The price and look of solar panels have been prohibiting factors in drawing energy from the sun to power homes. But what if those panels could be replaced by a system of tiny antennae that collects light and drives it to photovoltaic (PV) cells that convert it to electricity?
A chemical engineering team at the Massachusetts Institute of Technology in Cambridge has been nudging that process a little closer to reality. The team, led by associate professor Michael Strano, recently created a hair-like filament consisting of about 30 million intertwining nanotubes of carbon atoms. The fiber acts as a kind of antenna that can capture and absorb packets of energy known as photons, which excite the filament's electrons to a higher energy level. The energized electrons create holes known as excitons, and the difference in energy levels between the holes and the electrons is called a bandgap.
The fiber, which is only about 10 one-millionths of a meter long and four one-millionths of a meter wide, can concentrate 100 times more solar energy than a typical photovoltaic cell. It's made from two layers of nanotubes whose different electrical properties the fiber can control. That’s the real breakthrough, says Strano. “The filament has layers like an onion, with larger bandgaps on the outside. Excitons tend to want to transfer to tubes with smaller bandgaps, so we made a funnel [within the fiber] to concentrate the excitons” to get them to flow to the center of the filament.
The next step, says Strano, will be to create a device whose antennae can gather the photons and transfer them to a PV cell that will convert them to electrical current.
With the cost of nanotubes dropping, Strano thinks they’ll eventually be available for “pennies per pound, as polymers are.” He predicts that, within the next five to 10 years, PV modules using technology that concentrates energy could be built into products “that are easier to work with,” such as roof shingles. “Solar concentration has the advantage of being able to put PV on products more compatible with building materials.”
Among the sources financing this research is DuPont, which in 2005 made a $25 million, seven-year commitment through the DuPont-MIT Alliance. Wayne Marsh, a DuPont research manager who directs the Alliance, says the research that led to solar funnels started as an investigation into different types of nanotubes to determine their respective functionality.
Marsh concedes that solar funnels might never have commercial applications. But DuPont could still benefit from new discoveries through patents or by using the science to enhance an existing product. “We don’t know yet. We have to wait for the proof tests, then weigh the pros and cons.”