# Patent

## Title
Inverted metamorphic multijunction solar cells

## Abstract

A method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell comprising providing first substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on said substrate having a first band gap; forming a second solar subcell over said first subcell having a second band gap smaller than said first band gap; and forming a grading interlayer over said second subcell having a third band gap larger than said second band gap forming a third solar subcell having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell.

## Background
GOVERNMENT RIGHTS STATEMENT
This invention was made with Government support under Contract No. FA9453-04-2-0041 awarded by the U.S. Air Force. The Government has certain rights in the invention.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of solar cell semiconductor devices, and particularly to integrated semiconductor structures including a multijunction solar cell including a metamorphic layer.
2. Description of the Related Art
Photovoltaic cells, also called solar cells, are one of the most important new energy sources that have become available in the past several years. Considerable effort has gone into solar cell development. As a result, solar cells are currently being used in a number of commercial and consumer-oriented applications. While significant progress has been made in this area, the requirement for solar cells to meet the needs of more sophisticated applications has not kept pace with demand. Applications such as satellites used in data communications have dramatically increased the demand for solar cells with improved power and energy conversion characteristics.
In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important.
Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cells connected together in a series. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
Inverted metamorphic solar cell structures such as described in U.S. Pat. No. 6,951,819 and M. W. Wanlass et al., Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31.sup.st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005) present an important starting point for the development of future commercial products. The structures described in such prior art present a number of practical difficulties relating to the appropriate choice of materials and fabrication steps, in particular associated with the lattice mismatched layers between the “lower” subcell (the subcell with the lowest bandgap) and the adjacent subcell.
Prior to the present invention, the materials and fabrication steps disclosed in the prior art have not been adequate to produce a commercial viable, manufacturable, and energy efficient solar cell.
SUMMARY OF THE INVENTION
1. Objects of the Invention
It is an object of the present invention to provide an improved multijunction solar cell.
It is an object of the invention to provide an improved inverted metamorphic solar cell.
It is another object of the invention to provide in a multi-cell structure, an interlayer between a second subcell and a third lattice mismatched subcell that maximizes the energy efficiency of the solar cell.
It is still another object of the invention to provide a method of manufacturing an inverted metamorphic solar cell as a thin, flexible film.
Some implementations or embodiments of the invention may achieve fewer than all of the foregoing objects.
Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility.
2. Features of the Invention
Briefly, and in general terms, the present invention provides a solar cell including a semiconductor body having an upper surface; a multijunction solar cell disposed on the upper surfaces; a first solar subcell on the substrate having a first band gap; a second solar subcell disposed over the first subcell and having a second band gap smaller than the first band gap; and a grading interlayer disposed over the second subcell interlayer having a third band gap larger than the second band gap, and a third solar subcell over the second solar subcell such that the third solar subcell is lattice mismatched with respect to the second subcell and the third subcell has a fourth band gap smaller than the third band gap.
In another aspect, the present invention provides a method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell by providing a first substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on said substrate having a first band gap; forming a second solar subcell over said first subcell having a second band gap smaller than said first band gap; and forming a grading interlayer over said second subcell having a third band gap larger than said second band gap forming said at least one lower subcell over said middle subcell such that said at least one lower subcell is lattice mismatched with respect to said middle subcell and said third subcell has a fourth band gap smaller than said second band gap.
Some implementations or embodiments may incorporate or implement fewer of the aspects or features noted in the foregoing summaries.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of this invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:
FIG. 1 is an enlarged cross-sectional view of the solar cell according to some embodiments of the present invention at the end of the process steps of forming the layers of the solar cell;
FIG. 2 is a cross-sectional view of the solar cell of FIG. 1 after the next process step according to some embodiments of the present invention;
FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after the next process step according to some embodiments of the present invention;
FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after the next process step according to some embodiments of the present invention in which a surrogate substrate is attached;
FIG. 5A is a cross-sectional view of the solar cell of FIG. 4 after the next process step according to some embodiments of the present invention in which the original substrate is removed;
FIG. 5B is another cross-sectional view of the solar cell of FIG. 4 after the next process step according to some embodiments of the present invention in which the original substrate is removed;
FIG. 6A is a top plan view of a wafer in which the solar cells according to some embodiments of the present invention are fabricated;
FIG. 6B is a bottom plan view of a wafer in which the solar cells according to some embodiments of the present invention are fabricated;
FIG. 7 is a top plan view of the wafer of FIG. 6B after the next process step according to some embodiments of the present invention;
FIG. 8 is a cross-sectional view of the solar cell of FIG. 5B after the next process step according to some embodiments of the present invention;
FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after the next process step according to some embodiments of the present invention;
FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after the next process step according to some embodiments of the present invention;
FIG. 11 is a cross-sectional view of the solar cell of FIG. 10 after the next process step according to some embodiments of the present invention;
FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after the next process step according to some embodiments of the present invention;
FIG. 13 is a cross-sectional view of the solar cell of FIG. 12 after the next process step according to some embodiments of the present invention;
FIG. 14 is a cross-sectional view of the solar cell of FIG. 13 after the next process step according to some embodiments of the present invention;
FIG. 15 is a cross-sectional view of the solar cell of FIG. 14 after the next process step according to some embodiments of the present invention.



