RADIATION RESISTANT INVERTED METAMORPHIC MULTIJUNCTION SOLAR CELL

Abstract

A multijunction solar cell including a first solar subcell having a first band gap and a first short-circuit current; a second solar subcell disposed over the first solar subcell and having a second band gap greater than the first band gap and a second short-circuit current greater than the first short-circuit current by an amount in the range of 2% to 6%; a third solar subcell disposed over the second solar subcell and having a third band gap greater than the second band gap and a third short-circuit current less than the first short-circuit current by an amount in the range of 2% to 6%; and a fourth solar subcell disposed over the third solar subcell having a fourth band gap greater than the third band gap, and a fourth short-circuit current less than the third short-circuit current by an amount in the range of 6% to 10%, so that at an end of life state of the multijunction solar cell in an AM0 space environment the short-circuit current of each of the subcells are substantially identical.

Claims

1. A multijunction solar cell comprising: a first solar subcell composed of InGaAs and having a first band gap and a first short-circuit current; a second solar subcell composed of InGaAs disposed over the first solar subcell and having a second band gap greater than the first band gap and a second short-circuit current greater than the first short-circuit current by an amount in the range of 2% to 6%; a third solar subcell composed of GaAs disposed over the second solar subcell and having a third band gap greater than the second band gap and a third short-circuit current less than the first short-circuit current by an amount in the range of 2% to 6%; and a fourth solar subcell composed of InGaP disposed over the third solar subcell having a fourth band gap greater than the third band gap, and a fourth short-circuit current less than the third short-circuit current by an amount in the range of 6% to 10%, so that at an end of life state of the multijunction solar cell in an AM0 space environment the short-circuit current of each of the subcells are substantially identical.

2. A multijunction solar cell according to claim 1, wherein the end of life state corresponds to a period of use in an AM0 space environment of at least 15 years.

3. A multijunction solar cell according to claim 1, wherein the end of life state corresponds to exposure to a fluence of 110.sup.15 1 MeV electrons per square centimeter.

4. A multijunction solar cell according to claim 1, wherein the third solar subcell is lattice matched to the fourth solar subcell.

5. A multijunction solar cell according to claim 4, wherein a first graded interlayer is provided between the first and second solar subcells.

6. A multijunction solar cell according to claim 5, wherein a second graded interlayer is provided between the second and third solar subcells.

7. A multijunction solar cell for a space radiation environment, the multijunction solar cell having a plurality of solar sub-cells arranged in order of increasing band gap including: a first solar subcell composed of InGaAs and having a first band gap, the first solar subcell having a first short circuit current associated therewith; a second solar subcell composed of InGaAs and having a second band gap which is greater than the first band gap, the second solar subcell having a second short circuit current associated therewith; wherein in a beginning of life state the second short circuit current is greater than the first short circuit current such that the AM0 conversion efficiency is sub-optimal.

8. A multijunction solar cell according to claim 7, wherein the second short circuit current is greater than the first short circuit current by an amount in the range of 2% to 6%.

9. A multijunction solar cell according to claim 7, wherein said structure provides an AM0 conversion efficiency which varies over the life of the multijunction solar cell such that by the end of life of the multijunction solar cell the electrical energy generated is greater than for a multijunction solar cell having a structure which provides optimal beginning of life AM0 conversion efficiency.

10. A multijunction solar cell according to claim 9, wherein the end of life AM0 conversion efficiency is greater than 82% of the beginning of life AM0 efficiency.

11. A multijunction solar cell according to claim 10, wherein said end of life state corresponds to exposure to a fluence of 110.sup.15 1 MeV electrons per square centimeter.

12. A multijunction solar cell according to claim 8, wherein the plurality of solar sub-cells further includes: a third solar subcell composed of GaAs and having a third band gap which is greater than the second band gap, the third solar subcell having a third short circuit current associated therewith that is less than the second short circuit current; and a fourth solar subcell composed of InGaP and having a fourth band gap which is greater than the third band gap, the fourth solar subcell having a fourth short circuit current associated therewith which is less than the third short circuit current.

13. A multijunction solar cell according to claim 12, wherein the second short circuit current is greater than the first short circuit current by an amount in the range of 2% to 6%, wherein the third short circuit current is less than the first short circuit current by an amount in the range of 2% to 6%, and wherein the fourth short circuit current is less than the third short circuit current by an amount in the range of 6% to 10%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 schematically shows the main regions of a multijunction solar cell according to an embodiment of the invention;

[0011] FIG. 2 is a graph showing for each of four solar subcells of the multijunction solar cell illustrated in FIG. 1, the variation over time of the ratio of the short circuit current density for that solar subcell and the short circuit current for the solar subcell having the largest band gap;

[0012] FIG. 3 is a graph showing the variation in conversion efficiency during the lifetime of the multijunction solar cell illustrated in FIG. 1 in comparison with a current-matched multijunction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] 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 actual embodiments nor the relative dimensions of the depicted elements, and are not drawn to scale.

[0014] FIG. 1 schematically shows an inverted metamorphic four junction solar cell, hereafter referred to as an IMM4J solar cell. In particular, FIG. 1 shows an exploded view of the main layers of the IMM4J solar cell before removal of the growth substrate 1. It will be appreciated that the IMM4J solar cell shown in FIG. 1 is typically reverse-mounted onto a surrogate substrate and the growth substrate 1 is removed prior to use.

[0015] An InGaP solar subcell 3 is deposited on the growth substrate 1, and a GaAs solar subcell 5 is deposited on the InGaP solar subcell 3 such that the InGaP solar subcell 3 is between the growth substrate 1 and the GaAs solar subcell 5. The InGaP solar subcell 3 and the GaAs solar subcell 5 are lattice-matched to the growth substrate 1.

