High efficiency, large area and low cost solar cells are the drivers for Space Station solar array designs. The manufacturing throughput, process complexity, yield of the cells, and array manufacturing technique determine the economics of the solar array design. The cell efficiency optimization of large area (8 x 8 m), dielectric wrapthrough contact solar cells are described. The results of the optimization are reported and the solar cell performance of limited production runs is reported

Bower, M.

Hansen, N.

Khemthong, S.

English

NASA Technical Reports Server

N91-19215
Optimization and Performance of Space Station Freedom Solar Cells
S. Khemthong, N. Hansen and M. Bower
Applied Solar Energy Corporation
City of Industry, CA
High efficiency, large area and low cost solar cells are the drivers for Space Station
solar array designs. The manufacturing throughput, process complexity, yield of the
cells and array manufacturing technique determine the economics of the solar array
design. This paper describes the cell effÉciency optimization of large area (Scm by
8cm), dielectric wrapthrough contact solar cells. The results of the optimization are
reported and the solar cell performance of limited production runs is reported.
Introduction
The Space Station solar cell is a shallow diffused N on P silicon solar cell with
back surface field manufactured from P-type, boron doped, single crystal silicon with
a base resistivity of 7 to 14 Ohm-em. The cell size is 8cm x 8cm and the nominal
thickness is 8 mils.
Both N and P contacts are located on the back side of the solar cell. The N
contacts are wrapped through four interior holes, isolated from the P contacts by
SiO2 dielectric layer. The cell is equipped with a gridded back and a back side optical
coating, designed to optimize transmission of solar energy in tile range of 1.1 to 2.5
micrometer. This IR transmission allows the solar cell to operate at low temperature
and improve on-orbit electrical performance. The coverglass is a 5 mils thick ceria-
doped borosilicate glass. The top surface of the coverglass is optically coated with
UV reflecting coating designed to reject UV radiation and reduce solar absorptance.
The solar cell and coverglass are bonded together with an optically clear DC93-500
adhesive.
The original cell efficiency requirement was approximately 12.8 percent. Since
then, the requirement was raised to as high as 14.4 percent and the final efficiency
was negotiated to 14.1% (2420mA at 495mV). In early 1988, ASEC developed the
required solar cells. Since the standard cell for simulator calibration for this newly
developed cell was not available, ASEC chose one of the in-house standard cells to
calibrate the solar simulator to evaluate these new cells. In summer of 1988 a balloon
flight standard cell for this newly developed cell was flown by JPL. The cells were
then retested electrically using tile new balloon flight standard cell and found that
the solar cells were 3.4 percent below the requirement of the specifications.
400
https://ntrs.nasa.gov/search.jsp?R=19910009902 2020-03-19T18:46:59+00:00Z
Cell Efficiency Optimization
It was obvious that cell efficiency optimization was required to achieve the re-
quirement of the specifications. ASEC worked to optimize the cell efficiency. The
cell efficiency goal of 14.5% under production conditions was set. Table 1 shows the
performance status prior to optimization and the goal for optimization.
The approach for optimization was to fine-tune the existing process using produc-
tion equipment and personnel under close supervision by engineers and production
management. This process reduced production transfer time. The production per-
sonnel were part of the optimization team from the start and many of the potential
start-up problems were reduced.
Voc Optimization
The achieved Voc of 615.5 mV was considered good. It was felt that some marginal
improvement can be achieved by optimizing the back surface field process. Boron ni-
tride diffusion process is used to form the back surface field for this program. The
Voc optimization process was to vary the boron diffusion temperature while keeping
all other processes unchanged. Three (3) boron diffusion temperatures were cho-
sen, baseline temperature, 25°C above baseline temperature and 50°C above baseline
temperature. For simplicity, 2x4cm top-bottom contact cells were made for these
experiments. Two sets of experiments were carried out to assure repeatability. The
AMO electrical output for all three type of cells were evaluated to determine Voc
improvement.
Isc Optimization
It was determined that Is¢ improvement of 3 to 4% could be achieved. Is¢ opti-
mization included designing a new front grid mask to reduce gridline shadowing and
series resistance. This new front mask design included:
• Reduction of metalization dot size around the wrapthrough holes.
