Red wine mends solar cells

By Eric Smalley, Technology Research News

Researchers from the University of Toledo have found a way to increase energy production using red wine.

One challenge in making solar cells more efficient is countering the effects of bad spots in the large areas of semiconductor material used to harvest energy from light. These spots drain current, making devices like solar cells less efficient.

The researchers have found a way to use properties of the bad spots to seal them off from the working area of the cell. The researchers used the method to boost the efficiency of a cadmium telluride/cadmium sulfide solar cell from 2 percent to 11 percent, said Yann Roussillon, a researcher at the University of Toledo.

Solar cell efficiencies range from about seven percent for low-cost thin film materials to about 24 percent for high-quality silicon crystal. Large solar cells are usually made from polycrystalline or amorphous semiconductor rather than the high-quality crystal used to make computer chips.

In crystal semiconductors, atoms are arranged as a regular lattice. This gives a material like crystalline silicon good electrical properties, but makes it relatively expensive to produce. Amorphous materials contain disordered atoms, which makes them cheap to produce but less electrically efficient than crystal. Polycrystalline materials are made of aggregates of tiny crystals; they fall between crystals and amorphous materials in cost and efficiency. The University of Toledo method boosts the efficiency of the low-cost photovoltaic materials, which promises to make generating electricity from sunlight more cost-effective.

Solar cells consist of two layers of semiconductor material; one layer carries positive charges and the other negative. Sunlight is absorbed by the positive layer, and the photons excite the materials' electrons, which jump to the negative layer and are fed to an electrical circuit.

Amorphous silicon solar cells are made by spreading a thin film of silicon on a surface. Some polycrystalline semiconductor materials, including cadmium telluride, can also be made into thin films.

But thin-film polycrystalline solar cells usually have bad spots because it's nearly impossible to uniformly manufacture such large areas -- typically a square meter -- of the material, said Roussillon. Polycrystalline materials also have an intrinsically irregular molecular structure, including variations in chemical composition and crystal grain size.

These microscopic structures form microscale diodes, which allow electrical current to flow in only one direction. A photovoltaic cell is really a set of microdiodes connected in parallel, and the bad spots in the material are weak diodes, according to Roussillon. Surface voltage is lower at the weak diodes when photovoltaic material is exposed to light, he said.

The researchers' method takes advantage of the difference in electrical potential between weak and strong diodes to generate electrochemical reactions that block the weak diodes while leaving the strong ones intact. The researchers improved a photovoltaic panel by immersing it in an electrically conductive medium, or electrolyte, and exposing the panel to light. The variations in electrical potential at the surface result in currents in the electrolyte that work to even out those differences.

By choosing the right electrolyte, the researchers can focus electrochemical reactions where the currents are drawn to the weak diodes, according to Roussillon. The reactions "selectively block or etch the weak diodes and [leave] the good ones intact," he said.

One substance that can be used to cause these electrochemical reactions is red wine. One of the researchers, Victor Karpov, realized that microscopic particles in red wine should be attracted to the weak spots of the solar cell, said Roussillon. After they tested the process and found that red wine increased the solar cell's efficiency, the researchers switched to a mixture of an acid, water and aniline, a substance that turns solid in an acidic liquid and low electrical potential. "The problem is that red wine is a very complex chemical system," said Roussillon. "With the aniline system, it [was] easier to characterize what is going on on the surface of the device."

The researchers dubbed the process self-healing because the electrochemical reaction automatically acts on the weakest spots of the device.

The method could be used to improve other types of polycrystalline thin film photovoltaic devices, including light-emitting panels and liquid crystal displays, said Roussillon. The method could be used practically within two years, he said.

Roussillon and Karpov's research colleagues were Dean Giolando, Diana Shvydka and Alvin Compaan. The work appeared in the January 26, 2004 issue of Applied Physics Letters. The research was funded by the National Renewable Energy Laboratories (NREL).

Timeline:   3 years
Funding:   Government
TRN Categories:   Materials Science and Engineering; Energy
Story Type:   News
Related Elements:  Technical paper, "Blocking Thin-Film Nonuniformities: Photovoltaic Self-Healing," Applied Physics Letters, January 26, 2004


March 10/17, 2004

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Red wine mends solar cells

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