[0016] A first graded interlayer 7 is interposed between the GaAs solar subcell 3 and a first InGaAs solar subcell 9 on the side of the GaAs solar subcell 5 opposing the InGaP solar subcell 1. The first graded interlayer 7 is a metamorphic layer for bridging the difference between the lattice constants of the GaAs solar subcell 5 and the first InGaAs solar subcell 9.

[0017] A second graded interlayer 11 is interposed between the first InGaAs solar subcell 9 and a second InGaAs solar subcell 13 on the side of the first InGaAs solar subcell 9 opposing the first graded interlayer 7. The second graded interlayer 11 is a metamorphic layer for bridging the difference between the lattice constants of the first InGaAs solar subcell 9 and the second InGaAs solar subcell 13.

[0018] The InGaP solar subcell 3 has a band gap of 1.9 eV; the GaAs solar subcell 5 has a band gap of 1.4 eV; the first InGaAs solar subcell 9 has a band gap of 1.0 eV; and the second InGaAs solar subcell 13 has a band gap of 0.7 eV. Accordingly, the plurality of solar subcells are arranged in order of decreasing band gap from the growth substrate. In this way, when solar radiation impinges from the growth substrate side (following removal of the growth substrate 1), photons having an energy in excess of 1.9 eV are generally absorbed by the InGaP solar subcell 3, photons having an energy between 1.4 eV and 1.9 eV are generally absorbed by the GaAs solar subcell 5, photons having an energy between 1.0 eV and 1.4 eV are generally absorbed by the first InGaAs solar subcell 9 and photons having an energy between 0.7 eV and 1.0 eV are generally absorbed by the second InGaAs solar subcell 13. This results in a theoretical conversion efficiency of 40.8%.

[0019] A key requirement for solar cells intended for space applications is the ability to withstand exposure to electron and proton particle radiation. As mentioned previously, InGaAs solar cells are known to have lower radiation resistance than InGaP solar cells and GaAs solar cells. Accordingly, the short circuit current associated with InGaAs solar cells will fall at a faster rate than the short circuit current associated with InGaP solar cells and GaAs solar cells.

[0020] Conventionally, multijunction solar cells are designed such that at the beginning of the life of the solar cell, the short circuit currents for all the solar subcells are substantially identical. In this embodiment, to take account of the fact that the short circuit current for the first InGaAs solar subcell 9 and the second InGaAs solar subcell 13 will fall more quickly than that of the InGaP solar subcell 3 and the GaAs solar subcell 5, the short circuit currents for the solar subcells at the beginning of the life of the solar cell are mismatched such that the short circuit current at the end of life of the solar cell are substantially matched. In this way, the total energy conversion over the lifetime of the solar cell is improved.

[0021] FIG. 2 illustrates the conversion of the short circuit currents for the four solar subcells over the lifetime of the solar cell. In particular, the y-axis shows the value of the short-circuit current of each solar cell relative to the short circuit current of the InGaP solar cell 3. The variation of the short circuit currents of individual solar subcells over the lifetime was investigated using single junction cells which were fabricated to represent respective individual subcells, with surrounding subcell materials isotyped to produce the same device heat loads during growth as well as absorption characteristics for irradiation. The single junction cells were then exposed to 1-MeV electron radiation at fluences of 5E14 and 1E15 e/cm.sup.2.

[0022] As expected, the first InGaAs solar subcell 9 and the second InGaAs solar subcell 11 exhibit lower radiation resistance that the InGaP solar subcell 3 and the GaAs solar subcell 5. Surprisingly, however, the second InGaAs solar subcell 11 exhibits higher radiation resistance than the first InGaAs solar subcell 9. This was not expected because the expectation was that the higher InAs content in the second InGaAs solar cell 11 would result in a higher degree of degradation in that subcell in comparison with the first InGaAs solar subcell 9.

[0023] One theory that may explain the higher radiation resistance of the second InGaAs solar subcell 11 in comparison with the first InGaAs solar subcell 9 is that the diffusion length at the beginning of life of the second InGaAs solar subcell 11 is much longer than that of the first InGaAs solar subcell 9, which would be due to the higher minority carrier concentration in InAs relative to GaAs. Accordingly, although the change in diffusion length once subjected to electron radiation of the second InGaAs solar subcell 11 may be larger than that of the first InGaAs solar subcell 9, the net diffusion length at end of life is still longer in the second InGaAs solar subcell.

[0024] The desired current mismatch between the solar subcells at beginning of life may be accomplished by varying the subcell thicknesses and the subcell band gap. In this embodiment, the additional current required by the GaAs solar subcell 5 is achieved by thinning the InGaP solar subcell 3 in comparison to the width required for current matching, while the additional current required by the first and second InGaAs solar subcells is generated by a slight reduction in band gap, resulting in an increase in the absorption band in comparison to the band gaps for current matching.

[0025] Following optimisation of the fabrication procedure, the IMM4J solar cell exhibited at beginning of life an AM0 conversion efficiency of about 34%. This was a small decrease in comparison with an equivalent IMM4J solar cell that is current-matched at beginning of life but, as shown in FIG. 3, the end of life remaining factor for the IMM4J solar cell according to the invention is significantly better than that of the equivalent IMM4J solar cell that is current-matched at beginning of life. The structure of the IMM4J solar cell according to the present invention provides an AM0 conversion efficiency which varies over the life of the multijunction solar cell such that by the end of life of the multijunction solar cell the electrical energy generated is greater than for a multijunction solar cell having a structure which provides optimal beginning of life AM0 conversion efficiency.

[0026] Although the solar cell described above is a four junction solar cell, it is envisaged that the present invention can also apply to other multijunction solar cells, for example five junction or six junction metamorphic solar cells (IMM5J or IMM6J solar cells).