Mimicking nature to build a better solar cell

Charles T. Troy, Senior Editor, charley.troy@photonics.com

Leaves are nature’s quick-change artists. In summer, dressed in hues of green, they convert sunlight into chlorophyll, take in carbon dioxide and expel oxygen. Come fall – in northern latitudes, anyway – they turn from summer’s green to a riot of reds, yellows and oranges.

Researchers at North Carolina State University in Raleigh are hoping to mimic the chemical magic of leaves with devices they have dubbed “artificial leaves.” If their concept is successful, it could prove less expensive and more environmentally friendly than current silicon-based solar cells.

The bendable devices are composed of water-based gel infused with light-sensitive molecules – the researchers used plant chlorophyll in one of the experiments – coupled with electrodes coated with carbon materials, such as carbon nanotubes or graphite.

In the process, two photosensitive ions, DAS^— and [Ru(bpy)₃]²⁺, were used as photoactive molecules embedded in an aqueous gel. The hydrogel photovoltaic devices showed performance levels comparable with or higher than those of other biometric or ionic devices, the investigators say.

Shown is a prototype of the flexible hydrogel-based photovoltaic device with an active area of ~125 mm². Courtesy of Dr. Orlin Velev, North Carolina State University.

The light-sensitive molecules are excited by the sun’s rays to produce electricity in a way that is similar to how plant molecules are excited to synthesize sugars in order to grow, said the university’s Dr. Orlin D. Velev, Invista professor of chemical and biomolecular engineering and lead author of a report published online in September 2010 in the Journal of Materials Chemistry.

Velev said the research team hopes to “learn how to mimic the materials by which nature harnesses solar energy.” Synthetic photosensitive molecules can be employed here, but naturally derived products such as chlorophyll also can be integrated easily in these devices because of their water-gel matrix, he noted.

Commercialization of the artificial leaf device is not the immediate plan.

“While we are very excited about the biomimetic concept, we do not look for commercial development of such type of devices at this stage,” he said. “We foresee two research challenges that have yet to be solved before larger-scale commercial development.

“First, we still have to improve the efficiency of these devices, which is … very low. Second, we plan to replicate in such devices the ability of the natural leaves to regenerate and replace the organic dye, which will allow us to solve the problems with the long-term stability and performance that are common for all organic photovoltaic devices. We have clear plans how to address both of these challenges.”

The investigators are working on fine-tuning the water-based photovoltaic devices to make them resemble real leaves even more closely.

“The next step is to mimic the self-regenerating mechanisms found in plants,” Velev said. “The other challenge is to change the water-based gel and light-sensitive molecules to improve the efficiency of the solar cells.

“We believe this is one key advance that will be needed for the future development of usable biomimetic devices due to the photodegradation of the organic compounds in the devices. Note that natural leaves are a subject of continuous regeneration. We have a working plan on how to approach this problem, and we hope to reveal its results in the future.”

He said that someday roofs could be covered with soft sheets of similar electricity-generating artificial-leaf solar cells. He and his team believe that, over time, such devices could be made less expensively and on a larger scale than silicon cells. Plus, they are more environmentally friendly and easy to dispose of at the end of their life cycle.

“We do not want to overpromise at this stage, as the devices are still of relatively low efficiency and there is a long way to go before this can become a practical technology,” Velev said. “However, we believe that the concept of biologically inspired ‘soft’ devices for generating electricity may in the future provide an alternative for the present-day solid-state technologies.”