Overview of EMCORE`s Multi-junction Solar Cell

Overview of EMCORE’s Multi-junction Solar Cell
Technology and High Volume Manufacturing Capabilities
David Danzilio
EMCORE Photovoltaics, 10420 Research Road SE, Albuquerque, NM 87123
[email protected]
Keywords: Multi-junction, Solar, Photovoltaic, Cell
Abstract
Over the last decade, III-V multi-junction solar cells have
effectively displaced silicon solar cells for generating power on
the majority of commercial and military satellites. This
technology shift was driven by the substantially higher
conversion efficiency (28.5% for muti-junction vs. 17% for Si),
superior radiation tolerance and the potential for continual
performance advances offered by InGaP/InGaAs/Ge solar cells.
These advantages enable satellite solar arrays with higher
specific power (watts/kg of array weight), reduced launch costs
and longer satellite service life (>15years in geosynchronous
orbit). These are important factors that favorably influence the
economics of satellite manufacturing and the satellite services
industry. This technology is also finding broad application in
utility-scale concentrating photovoltaic systems where multijunction solar cells provide a substantial cost and performance
advantage over silicon solar cells. Furthermore the worldwide
multi-junction solar cell industry is extremely price competitive
with annual price erosion of 7-10% being common. Key to
EMCORE’s success in this highly competitive industry is its
well-developed compound semiconductor solar cell technology
and mature manufacturing capability. This multi-million dollar
investment in technology and production capacity has enabled
EMCORE Photovoltaics to become the largest manufacturer of
high efficiency multi-junction compound semiconductor solar
cells in the world.
INTRODUCTION
EMCORE’s Photovoltaics Division was founded in 1998
in response to increasing demand for high efficiency solar
cells in support of a growing commercial satellite market.
The 70,000-ft2 factory, located in Albuquerque NM, finished
construction in October 1998 and first commercial shipment
of the highest efficiency dual-junction solar cells ever
produced (23%) occurred at the end of 1999. Since the
commercial release of the dual junction product, EMCORE
has continuously improved upon the multi-junction
architecture to meet the ever increasing requirements of its
customers, releasing a 26% efficient triple junction cell (TJ)
in 2000, an advanced triple junction cell (ATJ) with 27.5%
average efficiency in 2001, incorporation of an on-board
monolithic bypass diode into the ATJ product family
(referred to as the ATJM) in 2002, deployment of a triple
junction solar cell for concentrator applications (CTJ, 35%
efficiency under concentrated illumination), and most
recently the release in 2006 of the BTJ/BTJM product family
that provides the highest commercially available conversion
efficiency of 28.5%, and is offered both with and without a
monolithic bypass diode.
In addition to producing state of the art multi-junction
solar cells for space and terrestrial concentrator applications,
EMCORE Photovoltaics also produces Coverglass
Interconnected Cells (CICs), terrestrial receiver modules for
concentrator systems, and fully integrated solar panels for
satellite applications. These products are built in our state of
the art, highly automated solar cell Fab and solar panel
manufacturing operation with the capacity to produce over
200,000 4-inch diameter wafers and more than 150kW of
solar panels annually.
MULTI-JUNCTION SOLAR CELLS
As the name suggests, multi-junction solar cells are
comprised of three junctions in series, with each junction
optimized for a different segment of the solar spectrum.
EMCORE’s solar cells are based on the InGaP/InGaAs/Ge
triple junction architecture, a schematic of which is shown in
figure 1. In this configuration, each junction is defined in a
different semiconductor layer, possesses a different bandgap
Window
Emitter
Base
BSF
Tunnel Junction
Window
Emitter
Base
BSF
Tunnel Junction
Buffer/Nucleat : ion
Emitter
Base
Substrate
n+
n+
p
p+
p++/n++
n+
n+
p
p+
p++/n++
n
n+
p
p
Metallization
Top
Window
InGaP
InGaP
BSF
Middle
Window
InGaP
InGaAs
BSF
Bottom
GaAs
Ge
Ge
Ge
140 µm
Figure 1 Cross Section of a MJ Solar Cell
CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA
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and is therefore tuned to a different wavelength segment of
the solar spectrum. The materials are arranged such that the
bandgap of the junctions become progressively narrower
from the top junction to the bottom junction. Thus highenergy photons are absorbed in the top junction, generating
electron-hole pairs, and less energetic photons pass through
to the lower junctions where they are absorbed and generate
additional electron hole pairs. The current generated in the
junctions is then collected at ohmic contacts formed at the top
and bottom of the solar cell.
By employing compound semiconductor materials in this
arrangement, the solar cell makes more efficient use of the
available energy in the solar spectrum and yields substantially
higher conversion efficiency than single junction solar cells
formed in silicon. EMCORE’s latest generation triple
junction solar cell, called the BTJ, exhibits a minimum
average conversion efficiency of 28.5% under the air mass
zero (AM0) conditions of space. This solar cell technology
has been adapted for use in terrestrial concentrator systems
designed for large-scale (1-100MW) solar power stations and
exhibit peak conversion efficiency of over 36% under
concentrated illumination. The application of triple junction
solar cells in concentrating photovoltaic systems represents a
substantial growth area, and to meet this market opportunity,
EMCORE has made, and continues to make substantial
investments in production capacity to satisfy present and
projected demand.
