Electronic Design

Spinach-Powered Portables Take "Green" To A New Level

Popeye would be proud. Just a little dash of spinach could help portable devices recharge in the sun, thanks to the world's first solid-state photosynthetic cell. While plants convert sunlight into energy quite efficiently, they require water and salt to survive—two materials that aren't compatible with electronics. So, researchers from the Massachusetts Institute of Technology, the University of Tennessee, and the U.S. Naval Research Laboratory focused on a protein derived from spinach chloroplasts known as Photosystem I (PSI).

The team ground up ordinary spinach and purified it with a centrifuge to isolate the protein. Researchers then took the resulting dark green pellets and purified them still before coaxing them into a water-soluble state. Next, they integrated the PSI into a membrane of peptide surfactants, a substance similar to the main ingredient in soap. This membrane helped the photosynthetic complexes self-assemble and stabilize while the researchers fabricated the circuit. In fact, the substance kept the protein complexes stable in a dry environment for at least three weeks.

The circuit's bottom layer is transparent glass coated with a conductive material. A thin layer of gold helps the chemical reaction that assembles the spinach chlorophyll PSI complexes. By evaporating a soft organic semiconductor, researchers prevented electrical shorts and protected the protein complexes from the layer of metal that completes the "spinach sandwich." When the researchers shone a laser light on the device, most of the optical excitation passed straight through without being absorbed. Yet the circuit converted about 12% of the light it did absorb to charge.

The researchers hope to achieve a power-conversion efficiency of 20% or more, which would provide an extremely efficient power source, by creating multiple layers of PSI or assembling them on rough or 3D surfaces for more surface area. Eventually, such circuits could be used in solar chargers for portable devices.

For more information about the cell, go to www.mit.edu.

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