TECH TALK TUESDAY – A Natural Approach To Electricity

nancy Techie Tues 9-2-97

In a bio-solar cell, a photon of light excites a molecule of pigment. The energy causes an electron to be boosted into a higher orbit and enters an electric circuit. Some scientists now think those green solar collectors offer interesting lessons in solar cell design.

A Natural Approach to Electricity 
by David Tenenbaum

   Plants do it. Solar cells do it. Why can’t solar cells do it like plants do? Logical question — once you realize that “it” means using sunlight to create free electrons.

Plants use the free electrons to make carbohydrates — the original carbo loading. Solar cells use the electrons to make streams of electrons. That’s called an electric current — handy stuff.

Call it artificial photosynthesis. Call it plants-on-a-plate. We call it an effort to work smarter by working biologically. “Nature has had roughly three billion years to perfect the process, so it’s very efficient,” says Robert Donohoe, a scientist in Los Alamos National Laboratory’s biological science and biotechnology group.

Today’s solar cells — called photovoltaic cells — are made from silicon that’s treated with various other elements. When a photon — a particle of light — strikes the cell, it excites electrons, which are siphoned off by tiny conductors that gather them into useful quantities of electricity.

Plants do it differently. Instead of silicon, they use a molecule of a green pigment called chlorophyll that gets excited if it’s struck by a photon. Chlorophyll responds to this excitement by raising an electron into a higher orbit, further from the nucleus. This electron is usually pulled away from the chlorophyll molecule to a positively charged site called an “electron trap.” The trapped electrons then drive chemical processes making glucose and other carbohydrates.

One handy chemical reaction
Photosynthesis is the basis of virtually all food chains on Earth and the source of all oxygen in the atmosphere. Here’s the basic formula by which photosynthesis converts carbon dioxide and water into carbohydrates (glucose is shown), oxygen and water:
6CO2 + 12H2O RIGHTWARD ARROW C6H12O6 + 6O2 + 6H20
To understand the process, think of juggling steel ball bearings (representing the electrons) under a magnet (representing the electron trap). If you (playing the chlorophyll molecule in our little skit) throw the balls higher (excite the electrons), the magnet is more likely to pull them away from you.

Since a similar process of exciting and trapping electrons occurs in a solar cell, the Los Alamos researchers are trying to apply lessons from biology to making better solar cells. The research is young, says post-doctoral fellow Greg Van Patten, who is looking at the physics of thin films of a class of dyes related to chlorophyll called porphyrins.

The plan is to coat a surface with dyes that absorb various colors of light; the combination of dyes should trap almost all of the incoming solar energy. Between each layer of dye, a polymer would conduct electrons as they are produced.

To make an environmental-friendly technology, the researchers plan to use a simple dipping technique that would eliminate the noxious solvents now used in most coating processes.

Looking at the basics
Instead of actually producing solar cells, the researchers are making basic observations on the dye system. Thus, they are measuring fluorescence, which is one way excited molecules can shed energy. By measuring the ratio of incoming to outgoing energy, Van Patten can detect how energy is leaving the system, which should lead to better electron trapping.

Ultimately, he says, the goal is to apply existing knowledge of how pigment molecules behave in solution to the solid conditions that will be found in solar cells.

And while there’s no proof that bio-solar cells will be better than existing models, Donohoe thinks the process has potential. “We already have synthetic pigments that will absorb some energy. If there’s a good matchup with the solar spectrum, you can get a reasonable expectation that you will harvest energy.”

The real attraction of working with dyes related to chlorophyll is simple: we know chlorophyll is highly efficient — almost all photons that strike a leaf are converted to electricity. Plants have lots of experience using light to liberate electrons. Now it’s time for people to start learning from the plants.

For a good review article on the topic, see Photoinduced Electron Transfer in Supramolecular Systems for Artificial Photosynthesis, Michael Wasielewski, Chemical Reviews, 1992, pp. 435-461.

ORBITALS!

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