|Thin films seek a solar future
|Despite setbacks, the technology may
|by Ineke Malsch
Energy from the sunavailable everywhere, for everybodyhas
motivated research on solar-energy technologies for about
three decades. The U.S. Photovoltaic Industry Roadmap, intended
to guide companies in developing solar-energy systems, takes
a more prosaic but realistic view of the next three decades.
It aims for solar energy to provide 10% of U.S. peak generation
capacity and supply a considerable share of foreign markets
Most photovoltaic (PV) solar technologies rely on semiconductor-grade
crystalline-silicon wafers, which are expensive to produce
compared with energy from fossil fuel sources. However, potentially
less costly thin-film alternatives may make major inroads
in the world market in five years, suggests Franz Karg, research
manager at the Shell Solar facility in Munich, Germany. Or
maybe not. Thin-film solar panels are hard to mass-produce
cost-effectively because of the difficulty of coating large
areas of glass. It is my opinion that crystalline- silicon
technologies will dominate for at least the next 10 years,
says Jeffrey Mazer of the U.S. Department of Energy (DOE)
Office of Solar Energy Technologies (Washington, DC).
|Figure 1. The main entrance
facade of Technology Place, a research incubator facility
for emerging high-tech companies on the Burnaby campus
of British Columbia Institute of Technology, incorporates
sufficient thin-film solar panels to power all lighting
in the building.
(British Columbia Institute of Technology)
Solar-energy systems pose many challenges for developers, particularly
in the current world economy. Last October, Shell Solar (Amsterdam,
The Netherlands) announced the closing of two production operationsin
Helmond, The Netherlands, and in Munichin a restructuring
meant to make the c o m p a n y more competitive. The next month,
BP Solar (Linthicum, MD) decided to close down production of its
thin-film amorphous silicon and cadmium telluride (CdTe) solar panels
to focus on crystalline- silicon technologies. While the thin-film
technology continues to show promise, lack of present economics
does not allow for continued investment, said Harry Shimp,
BP Solars president and chief executive officer.
Figure 2. Copper and indium
are deposited by magnetron sputtering, followed by selenization
to form the high-absorbing ptype semiconductor CuInSe2,
which is combined with an n-type electrode of ZnO to
create thin-film solar modules.
(Ian Worpole/Shell Solar,
A=Barrier/Mo deposition; B=
Laser patterning; C=Cu/In/Se deposition; D=Heat
treatment 500°C; E=Chemical deposition
F=Patterning; G=2nd deposition 200°C;
BPs decision is a setback to the marketing of new thin-film
solar technologies. However, First Solar, LLC (Perrysburg, OH),
a major maker of CdTe solar cells, remains strongly committed to
the technology. Shell Solar continues development of its thin-film
technologies. And DOEs National Renewable Energy Laboratory
(NREL) in Golden, Colorado, continues to provide funds for thin-film
PV research to its 40 industrial and university partners.
Why solar power?
In 2001, the global market for PV panels and equipment was valued
at $2 billion. Worldwide in 2000, solar, geothermal, wind, combustible
renewables, and burning garbage and other wastes collectively provided
1.6% of electricity production, according to the International Energy
Agency (IEA) in Paris. In its World Energy Outlook 2002,
IEA described two scenarios for world energy demand and supply until
2030. One scenario assumes only the continuation of current government
measures to stimulate sustainable-energy supply and demand. In it,
fossil fuels continue to meet more than 90% of energy demand. All
renewables except hydropower grow by 3.3% annually, but they will
not meet a large share of the total energy demand because of their
low-percentage base. Carbon dioxide (CO2) emissions will grow 70%
by 2030 to 38 billion metric tons annually.
In the alternative scenario, governments will implement policies
such as promoting energy efficiency, the use of cleaner energy sources,
and reducing the environmental impact of producing and burning fossil
fuels. Compared with1990, those changes would result in an estimated
16% fewer CO2 emissions in 2030a year when the
United Nations estimates the world population will be about 8.3
billionin part because renewable energy sources will grow
Current applications of PV solar panels include providing power
to spacecraft and isolated villages in developing countries, solar-energy
systems in homes and buildings in Western countries (Figure 1),
and even powering the lamps of remote lighthouses. Especially
where there is no connection to the grid, solar energy is easily
cheaper than small-scale electricity production with a diesel generator,
to give an example, Karg says. For electricity production
in rural areas in developing countries, solar is the cheaper alternative.
To achieve more, we need breakthroughs in large-scale storage of
electricity, and solar must be developed in combination with wind,
biomass, energy-storage systems, and fossil fuels.