SOLAR CELL FABRICATION
The process flow for EMCORE’s ATJM family of high
efficiency multi-junction solar cells is shown in figure 2. The
ATJM product incorporates a patented monolithic by-pass
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Figure 2 ATJM Process Flow
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#
diode for shadow protection. The manufacturing process
begins with procurement and qualification of 100-mm
diameter, 145µm (5.5 mil) thick Ge wafers. Substrate
qualification is required as the bottom junction is formed at
the substrate/epi interface, and wafer quality has a strong
influence on the performance of the bottom subcell. Once
qualified for use, the multi-junction epitaxial layer structure
is grown in EMCORE designed multi-wafer (13-14
wafer/run) MOCVD systems. The epitaxial structure is
comprised of more than 50 distinct layers and is on the order
of 10µm thick. The epitaxial structure is then characterized
using a suite of characterization tools (X-ray, PL,
electrochemical CV etc.). Although this characterization data
provides feedback on the growth process, historically the
only method of determining the quality of the epitaxial
structure (and hence whether a reactor should be turned over
to production) was to fully process and test solar cells.
Recently, EMCORE has developed a measurement technique
that enables the quantitative assessment of the full epitaxial
structure immediately after the first metal step. This
technique enables rapid feedback to the growth engineers
enabling faster reactor tuning, reduced downtime and higher
reactor availability.
The fabrication process proceeds with a lithography/etch
process to define the bypass diode and employs a selective
etch to define the area of the diode which has been grown on
top of the triple junction epitaxial structure. The diode etch is
followed by the Mesa lithography/etch process and etches
through the entire epitaxial structure, into the Ge substrate
and defines the active area of the solar cell. TEL MarkV
coat/develop tracks along with Canon 1x projection aligners
are utilized for the photolithographic operations in the above
steps (as well as all others). As MJ space solar cells are large
area devices (27-31 cm2 each – only 2 per wafer!), the
exposure field required (i.e., the entire wafer) dictates the use
of 1x projection systems as the field size of reduction
steppers is not adequate to pattern even one space cell.
Once the diode and Mesa are defined, the front side
metal grid is formed using industry standard photo patterning,
metal deposition and lift-off techniques. With the grids
defined on top of a highly doped n+ cap (to ensure ohmic
behavior), the cap layer is patterned and selectively etched to
remove this material from between the grid fingers and reveal
the window for the top junction. This etch is quite critical as
it has a large influence on the absorption of incoming photons
by the multi-junction structure. The process continues with
the reactive deposition of an anti-reflection coating of a
TiOx/Al2O3 dielectric stack whose spectral characteristics are
optimized to minimize reflections as well as maximizing the
end-of-life performance of the solar cells.
The solar cell manufacturing process concludes with the
deposition of the backside metal to form the p-contact and is
followed by an anneal step to ensure ohmic behavior. The
CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA
space solar cells are then on-wafer tested under AM0
conditions using a multi source solar simulator, and terrestrial
cells are tested under concentrated illumination (500x1000x). During this test step, full I-V curves are taken from
which relevant figures of merit are calculated (i.e. conversion
efficiency, output voltage, output current etc.). Wafers are
then marked with both human readable characters as well as
2-D barcodes to enable easy identification and traceability.
Once tested and marked, the solar cells are mounted onto film
frames and diced using industry standard methods. After
dicing, the solar cells are picked from the wafer and visually
inspected to rigorous criteria as these products are destined
for use in space where they must survive for >15 years in a
harsh environment, without failure. A completed BTJM solar
cell with silver plated kovar interconnects attached and
coverglass applied is shown below in figure 3.
Figure 3 Completed ATJM CIC
Although the manufacturing of multi-junction solar cells
appear to be relatively simple in comparison to FET/HEMT
or HBT based III-V devices/circuits, one should not conclude
that these products are easy to produce. While many
requirements of the solar cell manufacturing are relaxed in
comparison to mainstream FET/HEMT/HBT processing (e.g.,
5-10µm minimum geometries, one front side metal layer,
minimal process steps) there are numerous unique
requirements that make these products extraordinarily
difficult to manufacture in volume. Driving the degree of
manufacturing difficulty is the fact that, for space use, many
of these solar cells have total areas in the range of 27-31cm2
with nearly all the area being electrically active
semiconductor (try making an HBT of that size!). As one
100-mm wafer generates only two space solar cells, large
emphasis is placed on defect control and yield management
within the manufacturing process. This is particularly
important in the area of epitaxial growth, as a single large epi
defect will result in the loss of an entire cell (and half of a
wafer). Furthermore, small epitaxial defects can generate
recombination centers that reduce electrical performance and
adversely affect product yield. Near perfection is required in
the epitaxial growth process to result in electrically
conforming material on a consistent basis. These factors,
along with numerous others, result in a very challenging
technology that requires the highest degree of engineering
expertise to produce these products in high volume.
