Photovoltaic device with three dimensional charge separation and collection

• US 9,660,126 B2
• Filed: 12/06/2011
• Issued: 05/23/2017
• Est. Priority Date: 12/06/2010
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Walukiewicz, Wladyslaw, Reichertz, Lothar A., Gherasoiu, Iulian
• Primary Examiner(s): Mershon, Jayne
• Application Number:
  US13/312,780
• Publication Number:
  US 20120125417A1
• Time in Patent Office: 1,995 Days
• Field of Search: 136244, 136256, 136262
• CPC Class Codes: H01L 31/03048 comprising a nitride compounds, e.g. InGaN
• CPC Class Codes: H01L 31/065 the potential barriers being only of the graded gap type
• CPC Class Codes: H01L 31/072 the potential barriers being only of the PN heterojunction type
• CPC Class Codes: Y02E 10/544 Solar cells from Group III-V materials
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product

Abstract

A photovoltaic device having three dimensional (3D) charge separation and collection, where charge separation occurs in 3D depletion regions formed between a p-type doped group III-nitride material in the photovoltaic device and intrinsic structural imperfections extending through the material. The p-type group III-nitride alloy is compositionally graded to straddle the Fermi level pinning by the intrinsic structural imperfections in the material at different locations in the group III-nitride alloy. A field close to the surfaces of the intrinsic defects separates photoexcited electron-hole pairs and drives the separated electrons to accumulate at the surfaces of the intrinsic defects. The intrinsic defects function as n-type conductors and transport the accumulated electrons to the material surface for collection. The compositional grading also creates a potential that drives the accumulated separated electrons toward an n-type group III-nitride layer for collection. The p-type group III-nitride alloy may comprise an alloy of InGaN, InAlN or InGaAlN.

Claims

1. A solar cell, comprising:

a layer of a p-type Group III-nitride alloy for absorbing a portion of the solar spectrum upon exposure to light to generate photoexcited electron-hole pairs, wherein the p-type Group III-nitride alloy includes a top surface and a bottom surface, wherein the p-type Group III-nitride alloy includes a conduction band having a conduction band edge and a valence band having a valence band edge having a band gap there between the conduction band edge and the valence band edge;

a plurality of structural imperfections within the p-type Group III-nitride alloy, each of the structural imperfections having a surface that extends over a full distance from a point on the top surface to a point on the bottom surface of the p-type Group III-nitride alloy, wherein a Fermi level stabilization energy, E_FS, exists representing a Fermi level pinning position at the surfaces of the structural imperfections;

wherein the p-type Group III-nitride alloy is compositionally graded along the plurality of structural imperfections between at least a first area and a second area of the p-type Group III-nitride alloy such that the first area possesses a first composition of the p-type Group III-nitride alloy having its conduction band edge beneath the Fermi level stabilization energy, E_FS, and the second area possesses a second composition of the p-type Group III-nitride alloy having its conduction band edge above the Fermi level stabilization energy, E_FS; and

a layer of n-type Group III-nitride alloy.

2. The solar cell of claim 1, further comprising an electrostatic field between each of the plurality of structural imperfections and the p-type Group III-nitride alloy for separating the photoexcited electron-hole pairs and accumulating electrons around the structural imperfections in the first area possessing a first composition of the p-type Group III-nitride alloy having its conduction band edge beneath the Fermi level stabilization energy, E_FS.

3. The solar cell of claim 2, wherein the compositional grading of the p-type Group III-nitride alloy creates a potential that drives the electrons accumulating around the structural imperfections to the layer of n-type Group III-nitride alloy for collection.

4. The solar cell of claim 3, wherein the charge separation of the photoexcited electron-hole pairs is performed separately from collection of the electrons at the n-type Group III-nitride alloy.

5. The solar cell of claim 2, wherein the p-type Group III-nitride alloy layer comprises In_xGa_1-xN, where 0≦

x≦

1.0, wherein the first composition of the p-type Group III-nitride alloy having its conduction band edge beneath the Fermi level stabilization energy, E_FS, comprises In_xGa_1-xN, where 0.35<

x≦

1.0 and wherein the second composition of the p-type Group III-nitride alloy having its conduction band edge above the Fermi level stabilization energy, E_FS, comprises In_xGa_1-xN, where x<

0.35.

6. The solar cell of claim 2, wherein the p-type Group III-nitride alloy layer comprises In_yAl_1-yN, where 0≦

y≦

1.0, wherein the first composition of the p-type Group III-nitride alloy having its conduction band edge beneath the Fermi level stabilization energy, E_FS, comprises In_yAl_1-yN where 0.57<

y≦

1.0 and wherein the second composition of the p-type Group III-nitride alloy having its conduction band edge above the Fermi level stabilization energy, E_FS, comprises In_yAl_1-yN where y<

0.57.

