Development of advanced space solar cells at Spectrolab

Abstract

High efficiency multi-junction solar cells utilizing inverted metamorphic and semiconductor bonding technology are being developed at Spectrolab for use in one-sun space and near-space applications. Recently that effort has been extended to include low-concentration space applications. This paper will review the present state-of-the-art cell technologies at Spectrolab, with an emphasis on performance characterization data at both 1-sun and low-concentration operating conditions that these cells will experience in flight.. A cell coupon utilizing IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected and there is no performance degradation or mechanical issues after the test.

Full text

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
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.
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.
In a typical 3-junction IMM space cell three constituent
GaAs-based subcells are grown in an inverted
configuration. Large volume production MOVPE reactors
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 cm2)
as well as large area (26 cm2) cells for use in 1 sun AM0
environments have been fabricated and tested. Low
concentration small area 2.5 cm2 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 cm2 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
Growth
Direction
Ge or GaAs growth
substrate from which
active cells are removed
nucleation
GaInAs buffer
separation layer
Ge or GaAs growth
substrate from which
active cells are removed
nucleation
nucleation
GaInAs buffer
separation layer
Growth
Direction
Growth
Direction
Growth
Direction
Ge or GaAs growth
substrate from which
active cells are removed
nucleation
GaInAs buffer
separation layer
Ge or GaAs growth
substrate from which
active cells are removed
nucleation
nucleation
GaInAs buffer
separation layer

Figure 2. The LIV characteristic of a 1x1 cm2 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.
Figure 3. Typical external quantum efficiency
characteristics collected on the 3J IMM solar cells
shown in Figure 2.
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.
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
00.511.522.533.
5
V
olta
g
e
(
V
)
Jsc (Amp/cm2)
0
10
20
30
40
50
60
70
80
90
100
350 550 750 950 1150 1350
Wavelength (nm)
EQE (%)
0
0.1
0.2
0.3
0.4
0.5
01234
Voltage (V)
Current (A)
8e-11313-04

Figure 8. Photoluminescence maps of 1 eV
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 cm2 3J IMM
cells and LIV characteristics of large area 26 cm2 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.
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 cm2
cells.
Figure 12. The AM0 LIV characteristics of 1x1 cm2 4J
SBT cells.
Target parameters: V
OC
≈3.6 V, J
SC
≈16 mA/cm
2
, FF ≈84%, eff. 35% AM0
0.5
0.4
0.3
0.2
0.1
0.0
Current Density [mA/cm
2
]
18001600140012001000800600400
Wavelength [nm]
2.0 1.5 1.1 0.7 eV
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0 0.5 1 1.5 2 2.5 3 3 .5
4
Voltage (V)
Current density (A/cm2)
4J-SBT Target, 35% AM0
S1: 1.470 V, 15.6 mA, 0.856
S2: 1.081 V, 16.0 mA, 0.845
S3: 0.680 V, 16.8 mA, 0.817
S4: 0.350 V, 18.4 mA, 0.711
GaAs
s1
s2
InP
s4
s3
+=
GaAs
InP
+
bond
polish & reuse
remove
s2
s3
s4
s1
y
,y
Target parameters: V
OC
≈3.6 V, J
SC
≈16 mA/cm
2
, FF ≈84%, eff. 35% AM0
0.5
0.4
0.3
0.2
0.1
0.0
Current Density [mA/cm
2
]
18001600140012001000800600400
Wavelength [nm]
2.0 1.5 1.1 0.7 eV
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0 0.5 1 1.5 2 2.5 3 3 .5
4
Voltage (V)
Current density (A/cm2)
4J-SBT Target, 35% AM0
S1: 1.470 V, 15.6 mA, 0.856
S2: 1.081 V, 16.0 mA, 0.845
S3: 0.680 V, 16.8 mA, 0.817
S4: 0.350 V, 18.4 mA, 0.711
GaAs
s1
s2
InP
s4
s3
+=
GaAs
InP
+
bond
polish & reuse
remove
s2
s3
s4
s1
s2
s3
s4
s1
y
,y
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
01234
Voltage [V]
Current Density [A/cm2]
AM0 4J-SBT_B12: 3.394V 15.99mA 0.835 33.5%
AM0 4J-SBT_B6: 3.393V 15.94mA 0.833 33.3%

0
10
20
30
40
50
60
70
80
90
100
350 450 550 650 750 850 950 1050 1150 1250 1350 1450 1550 1650 175 0 185
0
Wavelength (nm)
Extern al QE (%)
4J-SBT_s1
4J-SBT_s2
4J-SBT_s3
4J-SBT_s4
SBT4J Reflectance
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.
ACKNOWLEDGEMENTS
The authors would like to thank the entire R&D team at
Spectrolab. Support from the Air Force Research
Laboratory Space Vehicles Directorate under Contracts
FA9453-09-C-0373 and FA9453-04-2-0042; and funds
(support) from The Boeing Company is gratefully
acknowledged.
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