# Patent

## Title
Inverted metamorphic multijunction solar cells

## Abstract

Multijunction solar cells that may include a first solar subcell with a first band gap, and a second solar subcell disposed over the first solar subcell and having a second band gap smaller than said first band gap. The solar cells may also include a grading interlayer disposed over the second solar subcell that may include a third band gap greater than the second band gap. The grading interlayer may not include phosphorus. The solar cells may also include a third solar subcell disposed over the interlayer that is lattice mismatched with respect to the second solar subcell. The third solar subcell may have a fourth band gap smaller than the third band gap.

## Background
REFERENCE TO RELATED APPLICATIONS
This application is a division of U.S. patent application Ser. No. 11/445,793, filed Jun. 2, 2006, and hereby incorporated by reference in its entirety. This application is also related to co-pending U.S. patent application Ser. No. 11/836,402, filed Aug. 9, 2007.


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 31st 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 band gap) 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 commercially viable, manufacturable 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 of the present invention 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 bottom 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 (TI). 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 band gap of layer 116 (i.e., throughout the series of layers) is 1.5 eV constant with a value slightly greater than the band gap 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. More broadly, the grading interlayer may be composed of InxGa1-xAlAs, with x such that the band gap remains constant at 1.50 eV throughout the graded step layers.
A window layer 117 is deposited on top of the cell B and more specifically, for example, above the metamorphic buffer layer 116. As similar to the window layer 111, the window layer 117 may be used in cell C to reduce recombination loss. The window layer 117 may also improve the passivation of the cell surface of the underlying junctions. 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. The p-type base layer 119 may compose In0.30GaAs, for example; although any other suitable materials consistent with lattice constant and band gap requirements may be used for the layers 119 and 118, respectively. On top of the cell C is deposited a BSF layer 120 which may perform the same function as the BSF layers 109 and 114. 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 one 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 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 510 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 of 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 cover glass 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, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

## Claims

1. A multijunction solar cell comprising:
a first solar subcell composed of InGa(Al)P and having a first band gap;
a second solar subcell disposed over said first solar subcell and having a second band gap smaller than said first band gap, the second solar subcell being composed of In0.015GaAs;
a grading interlayer composed of InGaAlAs disposed over said second solar subcell and having a third band gap that is constant throughout its thickness and greater than said second band gap; and
a third solar subcell disposed over said interlayer that is lattice mismatched with respect to second solar subcell and having a fourth band gap smaller than said third band gap, the third solar subcell being composed of In0.30GaAs,
wherein the grading interlayer achieves a transition in lattice constant from the second solar subcell to the third solar subcell.

2. A multijunction solar cell as defined in claim 1, wherein the grading interlayer includes layers composed of InxGa1-xAlAs with 0<x<1 such that the band gap remains constant at 1.50 eV throughout the layers.

3. A multijunction 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 with a lower layer having a lattice constant substantially the same as the second solar subcell and an upper layer having a lattice constant substantially the same as the third solar subcell, the grading interlayer having a band gap value slightly greater than the band gap of the second solar subcell.

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

5. A multijunction solar cell as defined in claim 1, wherein the grading interlayer includes at least five stepped layers.

6. A multijunction solar cell as defined in claim 5, wherein the band gap of each of the stepped layers is greater than the second band gap.

7. A multijunction solar cell as defined in claim 1, wherein the third band gap is slightly greater than the second band gap.

8. A multijunction solar cell as defined in claim 1 further comprising a buffer between the second solar subcell and the grading interlayer, wherein the composition of the buffer layer is different from the composition of the grading interlayer.

9. A multijunction solar cell as defined in claim 8 further comprising a tunnel diode layer between the second solar subcell and the buffer layer.

10. A multijunction solar cell as defined in claim 9 wherein the buffer layer is composed of InGaAs.

11. A multijunction solar cell as defined in claim 1 wherein the buffer layer has a thickness of about one micron.

12. A multijunction solar cell as defined in claim 9 wherein the tunnel diode is a p++/n++ tunnel diode.