News in Science

'Popeye' solar cells on the way

Spinach power The power of spinach is being used by researchers to help build cheaper and more efficient solar cells.

A team from Vanderbilt University in Nashville, Tennessee report their findings online this week in the journal Advanced Materials.

The researchers found that coating specially manipulated silicon with a plant protein that converts light into electrochemical energy produced more electrical current than has been reported by previous solar cells of this kind.

Researchers are interested in such "biohybrid cells" because of their potential to be manufactured affordably, says research team member, chemical engineer Professor Kane Jennings.

That's because their primary components, typically photosynthetic proteins from green plants, "are widely abundant and renewable in nature," he explains

Jennings and colleagues used a protein complex called photosystem-1 (PS1), which they extracted from spinach leaves. "PS1 is incredibly efficient in converting the light that it captures," Jennings says.

Scientists have known for decades that PS1 continues to function when it is extracted from plants, and that it converts sunlight into electrical energy with nearly 100 per cent efficiency.

But so far the amount of power that biohybrid cells have produced per square centimetre has been substantially below that of commercial photovoltaic cells.

Now, the Vanderbilt team has taken a leap forward by tailoring a silicon substrate to fit the electrical properties of the PS1 molecule. The trick was to implant electrically charged atoms in the silicon, a process known as "doping."

"The use of p-doped silicon causes the electrons to flow from silicon into the PS1 film and prevents the opposite flow of electrons," explains Jennings. "This opposite flow is a real problem on metal surfaces because our proteins are not all perfectly aligned at this stage of development, as they are in nature."

Some way to go

Jennings and colleagues report that their setup produced up to 875 microamps of current per square centimetre. The silicon modified with PS1 also retained about 80 per cent of photovoltage of the unmodified p-doped silicon, 0.28 volts compared with 0.35 volts.

Although this is an improvement on existing biohybrid cells, it leaves some way to go before they could compete with existing photovoltaic technologies, they say.

"These relatively new PS1-based wet cells are not ready to compete with mature dry cell technologies, including silicon-based photovoltaics, in terms of performance or overall efficiencies at this time," Jennings says.

However in the long term, the potential is great. PS1 can be extracted from almost any green plant, and the Vanderbilt team has already demonstrated success with kudzu, which is a fast growing, nuisance vine in the south-eastern US.

"We envision high performance solar cells that can be prepared affordably using the renewable natural resources of a region," says Jennings. "Such cells may some day have a profound impact on the economic development of solar energy strategies in third world countries where energy poverty is rampant."