DESCRIPTION OF THE PREFERRED EMBODIMENT
Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
FIG. 1 depicts the multijunction solar cell according to some embodiments of the present invention after formation of the three subcells A, B and C on a substrate. More particularly, there is shown a substrate 101, which may be either gallium arsenide (GaAs), germanium (Ge), or other suitable material. In the case of a Ge substrate, a nucleation layer 102 is deposited on the substrate. On the substrate, or over the nucleation layer 102, a buffer layer 103, and an etch stop layer 104 are further deposited. A contact layer 105 is then deposited on layer 104, and a window layer 106 is deposited on the contact layer. The subcell A, consisting of an n+ emitter layer 107 and a p-type base layer 108, is then deposited on the window layer 106.
It should be noted that the multijunction solar cell structure could be formed by any suitable combination of group III to V elements listed in the periodic table subject to lattice constant and band gap requirements, wherein the group III includes boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (T). The group IV includes carbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).
In the preferred embodiment, the substrate 101 is gallium arsenide, the emitter layer 107 is composed of InGa(Al)P, and the base layer is composed of InGa(Al)P.
On top of the base layer 108 is deposited a back surface field (“BSF”) layer 109 used to reduce recombination loss.
The BSF layer 109 drives minority carriers from the region near the base/BSF interface surface to minimize the effect of recombination loss. In other words, a BSF layer 109 reduces recombination loss at the backside of the solar subcell A and thereby reduces the recombination in the base.
On top of the BSF layer 109 is deposited a sequence of heavily doped p-type and n-type layers 110 which forms a tunnel diode which is a circuit element to connect cell A to cell B.
On top of the tunnel diode layers 110 a window layer 111 is deposited. The window layer 111 used in the subcell B also operates to reduce the recombination loss. The window layer 111 also improves the passivation of the cell surface of the underlying junctions. It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present invention.
On top of the window layer 111 the layers of cell B are deposited: the emitter layer 112, and the p-type base layer 113. These layers are preferably composed of InGaP and In0.015GaAs respectively, although any other suitable materials consistent with lattice constant and band gap requirements may be used as well.
On top of the cell B is deposited a BSF layer 114 which performs the same function as the BSF layer 109. A p++/n++ tunnel diode 115 is deposited over the BSF layer 114 similar to the layers 110, again forming a circuit element to connect cell B to cell C. A buffer layer 115a, preferably InGaAs, is deposited over the tunnel diode 115, to a thickness of about 1.0 micron. A metamorphic buffer layer 116 is deposited over the buffer layer 115a and is preferably a compositionally step-graded InGaAlAs series of layers with monotonically changing lattice constant to achieve a transition in lattice constant from cell B to subcell C. The bandgap of layer 116 (i.e., throughout the series of layers) is 1.5 eV constant with a value slightly greater than the bandgap of the middle cell B.
In one embodiment, as suggested in the Wanlass et al. paper, the step grade contains nine compositionally graded steps with each step layer having a thickness of 0.25 micron. In the preferred embodiment, the interlayer is composed of InGaAlAs, with monotonically changing lattice constant.
A window layer 117 is deposited on top of the cell B and more specifically, for example, above the metamorphic buffer layer 116. On top of the window layer 117 the layers of cell C are deposited: an n-type emitter layer 118 and a p-type base layer 119. On top of the cell C is deposited a BSF layer 120. A p+ contact layer 121 is deposited over the BSF layer 120 to allow a circuit to be formed with cell C.
FIG. 2 is a cross-sectional view of the solar cell of FIG. 1 after the next process step according to some embodiments of the present invention in which a metal contact layer 122 is deposited over the p+ semiconductor contact layer 121. The metal is preferably a sequence of Ti/Au/Ag/Au layers.
FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after the next process step according to some embodiments of the present invention in which an adhesive layer 123 is deposited over the metal layer 122. The adhesive is preferably GenTak 330 (distributed by General Chemical Corp.).
FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after the next process step according to some embodiments of the present invention in which a surrogate substrate, preferably sapphire, is attached. In the preferred embodiment, the surrogate substrate is about 40 mils in thickness, and is perforated with holes about 1 mm in diameter, spaced 4 mm apart, to aid in subsequent removal of the substrate.
FIG. 5A is a cross-sectional view of the solar cell of FIG. 4 after the next process step according to some embodiments of the present invention in which the original substrate is removed by a sequence of lapping and/or etching steps in which the substrate 101, the buffer layer 103, and the etch stop layer 104, are removed. The etchant is growth substrate dependent.
FIG. 5B is a cross-sectional view of the solar cell of FIG. 5A from the solar cell of FIG. 5A from the orientation with the surrogate substrate 124 being at the bottom of the Figure.
FIG. 6A is a top plan view of a wafer in which the solar cells according to some embodiments of the present invention are implemented.
FIG. 6B is a bottom plan view of the wafer with four solar cells shown in FIG. 6A. In each cell there are grid lines 501 (more particularly shown in FIG. 10), an interconnecting bus line 502, and a contact pad 503.
FIG. 7 is a bottom plan view of the wafer of FIG. 6B after the next process step in which a mesa 510 is etched around the periphery of each cell using phosphide and arsenide etchants.
FIG. 8 is a cross-sectional view of the solar cell of FIG. 5B after the next process step according to some embodiments of the present invention in which the sacrificial buffer layer has been removed with 4 citric 1 H2O2 solution.
FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after the next process step according to some embodiments of the present invention in which the etch stop layer 104 is removed by HCl/H2O solution.
FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after the next process step according to some embodiments of the present invention in which a photoresist mask (not shown) is placed over the contact layer 105 as the first step in forming the grid lines 501. The mask 200 is lifted off to form the grid lines 501.
FIG. 11 is a cross-sectional view of the solar cell of FIG. 10 after the next process step according to some embodiments of the present invention in which grid lines 501 are deposited via evaporation and lithographically patterned and deposited over the contact layer 105. The grid lines are used as a mask to etch down the surface to the window layer 106 using a citric acid/peroxide etching mixture.
FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after the next process step according to some embodiments of the present invention in which an antireflective (ARC) dielectric coating layer 130 is applied over the entire surface of the “bottom” side of the wafer with the grid lines 501.
FIG. 13 is a cross-sectional view of the solar cell of FIG. 12 after the next process step according to some embodiments of the present invention in which the mesa 501 is etched down to the metal layer 122 using phosphide and arsenide etchants. The cross-section in the figure is depicted as seen from the A-A plane shown in FIG. 7.
One or more silver electrodes are welded to the respective contact pads.
FIG. 14 is a cross-sectional view of the solar cell of FIG. 13 after the next process step according to some embodiments of the present invention after the surrogate substrate 124 and adhesive 123 are removed by EKC 922. Perforations are made over the surface, each with a diameter is 0.033 inches and separated by 0.152 inches.
The perforations allow the flow of etchant through the surrogate substrate 124 to permit its lift off.
FIG. 15 is a cross-sectional view of the solar cell of FIG. 14 after the next process step according to some embodiments of the present invention in which an adhesive is applied over the ARC layer 130 and a coverglass attached thereto.
It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a multijunction solar cell, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

