Biomimicry -- getting ideas from nature for the way we make or do things -- isn't just for robots and velcro. Plant leaves and sea sponges are inspiring researchers and companies to invent better photovoltaic cells; one by building the cells the way nature does, the other by having photovoltaics work more like photosynthesis.
Built Like Nature
Daniel Morse at the University of California Santa Barbara has been getting inspiration from sea sponges to make efficient solar cells. Manufacturing silicon solar cells is currently done the way all semiconductor devices are made; the process requires very high temperatures, plasmas and vacuum chambers, and many nasty chemicals. Sponges, on the other hand, self-assemble complex nano-structured silicon materials (their skeletons) out of protein and seawater at ambient temperature and pressure. And there's no need to worry about wafer shortages: As a university write-up of the research says, "Nature produces silica on a scale of gigatons." The sponge's secret is molecular templating, which Morse and colleagues are learning to imitate. Technology Review reported that "Morse and colleagues have made more than 30 types of semiconductor thin films and tested their photovoltaic properties. They are now working to incorporate the semiconductors into functional solar cells."
Works Like Nature
In status-quo photovoltaic cells, incoming light hits a doped semiconductor material, knocking electrons out of lower orbits into a free state, where they can be carried off by metal wires. New electrons come and fill the old holes via the same wires, so the material can absorb new photons. Pushing electrons around from one place to another like this is what generates a current.
The material properties require a tricky balance. The more conductive a material is, the harder it is to hold electrons in shells that are ready to be conveniently popped up by incoming light. But the less conductive a material is, the harder it is to get the electrons out to become useful electricity. In 1991, Michael Graetzel and colleagues developed what's now called the Graetzel cell (listed in Wikipedia as a dye-sensitized solar cell), which works more like photosynthesis in plants. It splits the process into three different steps and three different materials, using a little more chemistry than just solid-state physics. As explained on the web site of the Institute of Chemical Technology in Croatia,
In [a] natural solar cell the chlorophyll molecules absorb light (most strongly in the red and blue parts of the spectrum, leaving the green light to be reflected). The absorbed energy is sufficient to knock an electron from the excited chlorophyll. In the further transport of electron[s], other molecules are involved, which take the electron away from [the] chlorophyll. In [a] Graetzel cell, the tasks of charge-carrier generation and transport are also assigned to different species.
The "Graetzel cell" uses a thin coating of ruthenium and organic bipyridine molecules for light absorption, kicking electrons up into higher orbits but not quite all the way to being free electrons. This coating sits on a framework of titanium dioxide nano-crystals that carry the electrons away. A separate electrode replenishes the coating with more electrons (so it can absorb more photons), with the electrons carried from the electrode to the coating by a liquid electrolyte of dissolved iodine in which the entire coated framework sits.
These cells are not very efficient yet. However, they're far cheaper than silicon solar cells, because even though they are not manufactured in a biomimetic way (like Morse's cells), they also do not require the high vacuum and plasma and other difficulties of traditional PV manufacturing. We've mentioned before that the company Konarka has been selling these cells by the roll as "Power Plastic" since 2002, and have even made PV fabric. Power Plastic is currently about 3-5 percent efficient according to Machine Design, but they are hoping to jump to 20 percent efficiency by combining Graetzel cell technology with organic solar cells. Maybe at some point they'll combine their devices with the templating methods used by Morse to create PV cells that not only work more like plant leaves, but are made more like them as well.
Image Credits: UCSB's Convergence Magazine, Konarka
Unfortunately, the article implies that Dye Solar Cells (Graetzel Cells) have been available for some time, which is not the case. This is a technology just emerging now into the market place, and we are yet to see the impact that will have. Konarka was not selling "these cells by the roll as "Power Plastic"" in 2002, today is not selling, and seems to have no plans to sell. If you look at the article called "The Future Starts Now" by Jamais Cascio, March 15, 2005
you see the correct satement:
"A potential solution could come from Konarka, an energy technology firm in Lowell, Mass., which is developing inexpensive nanotechnology-based, high-efficiency, flexible, lightweight solar-power cells for electricity."
More recently, Konarka has licensed its patents to, and taken a minor share in, a company called G24 Innovations (www.g24i.com), which plans to commence sales next year. G24i will be the first company marketing Dye Solar Cells (DSC). Many other major organisations have DSC research programs and can be expected to follow G24i, so 2008 will be an exciting year, when we will begin to see changes in how broadly solar cells can be used.