October 2004

Spinach Power

By Gregory T. Huang

It may sound like something out of a Popeye cartoon, but MIT researchers are building a promising solar cell from spinach. In their Cambridge lab, bioengineer Shuguang Zhang and electrical engineer Marc Baldo shine a laser beam on a chip the size of a postage stamp. Out of a wire electrode hooked to the chip comes electricity -- a trickle now, but one day, perhaps, enough to power a cell phone or laptop. Instead of the silicon found in most solar cells, however, this chip uses proteins from plants that have evolved over millions of years to turn sunlight into usable energy.

The advance "is of tremendous importance," says Peter Peumans, an expert on organic electronics at Stanford University, because solar cells that draw on plants' natural photosynthetic ability could eventually be lighter, cheaper, and easier to repair than their conventional cousins.

Biological cells removed from plants and connected to electronic hardware typically die within hours. But Zhang and Baldo, collaborating with the University of Tennessee and the U.S. Naval Research Laboratory, took wholesale spinach and harvested just the proteins that absorb photons and generate free electrons during the process of photosynthesis. The researchers bathed the proteins with detergent-like molecules that would keep them working properly on a dry surface for weeks. They then placed the proteins on a gold-coated glass substrate and deposited a semiconductor layer on top to collect electricity.

So far, the chip's energy efficiency is far lower than that of existing solar cells. But "extensions of these methods could produce very important future energy conversion technologies," says MIT chemist Timothy Swager. To ratchet up the chip's efficiency enough that it could power a mobile device, says Zhang, the researchers plan to increase the area of its light-absorbing surface by building layers of proteins on wavy substrates. Zhang predicts the technology could be used commercially in five years.