NASHVILLE, Tenn. — Timothy Fisher is taking a Tiffany’s approach to
converting sunlight into electricity: with a $348,000 grant from
National Reconnaissance Office, the assistant professor of mechanical
engineering is exploring the use of polycrystalline diamond as a
replacement for the silicon solar cells currently used in many space

“Diamond has a number of potential advantages for use in outer space,”
says Fisher, who will be working on the project with Weng Poo Kang,
an associate professor of electrical engineering and computer science.

Fisher maintains that diamond films:

* Can withstand the high levels of radiation typical of the space
environment. By contrast, the performance of silicon cells degrades
by about 50 percent after 10 years in orbit.

* Can operate at high temperatures. As a result, they can be used with
low-weight inflatable solar collectors resulting in an energy system
that produces more electricity per pound, a critical factor in space

* Have a potential conversion efficiency of 50 percent as compared to
10 to 15 percent for silicon solar cells.

Surprisingly, diamond solar converters would not be much more expensive
to mass produce than silicon solar cells. “When you mention the word
‘diamond’ you have to address the question of cost,” Fisher acknowledges.
“But in large volumes you should be able to make this material for about
$1 per square centimeter.” That is because the system does not use
natural, gem-quality diamonds. Instead, it uses thin films made up of
millions of microscopic diamond crystals. Polycrystalline diamond films
can be made artificially from methane, the main ingredient in natural
gas, through a process called chemical vapor deposition.

So far, the advantages and costs of diamond solar systems are largely
theoretical. No one has tried to make a diamond solar converter before.
Fisher got the idea from the research of Vanderbilt colleagues Kang and
“Diamond” Jim Davidson, professor of electrical engineering, who have
been studying the use of polycrystalline diamond for electronics and
sensor applications for a number of years.

“When I saw that they had found that diamond film emits electrons
efficiently — you don’t have to use strong electric fields and a lot
of energy to pull them from the surface — I realized that it could
be used for energy conversion,” Fisher says.

Fisher’s idea has a definite “back to the future” twist.

His diamond solar converters are not photovoltaic devices like silicon
cells but solar thermal devices. That is, they do not convert light
directly into electricity. Instead, they convert light into heat and
heat into electricity. Diamond solar cells are very similar to
thermionic emission devices that were developed more than 40 years
ago. In fact, they are a close cousin to the vacuum tubes that powered
old-fashioned radios, televisions and even computers before the
development of the transistor. In thermionic devices, electrons are
released by heating. In diamond devices, however, electrons are
extracted by combining heating and an electric field.

What gives the device a futuristic element is Fisher’s use of diamond
films covered with millions of microscopic pyramids: about 10 million
per square centimeter. When heated, the tips of these pyramids, which
are only a few atoms across, emit streams of high-energy electrons. At
this extremely small “nanoscale,” the laws of physics can differ from
what they are at larger scales. In this instance, they favor the
efficient production of high-energy electrons.

“It is this nanoscale physics that makes the device work,” Fisher says.

In the new design, the bottom of the diamond film is laminated to a
metal sheet that acts as a cathode, or negative terminal, for the device.
Another sheet of electrically conducting material is placed on top of
the film with a very small gap in between from which almost all of the
air has been removed. The top sheet serves as the anode, or positive
terminal. A radiation absorber is attached to the bottom of the cathode.
When sunlight is directed on the absorber plate, it heats up the device
to about 1,000 degrees Celsius. When heated, the tips of the tiny
pyramids emit streams of electrons that flow across the intervening
vacuum to the anode, creating an electric current.

“This creates a large amount of current and a small voltage,” Fisher
says. Because moving electrical current at low voltages causes high
levels of line loss, the engineer has to add another step: a DC-to-DC
converter that increases the voltage and reduces the current. “This
can be done with about 90 percent efficiency,” he says.

Fisher and his colleagues have been working on a small test device with
a plain diamond film without the pyramids. “What we have seen increases
our confidence that the converter will work,” he says. The goal of the
nine-month project is to produce a prototype cell that is a square
centimeter in size and produces 10 watts of electrical power at 1,000
degrees Celsius.

The National Reconnaissance Office designs, builds and operates the
nation’s reconnaissance satellites for the Central Intelligence Agency
and the Department of Defense and is part of the nation’s 13-member
Intelligence Community.

The research is also partially supported by a National Science Foundation


Schematic, photomicrographs, photos available upon request.