Method of operating a solar cell
A method of operating a solar cell is provided in which strain balanced multiple quantum well stacks containing greater than 30 quantum wells disposed between bulk semi-conductor regions having a band gap differences between the deepest well of the stack and the bulk semi-conducting region of greater than 60 mev is irradiated with radiation having an intensity of greater than 100 suns. Photons are absorbed with and outside of the quantum well stack to generate electron hole pairs recombination of electrons and holes is substantially only via a radiative recombination mechanism.
1 . A method of operating a solar cell having a strain balanced multiple quantum well stack containing greater than thirty quantum wells and disposed between bulk semiconductor regions, a band-gap difference between a band-gap of a deepest well within said strain balanced multiple quantum well stack and a band-gap of said bulk semiconductor regions of the cell outside the multiple quantum well region being greater than 60 meV, said method comprises the steps of:
receiving incident radiation having an intensity of greater than one hundred suns concentration;
absorbing photons from said incident radiation both within and outside said quantum well stack to generate electron hole pairs;
recombining electrons and holes with a radiative recombination mechanism to form re-radiated photons that are re-absorbable within said solar cell to generate electrical energy; wherein
electrons and holes within said quantum well stack substantially only recombine via said radiative recombination mechanism.
2 . A method as claimed in claim 1 , wherein said quantum well stack has an absorption edge above 0.9 μm.
3 . A method as claimed in claim 1 , wherein said solar cell has a p region and an n region, said p region and said n region having band gap greater than a photon energy corresponding to an absorption edge of said solar cell so as to suppress Shockley recombination of electrons and holes.
4 . A method as in claim 1 , wherein said solar cell has one of a multiple-layer reflector or a Bragg stack beneath said solar cell to form a reflector operative to reflect radiation with an energy between an absorption edge of said quantum well stack and an absorption edge of said bulk semiconductor regions back to said quantum well stack.
5 . A method as claimed in claim 1 , wherein said solar cell is a tandem solar cell having a further absorption region beneath said quantum well stack and with a band gap such that said re-radiated photons are absorbed with high probability.
6 . A method as claimed in claim 5 , wherein said further absorption region is a further strain balanced multiple quantum well stack having greater than thirty quantum wells.
7 . A method as in claim 5 wherein said further absorption region is an active Germanium substrate.
8 . A method as claimed in claim 1 wherein said quantum well stack comprises GaAs 1-x P x /In y Ga 1-y As layers, where x and y are chosen so that an equilibrium lattice parameter of said quantum well stack as a free standing structure is substantially equal to a lattice parameter of a substrate of said solar cell for a given absorption edge and produce strain-balanced quantum well layers and quantum well barriers.
9 . A method as claimed in claim 1 , wherein said quantum well stack comprises Ga x In 1-x P/In y Ga 1-y As layers, where x and y are chosen so that an equilibrium lattice parameter of said quantum well stack as a free standing structure is substantially equal to a lattice parameter of a substrate of said solar cell for a given absorption edge and produce strain-balanced quantum well layers and quantum well barriers.
10 . A method as claimed in claim 1 , wherein said quantum well stack comprises GaAs x P 1-x /In y Ga 1-y AsN z layers, where x, y and z are chosen so that an equilibrium lattice parameter of said quantum well stack as a free standing structure is substantially equal to a lattice parameter of a substrate of said 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
11 . A method as claimed in claim 8 , wherein said solar cell has a multiple-layer reflector or Bragg stack grown beneath said solar cell which forms a reflector operative to reflect the radiation with energy between said absorption edge of said quantum well stack and an absorption edge of said bulk semiconductor regions back to said quantum well stack with high reflectivity over a large distribution of incidence angles.
12 . A method as claimed in claim 9 , wherein said solar cell has a multiple-layer reflector or Bragg stack grown beneath said solar cell which forms a reflector operative to reflect the radiation with energy between said absorption edge of said quantum well stack and an absorption edge of said bulk semiconductor regions back to said quantum well stack with high reflectivity over a large distribution of incidence angles.
13 . A method as claimed in claim 10 , wherein said solar cell has a multiple-layer reflector or Bragg stack grown beneath said solar cell which forms a reflector operative to reflect the radiation with energy between said absorption edge of said quantum well stack and an absorption edge of said bulk semiconductor regions back to said quantum well stack with high reflectivity over a large distribution of incidence angles.
14 . A method as claimed in claim 8 , wherein said solar cell has a further absorption region provided by an active Germanium substrate.
15 . A method as claimed in claim 9 , wherein said solar cell has a further absorption region provided by an active Germanium substrate.
16 . A method as claimed in claim 10 , wherein said solar cell has a further absorption region provided by an active Germanium substrate.
17 . A method as claimed in claim 5 , wherein said tandem solar cell contains a quantum well stack comprising Ga x In 1-x P/Ga y In 1-y P layers, where x and y are chosen to substantially minimise stress and said further strain balanced multiple quantum well stack comprises GaAs 1-x P x /In y Ga 1-y As layers where x and y are chosen so that a equilibrium lattice parameter of said further stack as a free standing structure is substantially equal to a lattice parameter of a substrate of said tandem solar cell for a given absorption edge.
18 . A method as claimed in claim 5 , wherein said tandem solar cell contains a quantum well stack comprising Ga x In 1-x P/Ga y In 1-y P layers, where x and y are chosen so that an equilibrium lattice parameter of said quantum well stack as a free standing structure is substantially equal to the lattice parameter of a substrate of said tandem solar cell for a given absorption edge and said further strain balanced multiple quantum well stack comprises GaAs 1-x P x /In y Ga 1-y AsN z where x, y and z are chosen to substantially minimise stress.
19 . A method as claimed in claim 17 , wherein said further strain-balanced quantum well solar cell has a multiple-layer reflector or Bragg stack grown beneath said tandem solar cell which forms a reflector designed to reflect radiation with energy between an absorption edge of the quantum well stack and an absorption edge of said bulk semiconductor region back to the quantum well stack with high reflectivity over a large distribution of incidence angles.
20 . A method as claimed in claim 18 , wherein said further strain-balanced quantum well solar cell has a multiple-layer reflector or Bragg stack grown beneath said tandem solar cell which forms a reflector designed to reflect radiation with energy between an absorption edge of the quantum well stack and an absorption edge of said bulk semiconductor region back to the quantum well stack with high reflectivity over a large distribution of incidence angles.
21 . A method as claimed in claim 17 , wherein a further absorbing region is provided by an active Germanium substrate.
22 . A method as claimed in claim 18 , wherein a further absorbing region is provided by an active Germanium substrate.