Window structure for solar cell

1. A photovoltaic cell comprising:

at least one subcell with a window structure, said at least one subcell comprising an emitter layer comprising InGaP with a first lattice spacing in the plane perpendicular to the direction of epitaxial growth;

said window structure being positioned above said emitter layer and in direct contact with said emitter layer;

said window structure having at least two window layers, a bottom window layer having an intrinsic material lattice spacing that is substantially the same as the intrinsic material lattice spacing of said emitter layer in the plane perpendicular to the direction of epitaxial growth, and at least one upper window layer overlying the bottom window layer and having a lattice spacing in the plane perpendicular to the direction of epitaxial growth that is substantially different from that of the emitter layer, wherein,

the bottom window layer is characterized by a thickness from 1 nm to 15 nm; and

the bottom window layer comprises Al x In 1-x P, wherein x is 0.52;

the at least one upper window layer comprises Al x In 1-x P, wherein x ≧0.60; and

the lattice spacing and in-plane strain of the emitter layer and the lattice spacing and in-plane strain of the bottom window layer differ by less than 0.3%; and

said at least one upper window layer having a band gap greater than a band gap of the bottom window layer.

2. The photovoltaic cell of claim 1 wherein said window structure has at least two upper window layers with intrinsic material lattice spacing differing from the intrinsic material lattice spacing of the bottom window layer in progressively increasing amounts.

3. The photovoltaic cell of claim 1 wherein a top window layer of said at least one upper window layer has a bandgap higher than a bandgap of any lower window layer.

4. The photovoltaic cell of claim 1 wherein at least a portion of said at least one upper window layer comprises an alloy characterized by a bandgap that increases continuously as a function of distance from said emitter layer.

5. The photovoltaic cell of claim 1 , wherein

the at least one subcell comprises a material selected from a group III-V material, a group V material, a group II-VI material, and a combination of any of the foregoing.

6. The photovoltaic cell of claim 1 , wherein the window structure is characterized by a thickness from 10 nm to 60 nm.

7. A photovoltaic cell comprising:

at least one subcell with a window structure;

the at least one subcell comprising an emitter comprising InGaP with a first lattice spacing in the plane perpendicular to the direction of epitaxial growth;

the window structure being positioned above the emitter and in direct contact with the emitter;

the window structure having at least one window layer comprising an alloy having an intrinsic material lattice spacing and a band gap that varies continuously across at least part of the window layer and wherein the intrinsic material lattice spacing of the window structure at an interface with the emitter in the plane perpendicular to the direction of epitaxial growth is substantially the same as the first lattice spacing, wherein,

the window structure is characterized by a thickness from 10 nm to 60 nm; and

the window structure is characterized by an Al x In 1-x P composition wherein x is 0.52 at the interface and varies continuously to x ≧0.60; and

the lattice spacing and in-plane strain of the emitter layer and the lattice spacing and in-plane strain of the bottom window layer differ by less than 0.3%.

8. The photovoltaic cell of claim 6 , wherein a bandgap at the top of the window structure is greater than a bandgap at the interface with the emitter.

9. The photovoltaic cell of claim 7 , wherein

the at least one subcell comprises a material selected from a group III-V material, a group V material, a group II-VI material, and a combination of any of the foregoing.

10. A photovoltaic cell comprising at least two subcells, wherein the at least two subcells comprise an upper subcell positioned above a lower subcell, wherein the lower subcell comprises:

an emitter layer comprising InGaP and characterized by a first lattice spacing in a plane perpendicular to the direction of epitaxial growth; and

a window structure positioned above the emitter layer and in direct contact with the emitter layer;

wherein the window structure comprises at least two window layers, wherein the at least two window layers comprise a bottom window layer characterized by an intrinsic material lattice spacing that is substantially the same as the intrinsic material lattice spacing of the emitter layer in the plane perpendicular to the direction of epitaxial growth, and at least one upper window layer positioned above the bottom window layer and characterized by a lattice spacing in the plane perpendicular to the direction of epitaxial growth that is substantially different from that of the emitter layer; and wherein

the at least one upper window layer is characterized by a band gap that is greater than a band gap of the bottom window layer;

the bottom window layer is characterized by a thickness from 1 nm to 15 nm;

the bottom window layer comprises Al x In 1-x P, wherein x is 0.52;

the at least one upper window layer comprises wherein Al x In 1-x P, ≧0.60; and

the lattice spacing and in-plane strain of the emitter layer and the lattice spacing and in-plane strain of the bottom window layer differ by less than 0.3%.

11. The photovoltaic cell of claim 10 , wherein the window structure comprises at least two upper window layers characterized by an intrinsic material lattice spacing differing from the intrinsic material lattice spacing of the bottom window layer in progressively increasing amounts.

12. The photovoltaic cell of claim 10 , wherein a top window layer of the at least one upper window layer is characterized by a bandgap that is higher than a bandgap of any lower window layers.

13. The photovoltaic cell of claim 10 , wherein at least a portion of the at least one upper window layer comprises an alloy characterized by a bandgap that increases continuously as a function of distance from the emitter layer.

14. The photovoltaic cell of claim 10 , wherein the window structure consists of two window layers.

15. The photovoltaic cell of claim 10 , wherein

each of the at least two subcells comprises a material selected from a group III-V material, a group V material, a group II-VI material, and a combination of any of the foregoing.

16. A photovoltaic cell comprising:

at least two subcells, wherein the at least two subcells comprise an upper subcell positioned above a lower subcell, wherein the lower subcell comprises:

an emitter layer comprising InGaP and characterized by a first lattice spacing in a plane perpendicular to the direction of epitaxial growth; and

a window structure positioned above the emitter layer and in direct contact with the emitter layer;

wherein the window structure comprises at least one window layer characterized by an intrinsic material lattice spacing and a bandgap that increases continuously as a function of distance from the emitter layer across at least part of the at least one window layer, and wherein

the window structure is characterized by a thickness from 10 nm to 60 nm;

the window structure is characterized by an Al x In 1-x P composition wherein x is 0.52 at the interface and varies continuously to x ≧0.60; and

the lattice spacing and in-plane strain of the emitter layer and the lattice spacing and in-plane strain of the window structure differ by less than 0.3%.

17. The photovoltaic cell of claim 16 , wherein

each of the at least two subcells comprises a material selected from a group III-V material, a group V material, a group II-VI material, and a combination of any of the foregoing.