Most people in solar energy consider government subsidies for R&D
and sales as necessary for its successful development and increased
usage. Indeed, the PV energy business is still largely dependent
on government intervention, and most U.S., European, and Japanese
projects are subsidized. The Bush administration seeks $79.7 million
from Congress for fiscal year (FY) 2004 to support solar-energy
research, up 0.1% from its amended FY 2003 request but down from
the $87.1 million that Congress appropriated in FY 2002.
PV solar panels convert sunlight directly into electricity.
A panel consists of several connected 0.6-V dc PV cells, which
are made out of a semiconducting material sandwiched between
two metallic electrodes. The photovoltaic effect refers
to the separation of minority carriers [electrons and holes]
by a built-in electric field, such as a pn-junction
or Schottky barrier, says DOEs Mazer. The cells are
usually encapsulated behind glass to weatherproof them. In
a PV array, several panels are connected to provide sufficient
power for common electrical applications such as household
electricity. The arrays can be connected to an electricity
grid or work as standalone systems.
Researchers at what is now Lucent Technologies Bell
Laboratories first demonstrated silicon solar cells in 1954,
and most PV systems today use mono- or multicrystalline silicon
as the semiconducting material. We obtain monocrystalline
wafers by sawing them from silicon rods, which we grow by
the Czochralski growth process, explains Ronald van
Zolingen, professor of sustainable energy at the Technical
University in Eindhoven, The Netherlands, and a senior business
advisor to Shell Solar. In this process, we pull a monocrystalline
rod from a liquid, starting with a small crystal. The growth
speed is relatively low. but we obtain excellent material.
Monocrystalline silicon solar cells have the advantage of
a high efficiency, about 15%, which is an advantage for specific
|Figure 3. A scanning electron
micrograph shows a cross section of the p-type semiconductor
CuInSe2 film, about 1.5 µm thick,
and the n-type layer of ZnO.
(Shell Solar, Munich, Germany)
We obtain multicrystalline wafers from ingots grown by casting
liquid silicon in a large container followed by controlled cooling,
van Zolingen adds. This technique is less complicated than
the pulling of single-crystalline rods. Multicrystalline-silicon
solar cells have a slightly lower efficiency than monocrystalline,
about 13.5%. Worldwide, the production of multicrystalline-silicon
solar cells outpaces that of monocrystalline-silicon solar cells.
|Figure 4. Optical-beam-induced
current imaging (in false color) of these four CdS/CuInSe2
thin-film solar cells connected in parallel gives a high-resolution
photocurrent map, in which defects appear dark.
(University of Waterloo, Ontario,
Among the major bottlenecks to the output of crystalline-silicon
PVs is the high loss of materials during production of the
wafers. In addition, we need to saw the silicon. We
typically lose 0.2 mm at the kerf, says van Zolingen.
Despite these problems, crystalline silicon remains the dominant
solarcell material. One reason for this is the support provided
by the federal governments PV program since the 1970s
for R&D projects focused on crystalline-silicon technologies.
Thin-film alternatives to standard PV solar cells are already
available or in development. Amorphous silicon, the most advanced
of the thin-film technologies, has been on the market for
about 15 years. It is widely used in pocket calculators, but
it also powers some private homes, buildings, and remote facilities.
An amorphoussilicon solar cell contains only about 1/300th
the amount of active material in a crystalline-silicon cell.
Amorphous silicon is deposited on an inexpensive substrate
such as glass, metal, or plastic, and the challenge is to
raise the stable efficiency. The best-stabilized efficiencies
achieved for amorphous-silicon solar panels in the U.S. PV
program are about 8%. The goal is to produce a stable device
with 10% efficiency. United Solar Systems Corp. (Troy, MI)
pioneered amorphous-silicon solar cells and remains a major
Thin-film crystalline-silicon solar cells consist of layers about
10 µm thick compared with 200- to 300-µm layers
for crystalline-silicon cells. Researchers at NREL use porous polycrystalline
silicon on low-cost substrates and trap light in the silicon to
enable total absorption. They have fabricated working solar cells
with this material.
New thin films
Copper indium diselenide (CIS) is a more recent thin-film PV material.
Siemens Solar developed a process for depositing layers of the three
elements on a substrate in a vacuum, and Shell Solar later acquired
the technology when it bought Siemens Solar (Figures 2 and 3). CIS
modules currently on the market reach stable efficiencies of more
than 11%. In the laboratory, NREL scientists have achieved cell
efficiencies of 19.2% with the semiconductor. Research now focuses
on increasing efficiency (Figure 4), reducing costs, and raising
the production yield of CIS panels. Karg predicts that thin-film
technology will eventually halve the present production cost per
unit kilowatt peak (kWp), which is the peak power that a solar panel
can produce at optimum intensity and sun angle (90°). This implies
a cost reduction for a complete system of 35% or more.
