Researchers, companies aim for efficient ultra-thin
PV cells
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| Sliver technology uses innovative manufacturing
techniques to produce individual cells of less than 70 microns
thick—thinner than a human hair. The cells use 90
percent less silicon than current conventional solar PV
modules. (photo courtesy of Origin Energy) |
In spite of the benefits renewable energy
has to offer, utilities often cite the cost of alternative technologies
as a barrier to developing more distributed generation projects.
Many renewable energy experts believe that the key to making
renewable resources, such as solar power, more competitive with
fossil fuels is to bring down the high cost of materials and
production. Several companies are working on new techniques
to make photovoltaic cells a more affordable—and versatile—option.
For conventional solar
cells made of crystalline silicon, material cost is the
most expensive part of the unit. Also, the silicon must be processed
in elaborate clean-room facilities, like computer chips, adding
to the production cost. The most promising solutions fall mainly
into two categories: develop a manufacturing process that uses
less crystalline silicon or make cells out of a different material.
Manufacturers believe thin
is in for crystalline silicon solar cells
The Australian company Origin
Energy is one of the leaders in the "less-material" camp
with its patented Sliver Cell technology, developed in cooperation
with Australian National University. According to Origin, the
solar cells are less than 70 microns thick and convert 19.5
percent of the solar energy that hits them into electricity.
Conventional industrial cells are about 300 micrometers thick
and have an efficiency value of about 16 percent.
"Sliver Cell technology needs the equivalent
of two silicon wafers to convert sunlight to 140 watts of power,"
explained Professor Andrew Blakers, director of the ANU
Centre for Sustainable Energy Systems. "By comparison, a
conventional solar panel needs about 60 silicon wafers to achieve
this performance."
The manufacturers believe that the economy and
flexibility of the paper-thin PV material will advance the adoption
of solar power. Besides conventional uses, Sliver lends itself
to such applications as flexible, roll-up solar panels, high-voltage
solar-panels and solar powered aircraft, satellite and surveillance
systems.
In late 2003, Origin started construction on
a $20 million plant in Adelaide, Australia. The plant will produce
up to 5 MW of PV modules per year initially, expandable to 25
MW per year to meet the potential export market. Sliver technology
PV modules are expected to be available to market by January
2005.
Researchers at the Fraunhofer
Institute for Solar Energy Systems in Germany are slicing
their crystalline silicon even finer—nearly half the thickness
of the Origin cells. The Fraunhofer ISE scientists claim to
have produced a crystalline silicon solar cell that is only
37 microns thick and achieves a solar energy conversion efficiency
of 20.2 percent.
The key to producing wafer-like cells is laser-fired
contacts technology, an inexpensive process for attaching electrical
connections to the cells. The process requires only one second
per solar cell and works for all wafer thicknesses, making it
ideal for industrial mass production. Stefan
Glunz, the institute's coordinator for monocrystalline silicon
solar cell research, said that considerable research is still
needed to economically manufacture the extremely thin wafers.
Tokyo-based PV specialistsMSK
achieve savings by making solar cells that are a little thinner—200
micrometers—rather than a lot thinner. The company's recently
opened module production factory in Nagano, Japan, is geared
to handle the thinner cells and keep pace with other advances
in PV cell technology. The facility also has the economy of
scale working for it with the capability to produce 45,000 solar
modules per month. It is the world's largest module production
capacity at a single site.
Academic research focuses on new PV materials
Flexible, "organic" PV material like malleable polymers offer
a vastly less expensive and more versatile alternative to crystalline
silicon, researchers say, although one that so far is considerably
less efficient. Such PV "films" have existed since 1986, according
to Princeton electrical engineering professor Stephen
Forrest, but have efficiency rates of only 1 percent.
Forrest heads a team of university electrical
engineers that invented a technique for producing a new class
of ultra-thin, organic photovoltaics. As reported in the Sept.
11 issue of Nature, the researchers broke the one-percent
efficiency barrier by changing the organic compounds used to
make their solar cells, yielding devices with efficiencies of
more than three percent.
The team's most recent advance involved finding
a new method for forming the organic film, "which increased
the efficiency by 50 percent," Forrest told the scientific journal.
The innovation, he continued, could lead to cells being manufactured
in a process something like printing or spraying the materials
onto a roll of plastic, and then applying it to large surfaces.
The next phase of research will combine the new
materials and techniques, with hopes of yielding at least five-percent
efficiency. "We think we have pathway for using this and other
tricks to get to 10-percent reasonably quickly," said Forrest.
On the other side of the country, researchers
at the Berkeley campus of the University of California are working
with nanocrystals of cadmium selenide, a light-sensitive semiconductor.
Forbes magazine reported that chemistry graduate students
at the university used the material to create a postage stamp-sized
solar cell that generates 1.5 milliwatts, or 15 watts per square
meter.
Ultra-thin PV films give new meaning
to portable power source
The UC Berkeley
chemistry department turned much of its research over to
the nanotechnology firm Nanosys,
Inc. in hopes that the company's resources will move development
closer to commercialization. The company's goal is to embed
nanocrystals of semiconductor material in cheap, bendable plastic
sheets to create solar cells with 10 percent efficiency generating
energy at $1 per watt.
Michigan-based Uni-Solar
produces easy-to-handle sheets of solar material using vapor-deposited
amorphous silicon alloy material. Because amorphous silicon
absorbs light more efficiently, the cell's thickness can be
up to 100 times thinner than a conventional cell, greatly reducing
material costs. The material's advantages are in versatility,
sturdiness and scalability. However, like other thin-film PV
cells, the sheets don't approach the efficiency of some crystalline
products.
The potential applications for flexible,
efficient and affordable thin-film PV are so broad that the
U.S. Army is partnering with industry to improve the technology.
In August, the Army awarded a new round of funding to
Konarka Technologies to develop a lightweight, portable
power source to reduce the number of batteries soldiers must
carry into combat.
The technology uses dye-coated titania
nanocrystals applied to a flexible metallic foil or plastic
base with the low-temperature sintering process that launched
the company four years ago. The foil-based units boast efficiencies
of about 7 percent while the less expensive plastic cells are
about 5 percent efficient.
The color flexibility of the dye combined
with Konarka's recent development of an actual PV fiber puts
the possibility of a wearable power source within reach. In
the future, troops may power sophisticated military electronic
equipment in the field with solar panels woven into their uniforms.
The same type of rollout recharging units will be available
for civilian applications such as camping equipment. Konarka
is targeting early 2005 for the commercialization of its first
product.