SOLAR TODAY
MARCH 2009
VOL. 23, No. 2
Breakthrough technologies will enable
the global transition to carbon-free solar
energy, providing plentiful, low-cost energy for all.
By DAN E. ARVIZU, Ph.D.
The lab’s core work includes the invention
and development of the gallium indium phos-phide/gallium arsenide (GaInP/GaAs) solar
cell, which is the foundation of all commercial
multijunction technology.
The initial patents were developed by Jerry
Olson, Ph.D., in 1984. By the mid-1990s, NREL
had transferred the multijunction technology
to the space PV industry, where it became the
standard. Most satellites today use these multijunction cells. The last 10 years have seen the
technology become commercially available for
terrestrial concentrator systems.
Almost all of today’s solar cell modules use
only what the sun produces naturally, what
researchers call “one sun insolation,” which
achieves an efficiency of 12 to 18 percent. By
using an optical concentrator, sunlight intensity can be increased, producing more electricity out of a single, more efficient solar cell. At
the utility scale, these systems, often located in
the desert, use inexpensive optical lenses and
mirrors to concentrate sunlight onto small,
highly efficient multijunction solar cells with
the potential to achieve more cost-effective
solar PV systems.
Most recently, NREL researchers invented
and developed the inverted metamorphic multijunction (IMM) solar cell, which produced
a world-record efficiency of 40. 8 percent at
326 suns concentration in April 2008. (The
the upside-down Solar cell
Fraunhofer Institute for Solar Energy Systems
Several decades ago, scientists developed ISE announced a new record in January: 41.1
a structure called the multijunction solar cell. percent efficiency at 454 suns concentration.)
This type of cell achieves a higher efficiency by This is the highest confirmed efficiency of any
capturing more of the solar spectrum. In a mul- PV device to date. NREL has transferred this
tijunction cell, individual cells are made of lay- new technology to industry, a success that
ers, where each layer captures part of the sun- earned a 2008 R&D 100 Award. NREL’s Mark
light passing through the cell. This allows the Wanlass, Ph.D., is the principal inventor of
cell to get more energy from the sun’s light. the IMM cell, and John Geisz, Ph.D., headed
Working with industry and other labora- the NREL team that achieved the 40. 8 percent
tories, NREL has played a central role in the efficiency record.
invention and deployment of these cells, and The IMM cell is a revolutionary advance,
in the transfer of the technology to industry. because it allows the cell’s attributes to be
triple-junction PV cell on its head by reversing the established order for depositing subcells, producing an ultra-high-efficiency, ultra-flexible cell. The other uses a process much
like printing to enable the direct inclusion
of PV cells into common building materials,
such as roof tiles and windows. I believe these
technologies represent a new wave of solar
cell breakthroughs that will significantly
increase the penetration of solar energy at
homes and businesses.
Basic research under way is equally exciting.
One NREL research project, for example, is
demonstrating how semiconductor nanotechnology might one day increase the efficiency
and lower the cost of solar cells. These submicroscopic dots of material are called “
quantum dots,” because they are small enough to
exhibit unexpected characteristics as a result
of their quantum mechanical behavior. For
instance, quantum dots made from the same
PV solar cell material can capture different frequencies of light, depending on their size. Research and development on a recently discovered effect called multiple-exciton generation
promises to produce solar cells that will generate more electric current than conventional
solar cells from the same amount of sunlight.
(Learn more in “The Next Generation of Solar
Cells,”at solartoday.org/solarrevolution.)
dan e. arvizu, Ph.d., is director of the
national renewable energy laboratory,
the u.S. department of energy’s primary
national laboratory for renewable energy and
energy-efficiency research and development.
Prior to joining nrel, arvizu was the chief
technology officer with cH2M Hill cos. ltd.
and previously was an executive with
Sandia national laboratories, where he
headed research efforts on alternative
energy technologies and materials. He started
his career as a member of the technical staff at
Bell telephone laboratories.
Heliovolt
helioVolt Corp. has developed a proprietary processing system that bonds metal-organic film
layers under heat and pressure, forming large-grain CiGS crystals. this solar printing process
could soon make it easier and more affordable
to construct buildings of PV-coated materials —
turning them into small power plants.
