Gel-based photovoltaics
US researchers have demonstrated that water-gel-based solar devices can act like "artificial leaves" heralding the possibility of soft matter solar energy conversion devices.
Soft matter solar cells have the potential to be a lot less costly to produce and more environmentally friendly than their semiconductor counterparts. Now, researchers from the Air Force Research Laboratory and Chung-Ang University in Korea, with funding from the Air Force Research Laboratory and the US Department of Energy, have shown how such devices might work. As part of North Caroline State University's university-wide nanotechnology program, Nano@NC State, the team explains that its proof of concept for making solar cells that more closely mimic nature could be a fruitful alternative to silicon-based photovoltaic materials.
Nanotube-coated electrodes
NC State's Orlin Velev, Invista Professor of Chemical and Biomolecular Engineering and the lead author of the research paper explains that the new bendable devices are composed of a water-based gel infused with light-sensitive dye molecules, in one test material the team used the plant pigment chlorophyll to get even closer to a natural approach. However, the natural-seeming technology is coupled to modern materials in its use of electrodes coated with carbon nanotubes, carbon black, or graphite. Replacing costly platinum counter-electrodes with carbon-coated copper drastically reduces price without loss of efficiency, the team says.
In their main experiments, the team used two photosensitive ions, DAS (4,4'-diamino-2,2'-stilbenedisulfonic acid) and [Ru(bpy)3]2+ as the active molecules embedded in an aqueous gel to produce what they refer to as hydrogel photovoltaic devices (HGPVs). Measured performance was comparable to or higher than that observed with earlier biomimetic or ionic photovoltaic systems.
The team has suggested a possible mechanism for the light to electricity conversion process in which a synergetic effect plays a role as the two dye molecules work together to produce a photocurrent. The researchers used UV-Vis spectroscopy to track the behaviour of the materials involved.
Velev explains that the research team hopes to "learn how to mimic the materials by which nature harnesses solar energy." Although synthetic light-sensitive molecules can be used, Velev says naturally derived products, such as chlorophyll, are also easily integrated into their devices because of their water-gel matrix. Velev adds that the team is now working to improve the water-based photovoltaic devices. There are two more important aspects of real leaves that the time would like to mimic.
Optimising light
"The next step is to mimic the self-regenerating mechanisms found in plants," Velev explains. "The other challenge is to change the water-based gel and light-sensitive molecules to improve the efficiency of the solar cells." At the moment, the devices have rather low efficiency, the team concedes, and Velev does not wish to overpromise on what can be delivered at this stage. "There is a long way to go before this can become a practical technology," he says. "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."
"A new class of low cost and flexible photovoltaic cells made of biocompatible matrix was demonstrated," the team concludes. "Biologically derived photoactive molecules, such as chlorophyll and Photosystem II, were successfully operated in aqueous gel media of such HGPVs."
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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