DEVELOPMENTOFSPACESOLARCELLSATSPECTROLAB

DEVELOPMENT OF SPACE SOLAR CELLS AT SPECTROLAB

J. Boisvert, D. Law, R. King, D. Bhusari, X. Liu, S. Mesropian, D. Larrabee, R. Woo, K. Edmondson, D. Krut, D.

Peterson, K. Rouhani, B. Benedikt, and N. Karam

Boeing-Spectrolab, Inc., 12500 Gladstone Ave., Sylmar, CA 91342 U.S.A.

ABSTRACT

High efficiency Inverted Metamorphic (IMM) and

Semiconductor Bonded Technology (SBT) multi-junction

In a typical 3-junction IMM space cell three constituent

GaAs-based subcells are grown in an inverted

configuration. Large volume production MOVPE reactors

solar cells have been under development at Spectrolab for

use in space and near space applications. This paper will

review the present state-of-the-art of this technology at

Spectrolab with an emphasis on performance

characterization data at operating conditions that these

solar cells will experience in flight. Solar cell current-bias

characteristics under illumination (LIV) at AM0 28°C are

presented along with external quantum efficiency

measurements that are used to verify the X-25 solar

simulator LIV short circuit current density. A mechanical

and thermal stress model has been used to predict

mechanical stresses on a ultra-lightweight panel assembly

in orbit and will be discussed.

INTRODUCTION

High efficiency IMM and SBT multi-junction solar cells [1-5]

have been under development at Spectrolab for use in

space and near space applications. This paper reviews the

present state-of-the-art of this technology at Spectrolab

with an emphasis on performance characterization data at

operating conditions that these solar cells will experience

in flight.

Under the AFRL IBIS program a coupon utilizing large

area, low mass IMM solar cells has been assembled. A

cross section of an IMM solar cell is shown in Figure 1.

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Figure 1.

A typical IMM solar cell that is grown in an

inverted configuration on a Ge or GaAs substrate. The

grown structure is affixed to a handle and the growth

substrate removed.

are used to grow these solar cells on 100 mm substrates.

The widest bandgap alloy (top cell) is grown first followed

by the middle cell, buffer layers and finally a low bandgap

metamorphic cell lattice mismatched to the growth

substrate. Subsequent wafer processing places the

inverted multijunction solar cell in an upright configuration

and the growth substrate is removed. Processes typical of

standard, high-volume semiconductor wafer processing

are used to complete fabrication. Cell-Interconnect-

Coverglass (CICs) are then assembled based on typical

production assembly processes.

TEST ARTICLES

A variety of solar cell test articles have been constructed

for use in this technology development. Small area (1 cm

2

as well as large area (26 cm

2

)

) cells for use in 1 sun AM0

environments have been fabricated and tested. Low

concentration small area 2.5 cm

2

cells have also been

fabricated and will be reported on in this paper. Because

specific metal grid patterns vary with design, performance

data for the concentrator cells is reported for cell aperture

areas only.

CHARACTERIZATION DATA

Typical current-bias characteristics for Spectrolab 3J IMM

solar cell under 1 sun AM0 solar simulator illumination (LIV

data) are shown in Figure 2. These simulator data were

collected on an AX-25 solar simulator using calibrated IMM

Lear Jet flight standards; to date no IMM balloon flight

standards have been flown. Typical spectral response

measurements for these cells are shown in Figure 3 and

are agree with the measured Jsc data shown in Figure 2.

Low concentration IMM cells were subjected to additional

characterization at both 1 sun and ~12X concentration.

Typical LIV characteristics at 25 °C are shown in Figure 4.

Low mass 26.62 cm

2

IMM cells and Coverglass-

Interconnect-Cells (CICs) have been fabricated for use on

the AFRL IBIS program. Figure 5 displays the backside

handle of this cell. A cavity structure has been fabricated in

the handle to reduce the total cell mass. A finite element

mechanical and thermal analysis of an IBIS panel

consisting of these CICs affixed to a metalized Kapton

substrate has been performed. This analysis predicts that

the structure will withstand exposure to -180 °C which a

space solar panel can be subjected to on orbit. The stress

distribution for the half plane of this structure is shown in

Figure 6. The maximum anticipated stresses on the

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Figure 2. The LIV characteristic of a 1x1 cm

2

3J IMM

cell at 28 °C. This cell was designed for 1 sun AM0

applications and has an AM0 conversion efficiency of

32.6%. The reported efficiency is based on Lear Jet

IMM calibration standards.