In 2000, CdTe solar panels were field-tested on a large scale in
the United States. NREL researchers consider CdTe a promising material
because of its lower cost of production, which uses techniques that
include electrodeposition and high-rate evaporation. Prototype CdTe
panels have reached 11% efficiency, and research now focuses on
improving efficiency and reducing panel degradation at the electrode
contacts. Studies at Brookhaven National Laboratory strongly
suggest that CdTe modules can be safely made in a large-scale manufacturing
environment, and that CdTe can be safely disposed of when the modules
are eventually retired, Mazer says.
|Figure 5. Examples of local photovoltaic
installations include, left to right: Tibetan home with 20-W
panel and 500-W wind machine (Simon Tsuo); water-pumping station
in West Bengal, India (Harin Ullal); and the world's largest
residential project, in Amersfoort, The Netherlands (BP Solarex).
Progress in solar PV research and the development of new applications
are guided by national and international collaborations between
industry and government, such as those described in the U.S. PV
roadmap and carried out by national research teams organized by
NREL. Japan and Germany have similar ongoing programs, and leading
manufacturers are collaborating with other companies to install
solar panels on commercial buildings. For example, a recent agreement
between Volkswagen and BP Solar calls for installing solar-energy
systems on the roofs of the automaker's dealerships throughout Germany.
Each company is investing in several different technologies.
Solar's big four
Which PV technologies will dominate future solarenergy markets may
depend on the companies developing, manufacturing, and selling them.
The four industry leaders are Sharp (Osaka, Japan), BP Solar, Kyocera
(Kyoto, Japan), and Shell Solar.
Sharp produces mono- and multicrystalline and amorphous silicon
solar cells. The monocrystalline modules have an efficiency of 17.5%,
and the multicrystalline cells have 16% efficiency. In 2001, the
company shipped 19.2% (75 MWp) of the world's total solar cells.
Last July, Sharp opened a new multicrystalline-silicon solar-cell
production plant in Nara Prefecture, Japan, and the company's total
production capacity now totals 200 MWp.
BP Solar also manufactures nearly 20% of the world's solar-electric
panels and systems, using technologies that include polycrystalline
solar cells. "We also have our own Saturn technology, which
is a highly efficient monocrystalline technology," said a spokesman
at BP Solar's U.K. office in Sunbury on Thames, England. The company
also sells amorphous silicon thin-film modules. BP developed its
proprietary PowerView thin-film silicon laminate partly with funds
from NREL. The PV coating converts part of the incoming light into
electricity while remaining transparent to the rest of the light.
The coating can be used to integrate a solar-energy-generating capacity
into building skylights and windowpanes to produce electricity and
reduce reliance on utility companies.
Kyocera focuses on off-grid solar systems for private homes in
developing countries, communication systems, water pumping, and
industry (Figure 5). It sells its own multicrystalline-silicon systems,
and amorphous silicon systems produced by United Solar Systems.
Shell Solar makes mono- and multicrystalline silicon as well as
thin-film CIS solar systems. It produced solar panels with a total
capacity of about 50 MWp in 2002, and it expects to double its crystalline
production capacity by 2004. The company now employs about 900 people
in the United States, Canada, Portugal, and Germany after cutting
170 jobs last October.
Although the market growth rate for PV solar panels has declined
sharply after four years of annual growth of more than 30%, the
growth rate is predicted to be 15 to 20% this year and next. Worldwide
production capacity almost doubled last year to 760 MWp, up from
400 MWp in 2001. The main producers of these panels have different
business strategies. Shell Solar strongly believes in thin-film
alternatives, including its CIS technology. Other companies see
crystalline silicon as the dominant technology during the next decade.
Few people doubt solar energy's potential, but many wonder when
it will be reached. "In the long term, solar may well play
an important role," Karg says. "I personally expect a
contribution of 10 to 20% of the global electricity production,
mainly in the form of grid-connected systems." However, he
does not foresee that happening within the next 20 years.
Fairley, P. BP Solar Ditches Thin-Film Photovoltaics. IEEE Spectrum
Web only edition, Jan. 8, 2003; www.spectrum.ieee.org,
click on Newsletter Archive.
Photovoltaics: Energy for the New Millennium, the National
Photovoltaics Program Plan 2000.2004; NREL Office of Solar Energy
Technologies: Golden, CO, 2000; available
Solar Electric Power: The U.S. Photovoltaic Industry Roadmap;
NICH Report No. BR-520-30150, 2001; 36 pp.; available
Franz Karg's views on solar cells appeared in Shell Venster, September/October
2002, pp. 4-9, Shell Nederland, Den Haag, The Netherlands, and are
quoted with permission. Key world energy statistics are available
Ineke Malsch, a consultant
in technology and society, is director of Malsch
TechnoValuation in Utrecht, The Netherlands.