Los Alamos Researchers Working To Harness Photosynthesis

At the American Chemical Society's recent annual meeting (March 29, 1998) in Dallas, Greg Van Patten, a researcher in Los Alamos' Bioscience and Biotechnology Group, outlined how he and his colleagues are developing methods to use thin films of dyes to capture light and convert it into energy.

"We're studying energy transfer processes that are analogous to photosynthesis," said Van Patten. "We ultimately hope to develop a more efficient means of grabbing solar energy and converting it into electrical power."

Van Patten and his colleagues are developing a simple, environmentally friendly method of depositing thin layers of dyes on a glass substrate to create a better solar collector. Each dye, able to absorb a certain wavelength of light, would be deposited on the substrate in sequence; although the plate would look dark to the human eye, a cross-section would reveal a rainbow of colored layers piled on top of one another.

By layering the dyes, Van Patten and his colleagues hope to capture as much available light as possible -- ultimately, more than can be collected with current semiconductor solar panels.

For example, a layer that appears green to the naked eye absorbs most of the spectrum except for the green wavelength, which is reflected back to the human eye to be seen. Other layers capture other portions of the spectrum and reflect back the portions that appear to the human eye as the layers' respective colors. To a human observer looking down on the plate, very little light would be reflected back, so the plate would appear dark, or black if it were absorbing all visible wavelengths.

Van Patten and his Los Alamos collaborators are using chemicals that will allow them to build up layers simply by dunking the substrate into a dye solution. This low-tech, low-cost method avoids the use of potentially hazardous organic chemicals common to some thin-film deposition techniques.

The simplicity comes from the chemicals themselves. In order to build up the layers, researchers coat the glass substrate with a polymer that has positively charged sites. Next, the substrate is dipped into a solution that contains negatively charged dye particles. The dye ions are attracted to the positively charged polymer layer and they stick to it. After the dye layer is applied, another layer of polymer is added and the collector is ready for dipping into another dye solution. The process can be repeated until all dye layers are applied.

Van Patten and his colleagues -- Victor Klimov, Duncan McBranch and Robert Donohoe of Los Alamos's Chemical Science and Technology Division -- are working to perfect the first dye layer. Van Patten and the Los Alamos group are using a dye that comes from a class of molecules called porphyrins.

"Porphyrins are the same class of dye as chlorophyll found in plants," said Van Patten.

The porphyrin molecule has four negatively charged branches radiating from a central cluster of chemical rings. Van Patten is experimenting with how different metal ion species added to the interior affect energy transfer. In this case, Van Patten and his colleagues are using porphyrins with zinc ions in the center.

When light hits the porphyrin, the molecule absorbs a photon and moves to an electronically excited state. Since molecules like to remain at the lowest stable energy level possible, the porphyrin will try to transfer its energy to a neighboring molecule of lower energy, which in turn will try to pass energy to its neighbor and so on - like a line of people passing a bucket of water from a well to a burning barn.

The idea behind Van Patten's light-harvesting multi-layer film is to get the molecules to pass their energy from one molecule to the next through all the different dye layers until the energy can be passed into a "trap."

A similar mechanism occurs in plants. When a plant porphyrin captures a photon, it passes its energy around to other porphyrins that are clustered around an energy center. Like a pinball bouncing off bumpers, the energy is transferred between porphyrins until it strikes the energy center. The plant then can "digest" and use the energy.

Zinc-space porphyrins transfer energy well while in solution, but they behave differently on the film. Van Patten believes that part of the problem is the way in which the molecules attach themselves to the substrate.

"We will experiment with the chemistry of the molecules to see if we can get them to line up on the substrate in an orderly fashion once the substrate is dipped into the dye solution," he said.

Van Patten and his colleagues are experimenting with other dyes to see how they behave on a substrate and how well they transfer energy.

If the Los Alamos research is successful, it may have applications beyond solar collectors. Energy-transferring films could be used in a number of applications, including devices that could use sunlight to transform toxic environmental contaminants into harmless substances.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.