilibrium lattice parameter of the further stack as a free standing structure is equal to a lattice parameter of a substrate of the tandem solar cell for a given absorption edge. The tandem solar cell contains a quantum well stack comprising GaxIn1-xP/GayIn1-yP layers, where x and y are chosen so that an equilibrium lattice parameter of the quantum well stack as a free standing structure is equal to the lattice parameter of a substrate of the tandem solar cell for a given absorption edge and the further strain balanced multiple quantum well stack comprises GaAs1-xPx/InyGa1-yAsNz where x, y and z are chosen to minimize stress.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The quantum well stack comprises GaAs1-xPx/InyGa1-yAs layers, GaxIn1-xP/InyGa1-yAs or GaAsxP1-x/InyGa~yAsNz layers, where x, y and z are chosen so that an equilibrium lattice parameter of the quantum well stack as a free standing structure is equal to a lattice parameter of a substrate of the solar cell for a given absorption edge and produce strain-balanced quantum well layers and quantum well barriers, and z represents the addition of a small proportion of nitrogen atoms. The tandem solar cell contains a quantum well stack comprising GaxIn1-xP/GayIn1-yP layers, where x and y are chosen to minimize stress and the further strain balanced multiple quantum well stack comprises GaAs1-xPx/InyGa1-yAs layers where x and y are chosen so that a equilibrium lattice parameter of the further stack as a free standing structure is equal to a lattice parameter of a substrate of the tandem solar cell for a given absorption edge. The tandem solar cell contains a quantum well stack comprising GaxIn1-xP/GayIn1-yP layers, where x and y are chosen so that an equilibrium lattice parameter of the quantum well stack as a free standing structure is equal to the lattice parameter of a substrate of the tandem solar cell for a given absorption edge and the further strain balanced multiple quantum well stack comprises GaAs1-xPx/InyGa1-yAsNz where x, y and z are chosen to minimize stress.