Solar Cells Get A Boost From Spinach
September 5, 2012

Spinach Gives Biohybrid Solar Cells A Boost

April Flowers for - Your Universe Online

Spinach is one of those leafy green vegetables that your mother always made you eat because it was good for you. Maybe Popeye was on to something when he sang, "I'm strong to the finich / Cause I eats me spinach." It turns out; spinach is good for more than just adding iron and calcium to your diet.

A team of researchers from Vanderbilt University has combined the photosynthetic protein that converts light into electrochemical energy in spinach with silicon, the material used in solar cells, in a fashion that produces substantially more electrical current than has been previously reported by "biohybrid" solar cells.

Vanderbilt has applied for a patent on the combination and has reported the results of the study online in the journal Advanced Materials.

"This combination produces current levels almost 1,000 times higher than we were able to achieve by depositing the protein on various types of metals. It also produces a modest increase in voltage," said David Cliffel, associate professor of chemistry, who collaborated on the project with Kane Jennings, professor of chemical and biomolecular engineering. "If we can continue on our current trajectory of increasing voltage and current levels, we could reach the range of mature solar conversion technologies in three years."

The team's next step is to build a functioning PS1-silicon solar cell using this new design. They estimate that a two-foot panel would put out at least 100 milliamps at one volt, enough to power a number of different types of small electrical devices. Jennings and a group of undergraduates won an Environmental Protection Agency award that will allow the students to build the prototype. They won the award at the National Sustainable Design Expo using a solar panel based on a two-year old design.

Scientists discovered over 40 years ago that one of the proteins involved in photosynthesis, called Photosystem 1 (PS1), continues to function when it is extracted from plants like spinach. PS1 converts sunlight into electrical energy with nearly 100 percent efficiency, compared to conversion efficiencies of less than 40 percent achieved by manmade devices. This discovery prompted researchers around the globe to begin trying to use PS1 to create more efficient solar cells.

Progress has been steady but slow. Efficient extraction from the leaves was developed, and it has been demonstrated that PS1 can be made into cells that produce electrical current when exposed to sunlight. So far, though, the amount of power that these biohybrid cells can produce per square inch has been substantially lower than that of commercial photovoltaic cells.

Another problem has been longevity. The performance of some early test cells deteriorated after only a few weeks. In 2010, however, the Vanderbilt team kept a PS1 cell working for nine months with no deterioration in performance.

"Nature knows how to do this extremely well. In evergreen trees, for example, PS1 lasts for years," said Cliffel. "We just have to figure out how to do it ourselves."

Biohybrid cells have another potential advantage in that they can be made from cheap and readily available materials. Other microelectronic devices require rare and expensive materials like platinum and indium. Most plants use the same photosynthetic proteins as spinach, and in fact, Jennings has been working on extracting PS1 from kudzu.

The Vanderbilt team reports that their PS1/silicon combination produces nearly a milliamp (850 microamps) of current per square centimeter at 0.3 volts. That is nearly two and a half times more current than the best level reported previously from a biohybrid cell.

The reason for this is that the electrical properties of the silicon substrate have been tailored to fit those of the PS1 molecule by implanting electrically charged atoms in the silicon to alter its electrical properties, a process called "doping." The protein worked extremely well with silicon doped with positive charges and poorly with negatively doped silicon.

The team extracted PS1 from spinach into an aqueous solution and poured the mixture on the surface of a p-doped silicon wafer to make the device. Then they put the wafer in a vacuum chamber in order to evaporate the water away leaving a film of protein. They found that the optimum thickness was about one micron, about 100 PS1 molecules thick.

The PS1 protein absorbs the energy in the photons and uses it to free electrons and transport them to one side of the protein when it is exposed to light. This creates regions of positive charge, called holes, which move to the opposite side of the protein.

In a leaf, all the PS1 proteins are aligned. But in the protein layer on the device, individual proteins are oriented randomly. Previous modeling work indicated that this was a major problem. When the proteins are deposited on a metallic substrate, those that are oriented in one direction provide electrons that the metal collects while those that are oriented in the opposite direction pull electrons out of the metal in order to fill the holes that they produce. As a result, they produce both positive and negative currents that cancel each other out to leave a very small net current flow. The p-doped silicon eliminates this problem because it allows electrons to flow into PS1 but will not accept them from protein. In this manner, electrons flow through the circuit in a common direction.

"This isn't as good as protein alignment, but it is much better than what we had before," said Jennings.