Qualification and Production of Emcore ZTJ Solar Panels for Space
Missions
Navid Fatemi1, John Lyons2 and Mike Eskenazi3
1
2
Emcore Photovoltaics, Albuquerque, New Mexico 87122, USA
NASA Goddard Space Flight Center (GSFC), Greenbelt, Maryland 20771, USA
3
ATK Space Systems, Goleta, California 93117, USA
Abstract — Emcore’s latest generation InGaP/InGaAs/Ge ZTJ
triple-junction space-grade high-efficiency solar cells have been in
volume production since 2009, with over 300,000 flight cells
produced to power more than 35 separate satellites. The ZTJ
cells, CICs (Coverglass-Interconnected-Cell) and solar panels
have also been characterized and qualified to both the AIAA-S111 and AIAA-S-112 standards. More than 10 life-cycle coupon
panels have been thermal cycled to temperature extremes
representing varied orbital conditions such as GEO
(geosynchronous), LEO (low-earth) and highly elliptical orbits.
In its larger 1-cell-per-wafer form factor, the ZTJ solar cell has
been used to manufacture solar panels for a dozen NASA and
other commercial spacecraft. Currently Emcore Photovoltaics is
under contract to NASA’s Goddard Space Flight Center (GSFC)
to build and deliver the solar panels for the Magnetospheric
Multiscale (MMS) Mission and to ATK Space Systems for the
Commercial Resupply (CRS) mission to the International Space
Station (ISS) and the AMOS-6 commercial telecommunication
satellite. In this paper, the results of the qualification, production
and testing of the life-cycle coupons and the solar panels for these
missions is presented. In addition, the initial on-orbit telemetry
performance results of NASA’s NuStar (Nuclear Spectroscopic
Telescope Array) spacecraft solar array will be presented. The
NuStar spacecraft was launched in June of 2012 and built by
Orbital Sciences Corp. It is powered by an ATK solar array
populated with ZTJ solar cells.
Index Terms — Multi-junction, space solar cells, MMS, CRS,
AMOS-6, NuStar
I. INTRODUCTION
Solar arrays populated with high-efficiency III-V based
multi-junction solar cells have been powering the payloads of
earth orbiting satellites and interplanetary spacecraft for well
over a decade. Due to their higher performance compared to
silicon cells, multi-junction cells have enabled the realization
of higher power and larger capacity satellites.
The conversion efficiency of the InGaP/InGaAs/Ge triplejunction (3J) solar cells, under Air-Mass Zero (AM0)
illumination at 28 C, has been steadily improved from about
23% to approximately 30% at beginning-of-life (BOL) since
the year 2000 [1-2]. The vast majority of the scientific,
military, exploration and telecommunication spacecraft now
use the highest efficiency 3J solar cells available. The power
requirements for these spacecraft range from ~1 kW to as
much as 25 kW at end-of-life (EOL).
One of the highest performance solar cells in volume
production is Emcore’s ZTJ cell, with an average AM0
conversion efficiency of ~29.5%. More than 300,000 ZTJ
flight cells (or ~350 kW at BOL) have been manufactured
since 2009 to power more than 35 spacecraft. An AM0
efficiency histogram of the ZTJ cell is shown in Fig. 1.
28.0
28.5
29.0
29.5
30.0
30.5
AM0 Efficiency
Fig. 1 – AM0 (one-sun at 28 C) Efficiency histogram of ZTJ
solar cells in production
Historically, the solar cells used to populate flight solar
panels have been of 2-per-wafer form factor, meaning that two
large-area cells are produced from a 100-mm dia. germanium
wafer. The area for these cells typically ranges from ~20 cm2
to ~30 cm2. In the past few years, however, larger 1-per-wafer
solar cells have been gaining acceptance with users due to
their lower cost of assembly and integration on a $/W basis.
These cells are typically square or nearly square and have
areas as large as ~61 cm2 [3].
To date, nearly 60 kW of 1-per-wafer ZTJ cells have been
manufactured at Emcore to power a dozen spacecraft. The ZTJ
1-per-wafer cell is expected to outpace the ZTJ 2-per-wafer
cell production in the coming years, as more missions begin to
take advantage of the cost saving benefits of larger area cells.
An example of an Emcore 1-per-wafer ZTJ solar cell is shown
in the photograph of Fig. 2.
