Cheap solar power on deck
Technology Research News
Solar cells are relatively pricey largely
because the silicon they are made from is expensive to manufacture --
it requires a clean room or high vacuum.
Solar cells harvest energy from light by capturing photons. The
way silicon's electrons are arranged allows the material to absorb photons
of visible light. The energy imparted by the photons excites the material's
electrons, causing electricity to flow.
Researchers from the University of California at Santa Barbara
have come up with a new type of solar cell that separates the process
into two steps. The cell uses one material to absorb light that generates
excited electrons, then passes the electrons on to a much cheaper semiconductor
than the crystalline silicon used in most current solar cell designs.
The researchers' prototype suggests that the devices would be
much less expensive to manufacture than today's solar cells and can be
improved to be nearly as efficient. "It's enormously cheaper... more than
a factor of 10," said Eric McFarland, a professor of chemical engineering
at the University of California at Santa Barbara.
Key to the design is a very efficient way to transport electrons.
The researchers were working on making sensors that use ballistic electronics
when it occurred to McFarland that ballistic transport could be used to
make cheap solar cells. "It was motivated and stimulated by us thinking
about how electrons move across thin metal films without losing energy,"
Ballistic transport occurs in extremely narrow wires or thin films
of metal, where electrons flow straight through. In larger, ordinary wires
electrons bounce around, losing energy.
Having an efficient way to transport electrons meant the researchers
didn't have to use silicon to absorb the light rays, but could use any
of a variety of light-absorbing substances to do so, then transport the
resulting excited electrons to a semiconductor layer that is both cheaper
than silicon and can be much thinner than the silicon layers in conventional
The researchers' prototype uses a dye to capture photons, and
there are several other options, including quantum dots, said McFarland.
Quantum dots are tiny bits of semiconductor that trap electrons. "There
are many options [and] we can have multiple absorbers," he said.
Absorbing light results in an excited electron in the dye, said
McFarland. "The photoreceptor collects the light and generates what we
call a hot electron, an energetic electron," he said.
The hot electron then moves ballistically across the thin metal
film and into a semiconductor's conduction band. "The purpose of the semiconductor
in our cell is just simply to separate the charges," said McFarland. Once
charges are separated there is potential for electricity to flow to bring
the charges together again.
The titanium dioxide semiconductor the researchers used to receive
the electrons is very inexpensive, said McFarland. "It's used for white
paint -- it's a terribly cheap material," he said.
The researchers' prototype has an internal quantum efficiency
of 10 percent, and it's probably possible to increase this above ninety
percent, said McFarland. Today's commercial solar cells have an internal
efficiency of about 95 percent, he said.
Internal efficiency is a measure of the number of all photons
absorbed versus the number of photons that make it to the electric circuit,
and is how researchers rate solar cells.
The researchers' prototype has an overall efficiency of less than
one percent compared to about 15 percent for today's solar cells. Overall
efficiency is a measure of the number of photons that hit a device versus
the number of photons that make it to the electric circuit.
Increasing the overall efficiency is an engineering problem, said
McFarland. "We need to make more surface area [and] coat the surface with
more dye," he said.
The researchers' design is just a step toward using different
mechanisms for converting solar energy into electricity, said McFarland.
"It's... simply another possible route," he said.
McFarland's research colleague was Jing Tang. The work appeared
in the February 6, 2003 issue of the journal Nature. The research was
funded by Adrena Inc.
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "A Photovoltaic Device
Structure Based on Internal Electron Emission," Nature, February 6, 2003.
March 12/19, 2003
Quantum chips advance
Cheap solar power on deck
Paper speeds video access
Chip device gets to
yields better light chip
disk drives on tap
tweeze every which way
logic promises speedy devices
has few degrees of separation
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link