Photovoltaic cell

1. A solar cell comprising a first photovoltaic junction, the first photovoltaic junction comprising: first and second doped bulk semiconductor regions; and an intrinsic region disposed between the doped bulk regions, the intrinsic region comprising a plurality of quantum wells formed of bulk-immiscible quaternary InGaAsP and separated by quantum barriers, wherein the relative proportions of As and P in the quaternary InGaAsP are given by As y and P 1−y , where 0.2<=y<=0.5, wherein the relative proportions of In, Ga, As and P are such that the InGaAsP is lattice matched to within 2% of the lattice constant of GaAs or Germanium.

2. The solar cell of claim 1 wherein the relative proportions of As and P are given by As y and P 1−y where 0.25<=y<=0.45.

3. The solar cell of claim 1 in which at least some of the quantum barriers are formed of a semiconductor material comprising Ga, In and P.

4. The solar cell of claim 3 wherein at least some of the quantum barriers are formed of tertiary GaInP.

5. The solar cell of claim 1 wherein at least one of the doped bulk semiconductor regions is formed of a doped semiconductor material comprising Ga, In and P.

6. The solar cell of claim 1 wherein some or all of the quantum wells are formed of InGaAsP in which the relative proportions of In, Ga, As, and P are such that these quantum wells are lattice matched to at least one of: the first doped bulk semiconductor region; the second doped bulk semiconductor region; and an underlying substrate.

7. The solar cell of claim 1 wherein the quantum wells and quantum barriers have compositions and thicknesses such that the plurality of quantum wells and barriers provide compensating compressive and tensile stresses which balance at a common lattice spacing matched to that of an underlying substrate and/or that of at least one of the doped bulk semiconductor regions.

8. The solar cell of claim 1 wherein the absorption band edge of the quantum wells has a wavelength of between 700 and 740 nm.

9. The solar cell of claim 1 wherein the intrinsic region comprises at least twenty of said quantum wells.

10. The solar cell of claim 1 wherein some or all of the InGaAsP quantum wells are less than 15 nm thick.

11. The solar cell of claim 1 wherein at least some of said quantum wells comprise a subwell of GaAs, separated from the barriers by the quaternary InGaAsP.

12. The solar cell of claim 1 , further comprising an underlying substrate.

13. The solar cell of claim 12 wherein the underlying substrate is one of: GaAs, Germanium, and a substrate substantially lattice matched to GaAs or Germanium.

14. The solar cell of claim 13 wherein the substrate is cut to an angle of more than 2 degrees from (100) to the <111> or <110> crystal plane.

15. The solar cell of claim 12 wherein the solar cell comprises a second photovoltaic junction disposed between the first photovoltaic junction and the substrate, to form a tandem cell of two or more photovoltaic junctions in which a common photocurrent passes through all the junctions.

16. The solar cell of claim 15 wherein the second photovoltaic junction has an absorption edge wavelength of at least 1000 nm, which is greater than that of the first photovoltaic junction.

17. The solar cell of claim 16 wherein the second photovoltaic junction comprises an intrinsic region containing a plurality of quantum wells.

18. The solar cell of claim 17 wherein at least some of the quantum wells of the second junction are formed of InGaAs.

19. The solar cell of claim 18 wherein the quantum wells of the second photovoltaic junction are disposed between quantum barriers at least some of which are formed of GaAsP, and the quantum wells and quantum barriers of the second photovoltaic junction have compositions and thicknesses such that the plurality of quantum wells and barriers provide compensating compressive and tensile stresses which balance at a common lattice spacing matched to that of the underlying substrate.

20. The solar cell of claim 12 wherein the first and second photovoltaic junctions are adapted such that the photocurrents of the junctions match for a predefined illumination condition.

21. A photovoltaic system for generating electrical power from solar illumination comprising: a solar cell as set out in claim 12 ; and a concentrator arranged to deliver concentrated sunlight at the surface of the solar cell.

22. The photovoltaic system of claim 21 wherein the concentrator delivers said sunlight concentrated by at least 50 times.

23. A method of forming a solar cell photovoltaic junction using epitaxial growth, comprising: growing a first doped bulk semiconductor layer; growing on top of the first doped bulk layer an intrinsic region comprising quantum well layers of quaternary InGaAsP separated by quantum barrier layers, the quantum wells being formed of bulk-immiscible quaternary InGaAsP; and growing a second doped bulk semiconductor layer on top of the intrinsic region;

wherein the quantum well layers have the relative proportions of As and P given by As y and P 1−y where 0.2<=y<=0.5, wherein the relative proportions of In, Ga, As and P are such that the InGaAsP is lattice matched to within 2% of the lattice constant of GaAs.

24. The method of claim 23 wherein some or all of the InGaAsP quantum wells are less than 15 nm thick.

25. The method of claim 23 wherein one or more of the barrier layers are formed of a semiconductor material comprising Ga, In and P.

26. The method of claim 25 wherein said one or more of the barrier layers are formed of tertiary GaInP.

27. The method of claim 25 wherein the transition from growth of a quantum barrier layer to a quantum well layer is carried out by the controlled introduction of an Arsenic source along with a corresponding controlled reduction of a Gallium source, and a subsequent transition to growth of a quantum well layer is carried out by the ceasing of the Arsenic source along with a corresponding controlled increase in the Gallium source.

28. The method of claim 23 wherein one or more of the doped bulk semiconductor layers are formed of a semiconductor material comprising Ga, In and P, or a related material having a band edge between 640 and 700 nm.

29. The method of claim 23 wherein growing one or more of the quantum well layers of InGaAsP comprises growing a subwell layer of GaAs separated from barrier layers by the InGaAsP material.

30. A solar cell photovoltaic junction comprising: first and second doped bulk semiconductor regions lattice matched to GaAs, at least one of which bulk regions is formed of a semiconductor material comprising Ga, In and P; and an intrinsic region disposed between the bulk regions comprising a plurality of quantum well layers separated by quantum barrier layers, the quantum well layers being formed of bulk immiscible quaternary GaInAsP, in which relative proportions of As and P are given by As y and P 1−y , where 0.2<=y<=0.5, wherein the relative proportions of In, Ga, As and P are such that the InGaAsP is lattice matched to within 2% of the lattice constant of GaAs or Germanium.

31. The photovoltaic junction of claim 30 wherein the quantum wells layers are separated by barrier layers formed of a semiconductor material comprising Ga, In and P.

32. A method of providing an extended absorption edge in a GaInP based photovoltaic junction lattice matched to GaAs or Ge, which includes an intrinsic region having multiple quantum wells therein, comprising forming at least some of the quantum wells from a bulk-immiscible quaternary InGaAsP material, separated by quantum barrier layers, wherein relative proportions of As and P in the quaternary bulk-immiscible InGaAsP material are given by As y and P 1−y , where 0.2<=y<=0.5, wherein the relative proportions of In, Ga, As and P are such that the InGaAsP is lattice matched to within 2% of the lattice constant of GaAs or Germanium.