III. SOLAR PANEL AND ARRAY CONFIGURATIONS
MMS
32 flight solar panels and 2 spare panels are built for the
four MMS satellites, capable of a total output power exceeding
5 kW at EOL at normal sun incidence. The solar array for
each spacecraft consists of 8 body-mounted panels. Each
panel is populated with 9 strings of 18, 1-per-wafer ZTJ solar
cells, for a total of 162 cells. Greater than 90% panel packing
factor is achieved using the ZTJ cells shown in Fig. 3.
Fig. 2 – Emcore 1-per-wafer ~8x8 cm ZTJ solar cell
II. FLIGHT MISSIONS BACKGROUND
Emcore is currently under contract to supply solar panels for
12 separate satellites populated with the 1-per-wafer ZTJ solar
cells. These include panels for NASA’s Magnetospheric
Multiscale (MMS) mission, managed by the Goddard Space
Flight Center (GSFC). The MMS mission is a Solar Terrestrial
Probes Program within NASA’s Heliophysics Division. It
consists of four identically instrumented spacecraft in a highly
elliptical orbit. It will use Earth'
s magnetosphere as a
laboratory to study magnetic reconnection, which is a
fundamental plasma-physical process that taps the energy
stored in a magnetic field and converts into heat and kinetic
energy in the form of charged particle acceleration and largescale flows of matter.
Emcore is also under contract with ATK Space Systems to
deliver solar panels for two other missions. These are
NASA’s 4th through 8th Commercial Resupply Service (CRS)
spacecraft to the International Space Station (ISS) and the
Affordable Modular Optimized Satellite (AMOS-6)
commercial GEO telecommunication satellite. ATK Space
Systems’ UltraFlex solar arrays are used for the CRS 4-8
spacecraft.
Under the CRS program, Orbital Sciences Corp. is to
provide a fleet of pressurized cargo missions to the ISS. CRS
will provide a U.S.-produced and operated automated cargo
delivery service for ISS logistics support, to complement the
existing Russian, European and Japanese cargo vehicles.
Israel’s AMOS-6 is a 5-ton class GEO satellite. The
communication payload includes 45 transponders in 3
frequency bands (Ku, Ka, S), which enable the satellite to
provide communication services that can include DTH (directto-home) satellite internet.
NuStar (Nuclear Spectroscopic Telescope Array) is a NASA
Explorer mission and is the first orbiting space-based X-ray
telescope to focus high energy X-rays from astrophysical
sources, especially for nuclear spectroscopy, and operates in
the range of 6 to 79 keV. It was successfully launched by
Orbital’s Pegasus XL rocket to orbit on June 13, 2012.
Fig. 3 – MMS solar panel
Due to MMS’ highly elliptical orbit, the panels were
qualified to the hot and cold extreme temperatures of +70 C
and -150 C, respectively.
The solar panels were also
manufactured to be electrostatically and magnetically clean
using a proprietary process.
CRS
The CRS solar arrays consist of 2 circular wings, with 10
triangle-shaped UltraFlex flexible blanket/gores per wing. A
total of 100 gores are built to power 5 Cygnus satellites. The
solar array for each spacecraft produces about 3.5 kW in orbit.
Due to the UltraFlex flexible gore, the CRS arrays are one of
the lightest space-qualified structures available in the market.
A photo of a CRS gore is shown in Fig. 4.
Fig. 4 – ZTJ 1-per-wafer solar cells on ATK’s UltraFlex CRS
panel
AMOS-6
The AMOS-6 solar array consists of 2 wings of 6 panels per
wing. The ZTJ solar cell assemblies are laid down on
conventional kapton-covered aluminum honeycomb core
substrates with CFRP Facesheet. Space qualified, proven
processes are applied to the panels to mitigate and protect the
array against ESD events in the GEO environment. More than
6,000 ZTJ cells (cell area ~60 cm2) were laid down on 12
panels to meet the >10 kW EOL power requirement for
AMOS-6’s 15-year mission.
followed by thermal cycling in vacuum (TVAC) or in ambient
pressure nitrogen atmosphere (APTC). A partial list of ZTJ
flight panel acceptance testing is shown in Table III.
TABLE I
ZTJ 2-PER-WAFER LIFE-CYCLE COUPONS
NuStar
The NuStar solar panels that were delivered to ATK were
populated with 2-per-wafer ZTJ solar cells. The solar array
consisted of 1 wing of 5 panels. A photograph of the stowed
wing inside the Pegasus rocket is shown in Fig. 5.
TABLE II
ZTJ 1-PER-WAFER LIFE-CYCLE COUPONS
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Fig. 5 – Partial view of ATK’s NuStar solar array inside
Orbital’s Pegasus Rocket
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IV. QUALIFICATION AND ACCEPTANCE TESTS
The ZTJ has been qualified and characterized at the bare
cell, CIC, life-cycle coupon and flight panel acceptance levels.
The bare cell , CIC assembly and coupon qualifications were
performed according to the requirements of the AIAA-S1112005 and AIAA-S112-2005 standards, respectively.
Additional S-112 qualification and characterization tests were
performed on the 1-per-wafer ZTJ CICs and panels consisting
of life-cycle coupons, solar absorptance, angle of incidence,
bypass diode, emissivity, UV radiation and ESD
characterization at the CIC and coupon levels.
In addition, extensive life-cycle coupon testing has been
performed to various orbital environmental temperature
extreme conditions in the past several years. The coupons were
populated with both the 2-per-wafer and 1-per-wafer ZTJ
cells. These are summarized in Tables I and II, respectively,
and encompass the qualification requirements of the MMS,
CRS, AMOS-6 and NuStar programs.
The flight panels also undergo environmental acceptance
thermal cycle testing post manufacturing before they are
delivered for integration into solar arrays. The most common
tests include a high-temperature thermal vacuum bake-out,
TABLE III
ZTJ FLIGHT PANEL ACCEPTANCE TESTING
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V. NUSTAR ON-ORBIT PERFORMANCE RESULTS
The NuStar spacecraft is orbiting at an altitude of ~600 km
at 6° inclination and represents the first flight of ZTJ solar
cells serving as primary power to the spacecraft. The
spacecraft’s on-orbit solar array operation has been nominal
since its launch on June 13, 2012, including both a Summer
solstice and a Winter solstice.
The solar array consists of one deployable wing of 5 solar
panels (see Fig 6) that produces 750W of power at EOL and
operates at a load voltage of ~32.8V. The predicted output
current of the solar array is compared to the on-orbit measured
telemetry data provided by ATK and Orbital in Fig. 7.
Fig. 6 – NuSTAR Spacecraft On-Orbit Configuration.
As shown in the figure, the on-orbit solar array performance
slightly exceeds predictions with power production varying
with earth sun distance and exhibiting minimal degradation as
expected.
This is consistent with other on-orbit results reported for
prior generations of Emcore triple-junction cells (i.e., TJ, ATJ
and BTJ), where the telemetry data showed on average about
1%-3% greater array output power than predicted.
VI. SUMMARY
The high-efficiency ZTJ triple-junction solar cells have been
in volume production at Emcore for the past 4 years. The ZTJ
is qualified and characterized at the bare cell and CIC/panel,
levels per the requirements of the AIAA-S111-2005 and
AIAA-S112-2005 standards, respectively. Several life-cycle
coupons have also been built and tested to the environmental
conditions of various orbits, with both the 1-per-wafer and 2per-wafer ZTJ form factors. The large-area 1-per-wafer ZTJ
cells were used in the manufacture of solar panels for the
MMS, CRS 4-8 and AMOS-6 missions. The on-orbit
performance results of NuStar solar array populated with ZTJ
cells were also presented and shown to be operating as
predicted. Finally, presently the ZTJ cell is baselined to power
more than 40 future satellites that are scheduled to launch in
the 2013-2018 time frame.
REFERENCES
[1] P. Sharps, et al., "Emcore high efficiency space solar cell
technology: 30% and beyond," in 38th European Space Power
Conference, 2011.
[2] M. Stan, et al., "Air Force mantech qualification of the 30% class
GaInP2/Ga(In)As/Ge solar cell to the AIAA S-111 standard:
results and recommendations," in 35th IEEE Photovoltaic
Specialist Conference, 2010, 657-S9
[3] B. Cho, M. Winter, I. Aeby, P. Patel, N. Fatemi and P. Sharps,
"The one-per-wafer ZTJ solar cell from Emcore – confidence
testing and volume manufacturing," in 38th IEEE Photovoltaic
Specialist Conference, 2012, 925.
Fig. 7 – NuStar solar array predicted and measured on-orbit
telemetry data