## Claims

1. A method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell, the method comprising:
providing a first substrate composed of GaAs or Ge for the epitaxial growth of semiconductor material;
growing a first solar subcell composed of InGa(Al)P on said first substrate having a first band gap;
growing a second solar subcell composed of In0.015GaAs over said first subcell having a second band gap smaller than said first band gap; and
growing a grading interlayer over said second solar subcell, said interlayer composed of InGaAlAs and having a third band gap greater than said second band gap, the band gap of the InGaAlAs grading interlayer being constant; and
growing a third solar subcell composed of In0.30GaAs over said grading interlayer having a fourth band gap smaller than said second band gap such that said third solar subcell is lattice mismatched with respect to said second solar subcell,
wherein the grading interlayer achieves a transition in lattice constant from the second subcell to the third subcell.

2. A method of forming a solar cell as defined in claim 1, wherein said first solar subcell is composed of an InGa(Al)P emitter region and an InGa(Al)P base region.

3. A method of forming a solar cell as defined in claim 1, further comprising depositing a bottom contact layer over said third solar subcell and making electrical contact therewith.

4. A method of forming a solar cell as defined in claim 3, further comprising attaching a surrogate second substrate over said bottom contact layer and removing the first substrate.

5. A method of forming a solar cell as defined in claim 4 and subsequent to removing the first substrate, further comprising;
forming a metal top contact layer over said first solar subcell; and
patterning said top contact layer into a grid.

6. A method of forming a solar cell as defined in claim 5, further comprising etching said solar cell so as to form a mesa on said surrogate second substrate, where the mesa is surrounded by a trough.

7. The method of forming a solar cell as defined in claim 4, wherein the surrogate second substrate is about 40 mils in thickness, and is perforated with holes to aid in subsequent removal of the second surrogate substrate.

