Brazilians joke that theirs is the country of the future—and always will be. Likewise, solar power has always been the ultimate green technology of the future. But maybe the sun is finally rising. The photovoltaic market, though small, has been growing briskly: by more than 60 percent in 2004. Plastering your roof with solar cells now runs as little as 20 cents per kilowatt-hour over the system's estimated lifetime, which is approaching what most households pay for electricity.
One especially promising technology that emerged in the 1990s was to make solar cells from plastic spiked with nanometer-scale crystals. Even those composite devices, though, were restricted to absorbing visible light. This year a group led by Edward H. Sargent at the University of Toronto coaxed them to absorb infrared light as well. A concoction of lead sulfide particles a few nanometers in size can absorb wavelengths as long as two microns. Thus able to harvest a wider swath of the solar spectrum, inexpensive plastic cells could rival the performance of pricey silicon ones.
Other avant-garde photovoltaic devices consist of nanoparticles coated with dye and doused in electrolyte, an approach pioneered by Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne a decade ago. The dye handles the job of absorbing photons and generating a current of electrons. Because the source of the electrons (the dye) is divorced from the matrix through which they flow in (the electrolyte) and out (the nanoparticles), electrons are less likely to get prematurely recaptured by atoms, a process that impairs current flow in conventional cells. Consequently, the dye-based cells work better under weak lighting conditions.
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Tsutomu Miyasaka and Takurou N. Murakami of the Toin University of Yokohama have extended the technique to create the world's first photocapacitor: a solar cell that both generates and stores electricity. Alongside the dye-coated particles, the researchers slapped down layers of activated carbon, which traps electrons and holds them until a switch completes the circuit. Under a 500-watt bulb, their latest design takes a couple of minutes to charge up to 0.8 volt. It has a capacitance of about 0.5 farad per square centimeter, which would give a typical solar panel the same energy storage capacity as the so-called ultracapacitors developed to replace or supplement batteries in hybrid cars and uninterruptible power supplies. In 2004 Miyasaka founded a company, Peccell Technologies, to commercialize this and other innovations.
Another way to store energy is in the form of hydrogen gas. In the late 1960s Japanese researchers Akira Fujishima and Kenichi Honda discovered that a solar cell can act like an artificial tree leaf, splitting water into its constituent elements. The trouble was that the materials involved, such as titanium dioxide, absorb mostly ultraviolet light. Restricted to such a narrow band of spectrum, the process was pitifully inefficient. Tinkering with their chemical properties allowed the cells to absorb visible light but also made them prone to corrosion.
Grätzel recently developed a way around this unhappy trade-off: put two solar cells together. The first contains tungsten trioxide or iron oxide, which soaks up the ultraviolet. The second is one of his dye-sensitized cells, which absorbs the rest of the visible spectrum and provides more electrons to aid the photolysis.
A year ago Hydrogen Solar, a British company trying to commercialize the work, made the announcement of a nearly 10-fold improvement in the efficiency of water splitting. It estimates that hydrogen produced this way would still cost about twice as much as hydrogen from natural gas but might become competitive if greenhouse gas emissions were restricted. You wouldn’t need to go to a gas station to refill your fuel-cell car; the solar panel on the roof of your house could be your private gas station.