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| Cutting
Edge Solar Cell Technology: The Fast-Growing Trend in a Slow Global Economy |
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Ed Wijaranakula, Ph.D.
Chief Technology Officer, Infotix
Systems, Inc. - January
5,
2009
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Commercial solar cells
are manufactured from p-type boron doped silicon
wafers sliced from either high purity single crystal
ingots grown using the Czochralski
(Cz) method or from low
cost multicrystalline silicon bricks or ingots produced by the casting
method. P-type silicon is used because the
diffusion length of the minority carrier in p-type
silicon is longer than that in n-type silicon. A Cz-single crystal silicon ingot
is "pulled" from 1410 °C molten silicon at a constant rate, about 1.0-1.5
mm/min., whereas cast silicon is produced by directly casting
molten silicon into a mold and
allowed to solidify into a multicrystalline
ingot.
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Rapid cooling during
the solidification of cast silicon introduces crystallographic defects
such as grain boundaries, dislocations and metal
precipitates, which degrade solar cell
efficiency. Although a Cz-silicon
ingot is free from slips and dislocations, grown-in
defects such as oxide microprecipitates and
vacancy clusters,
which may have an impact on solar cell efficiency, are
still present. L. J. Geerligs and D. Macdonald
suggested that B-O
recombination active complexes formed between intentionally
doped boron atoms and oxygen atoms unintentionally
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incorporated
into silicon during the crystal growth process, as well
as transition metals such as Fe, cause degradation
of the minority charge carrier lifetime and hence the
solar cell efficiency in
p-type silicon. The lifetime degradation associated
with these defect centers is less pronounced in n-type
silicon. Thus, n-type
silicon is
also being considered as a promising candidate
for future generations of high-efficiency commercial solar cells.
Despite
the high cost of Cz-crystal growth equipment and
about three times more electrical power consumption
than that of the casting method, the average retail
prices of
single crystal silicon solar modules are very competitive to those manufactured from
multicrystalline silicon solar cells. The conversion
efficiencies of commercial single crystal and
multicrystalline silicon solar cells, e.g. Q-Cells Q6LM and Q6LTT,
are about 16 and 14 percent,
respectively. In a laboratory environment such as at
UNSW,
single crystal silicon-based PV cells can achieve an
efficiency of more than 24 percent. There is a
strong correlation
between cell efficiency and back cell temperatures in
which an increase
in back cell ambient temperature leads to a decrease in cell
efficiency.
The first step of silicon-based
PV cell manufacturing is to form
a front p-n junction by
phosphorus diffusion, e.g. using phosphorus oxychloride
(POCl3)
as the diffusion source. Various time-temperature schedules can
be used to tailor the concentration profile of the
phosphorus. After the homogeneous
emitter junction is formed, an anti-reflection
coating (ARC) with a thin layer of dielectric nitride
film (SiN) is applied, e.g. using a
plasma enhanced chemical vapor
deposition (PECVD) from silane
(SiH4) and
ammonia (NH3)
based gases at around 400 °C.
Subsequently, screen-printed
contacts are applied to the front and back of the
cell. Screen-printing using silver and aluminum paste
is relatively simple, highly efficient and low cost.
A non-contact Aerosol Jet printing process,
developed for high yield printing on thin
silicon PV wafers, can produce narrow and high integrity collector lines,
which in turn improve the conversion efficiency by 2
to 4 percent, compared to
conventional screen-printed silicon solar cells.
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Thin Film Technology
(TFT) - Commercial thin film solar cells are
manufactured by depositing thin layers of semiconductor
materials including amorphous/ microcrystalline silicon,CdTe, copper indium gallium (CIS) and copper indium
gallium
diselenide (CIGS) onto flexible substrate
such as stainless steel or polyamide
using the continuous roll-to-roll manufacturing method or on a
large size glass substrate, up to about 1,000 x 1,400
mm. Thin film cell manufacturing uses less semiconductor
materials and hence is highly economical, compared to the conventional solar cell batch
process. Therefore, the thin film
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technology provides a low cost solution
for commercial scale high throughput solar PV cell manufacturing. According
to First
Solar,
CdTe thin film cells can be manufactured at an
average cost of about $1.01 per watt.
The conversion
efficiencies of
thin film solar cells are much lower than that of silicon-based PV
cells and degrade over time, depending upon the
types of thin film materials. A research study conducted by the USDA
found that the performance degradation of the CdTe thin film modules
occurred at a rate that was much more rapid than that of amorphous-silicon
modules over timescales of years.
Amorphous/Micromorph Silicon
Thin Film Technology - Amorphous silicon is a non-crystalline form of silicon having the same short
range order as the silicon crystal but lacks long
range order. The lattice of amorphous silicon contains
defects such as dangling bonds, which negatively
impact the
diffusion length of the minority carriers and hence
the cell
efficiencies. The dangling bonds can be neutralized by hydrogen
passivation in which hydrogen atoms are
intentionally introduced into amorphous silicon during
the thin film deposition.
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About the Author: Dr. Ed Wijaranakula is presently the
Chief Technology Officer at Infotix Systems, Inc. Prior to Infotix Systems, he has worked with
Intel, Hewlett-Packard, Micron, Motorola and Texas Instruments and has held senior as well
as managerial positions in semiconductor manufacturing companies. He has published over 80
technical papers and holds more than 12 U.S. and foreign patents. Dr.
Wijaranakula's portfolio does not hold any positions in any of the
financial products mentioned in the article.
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