8. A method of forming a solar cell as defined in claim 1, wherein said grading interlayer is a compositionally step-graded metamorphic buffer layer composed of a series of InGaAlAs layers with a monotonically changing lattice constant that achieves a transition in lattice constant from the second solar subcell to the third solar subcell.

9. A method of forming a solar cell as defined in claim 1, wherein the grading interlayer is composed of a plurality of layers with monotonically changing lattice constant.

10. A method of forming a solar cell as defined in claim 1, wherein the grading interlayer includes at least five stepped layers.

11. A method of forming a solar cell as defined in claim 10, wherein the band gap of each of the stepped layers is greater than the second band gap.

12. A method of forming a solar cell as defined in claim 1, wherein the fourth band gap of said third solar subcell is smaller than said third band gap.

13. A method of forming a solar cell as defined in claim 1, wherein the third band gap is greater than the second band gap.

14. A method of forming a solar cell as defined in claim 1, further comprising:
mounting a surrogate substrate on top of the third solar subcell; and
removing the first substrate.

15. A method of forming a solar cell as defined in claim 14, wherein the surrogate substrate is mounted with an adhesive.

16. A method of forming a solar cell as defined in claim 14, wherein the surrogate first substrate is removed by lapping.

17. A method of forming a solar cell as defined in claim 14, wherein the first substrate is removed by etching.

18. A method of forming a solar cell as defined in claim 14, further comprising subsequent to removing the first substrate:
forming a metal top contact layer over said first solar subcell; and
patterning said top contact layer into a grid.

19. A method of forming a solar cell as defined in claim 14, further comprising etching said solar cell so as to form a mesa on said surrogate substrate, wherein the mesa is surrounded by a trough.

20. A method of forming a solar cell as defined in claim 18, further comprising attaching a coverglass over the first solar subcell.

21. A method of forming a solar cell as defined in claim 14, wherein the surrogate substrate is about 40 mils in thickness, and is perforated with holes to aid in subsequent removal of the surrogate substrate.

22. The method of claim 1 further comprising growing a buffer layer over the second solar subcell prior to growing the grading interlayer, wherein the composition of the buffer layer is different from the composition of the grading interlayer.

23. The method of claim 22 further comprising growing a tunnel diode layer over the second solar subcell prior to growing the buffer layer.

24. The method of claim 23 wherein the buffer layer is composed of InGaAs.

25. The method of claim 24 wherein the buffer layer has a thickness of about one micron.

26. The method of claim 23 wherein the tunnel diode is a p++/n++ tunnel diode.

27. A method of forming a solar cell comprising:
providing a first substrate composed of GaAs or Ge;
depositing on the first substrate a sequence of layers of semiconductor material forming a solar cell including a metamorphic buffer layer composed of InGaAlAs and having a constant band gap, disposed between two subcells of the solar cell, wherein the solar cell comprises a first solar subcell composed of InGa(Al)P, a second solar subcell composed of In0.015GaAs, and a third solar subcell composed of In0.30GaAs, the first solar subcell having a first band gap, the second solar subcell having a second band gap smaller than the first band gap, the metamorphic buffer layer having a third band gap greater than the second band gap, the third solar subcell having a fourth band gap smaller than the second band gap, wherein the metamorphic buffer layer achieves a transition in lattice constant from a first one of the subcells to a second one of the subcells;
mounting a surrogate substrate on top of the sequence of layers; and
removing the first substrate.

28. A method of forming a solar cell as defined in claim 27, wherein the third solar subcell is lattice mismatched with respect to said second subcell.

29. A method of forming a solar cell as defined in claim 28, wherein said first solar subcell is composed of an InGa(Al)P emitter region and an InGa(Al)P base region.

30. A method of forming a solar cell as defined in claim 28, further comprising depositing a metal bottom contact layer over said third solar subcell and making electrically contact therewith.

31. A method of forming a solar cell as defined in claim 30 wherein mounting the surrogate substrate comprises attaching the surrogate substrate over said metal bottom contact layer and subsequently removing the first substrate.

32. A method of forming a solar cell as defined in claim 30 and subsequent to removing the first substrate, further comprising:
forming a metal top contact layer; and
patterning said top contact layer into a grid.

33. A method of forming a solar cell as defined in claim 30, further comprising removing said surrogate substrate.

34. A method of forming a solar cell as defined in claim 32, further comprising attaching a coverglass over the solar cell.

35. A method as defined in claim 28, wherein the metamorphic buffer layer is a compositionally step-graded InGaAlAs series of layers with monotonically changing lattice constant to achieve the transition in lattice constant from the second subcell to the third subcell.

36. The method of forming a solar cell as defined in claim 27, wherein the surrogate second substrate is about 40 mils in thickness, and is perforated with holes to aid in subsequent removal of the surrogate substrate.