New Solar Cell Kit Links PV, Plants and the Planet

By Greg Smestad

In December of 1997, President Clinton's negotiators traveled to Kyoto, Japan where both industrialized and developing nations signed a treaty binding them to a detailed plan of action that limits the emissions of Carbon Dioxide. The implications of this are expected to ignite a "heated" debate on the connections between issues as diverse as energy, economics, technology and environmental science. The challenge facing educators will be to explain these issues integrated into curriculum in physics, chemistry, mathematics and biology.

A new solar (photovoltaic, or PV) cell kit has been developed, using natural dyes extracted from berries, that provides an interdisciplinary context for students learning the basic principles of biological extraction, chemistry, physics, as well as environmental science and electron transfer. Far from exotic, electron transfer occurs in the mitochondrial membranes found in our cells, and in the thylakoid membranes found in the photosynthetic cells of green plants. Understanding exactly how fossil fuels like coal and oil were created by plants millions of years ago is the key to understanding concepts like oxidation and reduction, ecosystem function, renewable energy, carbon dioxide pollution, and the Greenhouse effect. The new kit draws on these concepts, and is based on the work on nanocrystalline dye sensitized solar cells that use an organic dye to absorb incoming light to produce excited electrons (see http://lpi.epfl.ch).

To fabricate the new cell, a titanium dioxide film that is coated on a conductive glass plate is dipped into a solution of a dye (for example blackberry, raspberry, or pomegranate juice). A single layer of dye molecules self assembles on each titanium dioxide particle and absorbs sunlight. To complete the device, a drop of liquid electrolyte containing iodide (similar to medicinal iodide) is placed on the film to enter the pores of the film. A counter electrode, made of conductive glass that has been coated with a catalytic layer, is then placed on top, and the two glass plates are clipped together using binder clips.

As the glass sandwich is illuminated, light excites electrons within the dye, and they are transferred into the film. These electrons are quickly replaced by the iodide in the electrolyte solution. The titanium dioxide serves the same role as the silver halide grain in color photography except that the electrons from the dye produce electricity rather than forming an image. The oxidized iodide becomes iodine or triiodide, and travels to the counter electrode to obtain an electron after it has flowed through the electrical load. The cycle is completed and electricity is generated. This operation mimics natural photosynthesis in which the electron acceptor is ultimately carbon dioxide, water is the electron donor, and the organic molecule chlorophyll absorbs the light. Students can easily fabricate the device, and determine the current-voltage and power output characteristics of the solar cell using a resistor. They can then relate this output to the chemical processes occurring in photosynthesis and in the biosphere. The sunlight-to-electrical energy conversion efficiency is between 1 and 0.5 %, but is enough to power a small motor or deflect the needle of a compass wrapped with transformer wire in a demonstration. The scientific aspects of the berry juice sensitized solar cell have been published (see N. J. Cherepy, G. P. Smestad, M. Grätzel and J. Z. Zhang, J. Phys. Chemistry, 101, 1997, "Ultrafast Electron Injection: Implications for a Photoelectrochemical Cell Utilizing an Anthocyanin Dye-Sensitized TiO2 Nanocrystalline Electrode").

Meanwhile, the simplified solar cell kit has been released by the American Chemical Society's Institute for Chemical Education. Conductive glass for the kit can be obtained separately from Hartford Glass Co. Inc. (e-mail: hartglas@netusa1.net). In initial tests, it has been successfully used in undergraduate chemistry classes at the California State University Monterey Bay, and as a demonstration to over 1000 high school science students attending the Illmac International congress for chemical techniques in Basel, Switzerland in November 1996. It was also presented at the International Symposium on New Materials for Hydrogen - Fuel Cell - Photovoltaic System - 1, August 31 - September 5, 1997 held in Cancun, Mexico. The kit has also been featured in 1997 on CNN's World Report.

On May 14, 1998, the kit was demonstrated as part of the Georgia Institute of Technology's Sherry Memorial Lecture. In July 1998, Lady Mary Archer demonstrated it to over 1000 students from around the United Kingdom at the Royal Institute in London. In a demonstration to 2nd graders in Pacific Grove, California, students were able to link the cycles found within the cell to the cycles of energy and materials which have existed on the Earth for billions of years. Humans have changed the physical state of the planet. The kit can teach that renewable energy is not merely an alternative for powering our society, but it is a search for our place in the biosphere.
 

For more information:

• M. Grätzel, "Low-cost solar cells", The World & I, pgs. 228-235 (1993).
   
• B. O'Regan and Grätzel, "A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2films," Nature , 737-739 (1991).