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35501350

Wavelength (nm)

Figure 3. Typical external quantum efficiency

characteristics collected on the 3J IMM solar cells

shown in Figure 2.

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Figure 4. 25 °C LIV characteristics of low concentration

IMM cells at 1X AM0 and ~12X AM0 concentration.

Figure 5. The low mass IBIS 3J IMM cell handle.

Figure 6. The low concentration 3JIMM half-plane

stress model.

Figure 7. An assembled IBIS coupon.

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Figure 8. Photoluminescence maps of 1 eV

Target parameters: V≈3.6 V, J≈16 mA/cm

2

, FF ≈84%, eff. 35% AM0

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=

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polish & reuse

0

metamorphic wafers show recent progress in material

quality.

structure are on the order of 10 MPa, much less than the

failure strengths of the cell materials.

A coupon of low mass IMM CICs was assembled for the

AFRL IBIS program and is shown in Figure 7. Spectrolab

continues to make progress on MOVPE growth of

metamorphic material quality. Figure 8 displays

photoluminescence maps of two wafers demonstrating that

progress. The PL signal shows much better uniformity on

the more recent wafer. That uniformity is reflected in 3J

IMM cell performance shown in Figure 9 along with large

area cell performance data collected solar cells fabricated

from similar material. Large area 3J IMM cells presently

display about 31% AM0 efficiency as measured on an AX-

25 solar simulator set up to a Lear Jet flight calibration.

Figure 9. A wafer uniformity map of 1x1 cm

2

3J IMM

cells and LIV characteristics of large area 26 cm

2

cells

built from similar material.

Spectrolab is also pursuing semiconductor bonded solar

cells for space applications. A 4J SBT solar cell is shown in

Figure 10. This cell has the advantage that all subcells can

be grown lattice-matched to independent substrates which

leads to improved material quality and higher subsequent

performance. The trade is that large area wafer bonds

require very low surface roughness to be held over the

entire wafer. Spectrolab has succeeded in fabricating large

area bonds as shown in Figure 11. 4J SBT cells have been

characterized and shown to have 33.5% AM0 conversion

efficiency as measured on an AX-25 solar simulator set up

to 3J IMM Lear Jet standards – Figure 12. The external

quantum efficiency (as measured using a spectrometer set

up to NIST-traceable calibration standards) of one of these

cells is shown in Figure 13 and demonstrates that all 4

subcells exceed 90% EQE.

OCSC

Figure 10. A 4J SBT utilizes lattice-matched subcells

grown on two different substrates (in this case GaAs

and InP) that are bonded together and subsequently

processed similarly to an IMM cell.

Figure 11. A 4J SBT wafer fabricated with 1x1 cm

2

cells.

0.020

AM0 4J-SBT_B12: 3.394V 15.99mA 0.835 33.5%

0.018

AM0 4J-SBT_B6: 3.393V 15.94mA 0.833 33.3%

0.016

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Figure 12. The AM0 LIV characteristics of 1x1 cm2 4J

SBT cells.

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ACKNOWLEDGEMENTS

The authors would like to thank the entire R&D team at

Spectrolab. Support from the Air Force Research

70

Laboratory Space Vehicles Directorate under Contracts

60

FA9453-09-C-0373 and FA9453-04-2-0042; and funds

50

(support) from The Boeing Company is gratefully

acknowledged.

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4J-SBT_s1

4J-SBT_s2

References

20

4J-SBT_s3

10

4J-SBT_s4

SBT4J Reflectance

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0

351501850

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Wavelength (nm)

Moriarty, R. J. Wehrer, and an, Proceedings of

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e

r

n

a

l

Q

E

(

%

)

Figure 13. The external

quantum

efficiency versus

wavelength as measured on a 4J SBT solar cell.

CONCLUSIONS

Spectrolab continues development of inverted

metamorphic solar cell technology for high efficiency space

and near space applications. IMM solar cells with 1X AM0

efficiency greater than 32.5% at 28 °C have been

demonstrated. These efforts have led to development of 3J

IMM low concentration cells. 3J IMM CICs have been

measured at 28 °C under concentration and have

demonstrated greater than 34% efficiency. A coupon with

3J IMM CICs has been fabricated under the IBIS program.

4J SBT solar cells have been fabricated and demonstrate

33.5% AM0 efficiency at 28 °C.

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