7. The solar cell of claim 2, wherein the p-type Group III-nitride alloy layer comprises an alloy of In_xAl_yGa_1-x-yN, where 0≦

x, y≦

1.

8. The solar cell of claim 1, wherein the structural imperfections comprise at least one of threading dislocations, grain boundaries, and/or columns in the p-type Group III-nitride alloy.

9. The solar cell of claim 2, wherein the structural imperfections are n-type conductors in the first area possessing a first composition of the p-type Group III-nitride alloy having its conduction band edge beneath the Fermi level stabilization energy, E_FS, to transport the accumulated photoexcited electrons to the layer of n-type Group III-nitride alloy.

10. The solar cell of claim 1, further comprising:

a first electrical contact coupled to the p-type Group III-nitride alloy layer;

a silicon base substrate; and

a second electrical contact coupled to the silicon base substrate.

11. The solar cell of claim 10, further comprising at least one buffer layer formed between the silicon base substrate and the n-type Group III-nitride alloy layer.

12. The solar cell of claim 10, wherein the p-type Group III-nitride alloy layer and the n-type Group III-nitride alloy layer form a top cell in the solar cell for absorbing a first portion of the solar spectrum, wherein the silicon base substrate is formed to possess a p-type region and an n-type region so as to form a bottom p-n junction cell in the solar cell for absorbing a second portion of the solar spectrum.

13. A solar cell, comprising:

a layer of a p-type Group III-nitride alloy for absorbing a portion of the solar spectrum upon exposure to light to generate photoexcited electron-hole pairs, wherein the p-type Group III-nitride alloy includes a top surface and a bottom surface;

a plurality of structural imperfections within the p-type Group III-nitride alloy, each of the structural imperfections having a surface that extends a full distance from the top surface to the bottom surface of the p-type Group III-nitride alloy;

wherein the p-type Group III-nitride alloy is compositionally graded along the plurality of structural imperfections between at least a first area and a second area of the p-type Group III-nitride alloy such that the first area possesses a first composition of the p-type Group III-nitride alloy having a conduction band edge beneath a Fermi level stabilization energy, and the second area possesses a second composition of the p-type Group III-nitride alloy having a conduction band edge above the Fermi level stabilization energy; and

a layer of n-type Group III-nitride alloy underlying the layer of p-type Group III-nitride alloy.

14. The solar cell of claim 13, further comprising a silicon substrate underlying the layer of n-type Group III-nitride alloy.

15. The solar cell of claim 13, wherein the p-type Group III-nitride alloy layer comprises In_xGa_1-xN, where 0≦

x≦

1.0, wherein the first composition of the p-type Group III-nitride alloy comprises In_xGa_1-xN, where 0.35<

x≦

1.0 and wherein the second composition of the p-type Group III-nitride alloy comprises In_xGa_1-xN, where x<

0.35.

16. The solar cell of claim 13, wherein the plurality of structural imperfections comprises a threading dislocation, and an electron accumulation area of the threading dislocation increases in size as the threading dislocation approaches the bottom surface of the p-type Group III-nitride alloy.

17. The solar cell of claim 13, wherein the p-type Group III-nitride alloy layer comprises In_yAl_1-yN, where 0≦

y≦

1.0, wherein the first composition of the p-type Group III-nitride alloy comprises In_yAl_1-yN where 0.57<

y≦

1.0 and wherein the second composition of the p-type Group III-nitride alloy comprises In_yAl_1-yN where y<

0.57.

18. The solar cell of claim 13, wherein the p-type Group III-nitride alloy layer comprises an alloy of In_xAl_yGa_1-x-yN, where 0≦

x, y≦

1.

19. The solar cell of claim 13, wherein the p-type Group III-nitride alloy layer has a thickness of at least 200 nm.

20. A solar cell, comprising:

a layer of a p-type Group III-nitride alloy for absorbing a portion of the solar spectrum upon exposure to light to generate photoexcited electron-hole pairs, wherein the p-type Group III-nitride alloy includes a top surface and a bottom surface, and wherein the p-type Group III-nitride alloy layer has a thickness of at least 200 nm;

a plurality of structural imperfections within the p-type Group III-nitride alloy, each of the structural imperfections having a surface that fully extends from the top surface to the bottom surface of the p-type Group III-nitride alloy, wherein the plurality of structural imperfections comprises a threading dislocation, and an electron accumulation area of the threading dislocation increases in size as the threading dislocation approaches the bottom surface of the p-type Group III-nitride alloy;

wherein the p-type Group III-nitride alloy is compositionally graded along the plurality of structural imperfections between at least a first area and a second area of the p-type Group III-nitride alloy such that the first area possesses a first composition of the p-type Group III-nitride alloy having a conduction band edge beneath a Fermi level stabilization energy, and the second area possesses a second composition of the p-type Group III-nitride alloy having a conduction band edge above the Fermi level stabilization energy; and

a layer of n-type Group III-nitride alloy underlying the layer of p-type Group III-nitride alloy.