Cheap solar power on deck

By Kimberly Patch, 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," said McFarland.

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 solar cells.

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.

Timeline:   Unknown
Funding:   Corporate
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

Page One

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