• Reduction of gridline width, and
• Increase gridline density to handle solar cell with shallow junction.
An improved photoresist process was proposed to be used in manufacturing of
Space Station cell. This improved photoresist process allows narrow and tall gridlines
to be formed on solar cells with clean and sharp edges, and reduced shadowing.
With new mask design and implementation of improved photoresist process, it was
estimated that cell efficiency improvement approximately 3% would be achieved.
The next area of Is_ optimization was to reduce the solar cell junction depth
further and to fine-tune the AR coating to match the junction depth. The junction
401
depth experiment was performed by diffusing the N + junction at 25 ° and 50 ° below
the baseline diffusion temperature. 2x4cm solar cells with baseline BSF were made
and evaluated electrically. Particular attention was paid to the improvement of short
circuit current without sacrificing the loss in curve filled factor.
CFF Optimization
The CFF Optimization included:
• Improved CVD dielectric
• Improved wrap-through hole geometry
The dielectric layer in and around the wrapthrough holes must fulfill several
functions. It must be impermeable to reduce any possible shunting paths caused
when the wrapthrough metal layers are deposited. In addition, the dielectric layer
which is present under the N welding pads must not degrade during the welding
pulse, or impair the adhesion of the welded contact. Three CVD dielectric systems,
APCVD, LPCVD and PECVD were considered. It was determined that an APCVD
system would be a suitable system for ASEC due to the nature of the CVD dielectric
process, and this system is now used.
It was also determined that the wrapthrough hole geometry needed improvement.
After laser drilling of the holes, the silicon damage silicon surfaces were removed by
chemical etching. This chemical etching also smoothed the surface and rounded
the wrapthrough holes allowing good coverage of dielectric material in and around
the wrapthrough hole, thus reduce shunting paths caused when wrapthrough metal
layers are deposited. Figure 1 shows a typical wrapthrough hole geometry after
optimization.
Production of Optimized Cell
Upon completion of the optimization process, limited prototype solar cells were
made to verify the optimization results. The verification process was successful and
documentations were established.
The first production quantity required for delivery was 970 assemblies. This
production run was carried out with full documentation. The production line was
closely monitored by engineers and supervision to assure smooth production flow.
402
Solar Cell Performance
Figure 2 showsthe open circuit voltage distribution of 558 solar cell assemblies
tested under AMO condition and 28°C. The solar simulator was calibrated using a
SpaceStation balloon flight standard cell flown late this summer. The open circuit
voltage ranges from 590mV to 630mV with an average of 618.9mV. The average
Voc met the established goal of 618 mV. Figure 3 shows the Isc distribution for the
same 558 cell assemblies. The Isc distribution is tightly controlled with an average of
2733mA (43.6mA/cm2). The average Isc is 5% above the established goal. Figure 4
shows the efficiency distribution of the same ,558 cell assemblies. The efficiency ranges
from 13.1% to 15.4% with an average of 14.6% (2495mA at 495 mV). The efficiency
of the cell is well above the requirement of the specification of 14.1%. Figure ,5
shows the typical spectral response of a typical Space Station solar cell showing good
response in both the blue and red regions. The Space Station solar cell has currently
undergone qualification tests. Electron radiation test and high temperature soak test
are completed and passed the qualification test. The remainder of the qualification
will be completed by the end of November. Figure 6 shows the results of 1MeV
electron radiation.
403
Table 1: Solar Cell Performance Status Prior to
Optimization and Optimlzation Goal
Electrical
Parameters
Voc (mV)
Isc (mA)
CFF (%)
EFF (%)
Status Prior to
Optimization
615.5
2516
74.7
13.7
Optimization
Goal
618
2600
76.5
14.5
FIGURE 1
Typical Wrapthrough Hole Profile of
Space Station Cell
404
I_-'_ ,,-_- PAGE IS
OF POOR QUALITY
Figure 2
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34
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cm 2
Distribution of Space Station
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18
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405
Figure 3
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406
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407
FIGURE 5
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408
FIGURE 6
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409


Optimization and performance of Space Station Freedom solar cells

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