PACKAGING – CICs and SOLAR PANELS
To be made useful for the intended application, whether
it be a solar panel for a geosynchronous satellite or a
concentrating PV system operating at 1000x, the solar cells
are packaged in a manner that will ensure they perform
reliably for the required operating life of the system they are
powering. In the case of a GEO satellite, the solar panels
must provide adequate power for a minimum of 15 years
while enduring large temperature swings (-180°C to +150°C)
and in a harsh radiation environment with little more than a
thin piece of glass for protection. Unlike a mobile phone, the
implications of failure are loss of a $100M-$200M asset (not
exactly a throw away item) and adverse financial implications
to the service provider, or in the case of a military satellite,
the loss of critical battlefield capability. No satellites
powered with EMCORE products have exhibited any onorbit anomaly, and EMCORE’s unique record is due to robust
cell design and rigorous control of the packaging process.
The process of integrating the solar cells into a solar
panel begins with the attachment of interconnects to the cell.
As shown in figure 3, silver-plated interconnects are attached
to multiple bond pads on the front side using a parallel gap
welding process. This is followed by the application of a thin
(4-6 mil) coverglass, and is bonded to the solar cell using a
transparent adhesive. The adhesive is then cured in a
convection oven and the CIC is electrically tested using a
solar simulator to ensure the addition of the cover glass
(which is covered with an anti-reflection coating) has not
shifted the performance of the solar cell outside of normal
limits.
It is well understood that interconnect to cell welding is
perhaps the most critical process in determining the on orbit
reliability of solar panels. This process must be robust and
well controlled, as these connections must survive tens of
thousands of thermal cycles (-180°C to +150°C for GEO
satellites, -110°C to +100°C for LEO) without failure. To
exceed the reliability requirements of our customers,
EMCORE has invested substantial resources to procure the
most advanced welding toolset available and to identify and
control the critical input variables to the welding process.
This effort has resulted in a process that exhibits pull
strengths of 1 to 2 kg (depending on the type of weld) and
process capability indices ranging from 2.0 to over 15. This
advanced toolset and process control methodology has been
built into an advanced pick and place tool that fully
CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA
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automates the placement of interconnects, executes the
welding step while exerting real time control of the input
variables, store all relevant process information, applies the
adhesive and places the coverglass onto the solar cell. The
automated pick and place tool, shown in figure 4, eliminates
variability and hence reliability risks posed by competing
approaches as well as reducing labor cost to a minimum.
Once assembled, the solar panels undergo extensive
environmental and electrical testing, which typically includes
temperature cycling under vacuum, electrical evaluation
using a large area pulsed solar simulator (LAPSS), ambient
thermal cycling and additional electrical testing using the
LAPSS.
Only upon successful completion of the
environmental testing and a thorough review of all test data
with the customer can the hardware be shipped for integration
into the solar array.
Due to space constraints, the above represents an
abbreviated description of the solar panel assembly and test
process. Many important details have been omitted and a
more comprehensive description will be provided during the
oral presentation.
CONCLUSIONS
EMCORE Photovoltaics is the world’s largest
manufacturer of high efficiency multi-junction solar cells for
space and terrestrial solar power applications. Through
continuous investment in production capacity and
engineering discipline, EMCORE has driven MJ junction
solar cells to commercial viability. This investment has
resulted in wide availability of high performance, ultra high
reliability solar power products for new markets.
EMCORE’s multi-junction technologies will also continue to
offer our customers the benefits of well-defined
product/technology roadmap to future improvement realized
in performance, cost and extended application.
Figure 4 Automated Tool for CIC Manufacture
Solar panel manufacturing proceeds by welding
completed CICs in series to form strings. The number of
CICs in a particular string is determined by the voltage
requirement of the mission, and all strings on the panel are
typically of the same length (voltage). Additionally, as each
series connected solar cell must pass nominally the same
current, all the CICs in a particular string are chosen from the
same current bin as determined in prior electrical test
operations. The strings are then arranged into circuits such
that the required number of strings is configured in parallel in
order to meet the current requirement of the satellite. The
circuits are then bonded to lightweight solar panel substrates
using an industry standard adhesive and cured under vacuum
to ensure that no air pockets are formed. This bonding
process is repeated several times until the entire panel has
been populated with solar cells in a geometric configuration
that maximizes the packing density. The circuits are then
wired together on both the frontside and backside of the solar
panel using NASA certified wiring techniques.
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ACKNOWLEDGEMENTS
The author would like to thank a number of people who
contributed to this article, particularly Jody Wood, Roger
Esra, Robert Gallegos, Navid Fatemi, Patrick Park, Brad
Clevenger, Paul Sharps and Viven Bercier.
ACRONYMS
AM0: Air Mass Zero
ATJ: Advanced Triple Junction
ATJM: Advanced Triple Junction with Monolithic Diode
BTJ: Best Triple Junction
BTJM: Best Triple Junction with Monolithic Diode
CIC: Coverglass Interconnected Cell
GEO: Geosynchronous Orbit
LAPSS: Large Area Pulsed Solar Simulator
LEO: Low Earth Orbit
MJ: Multi Junction
TJ: Triple Junction
CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA