Atwater will head a DOE-funded energy research center at Caltech, where scientists will work on developing materials that could enable thin-film photovoltaics to absorb sunlight more efficiently. These materials, whose microstructure is designed to interact with light in new ways, could be made using different types of semiconductors. Light that strikes solar cells made from them, Atwater says, can be forced to “turn a corner” and travel parallel to the surface of the thin film. As a result, the cell has a chance to absorb much more light than it would if the light passed through perpendicular to the surface.
Researchers elsewhere are hoping to overcome the challenges inherent in using disordered materials for photovoltaic cells. When light strikes the jumble of molecules in such materials, the excited electrons and the electron “holes” left behind when they’re knocked free form particle-like pairs called excitons. Excitons play a role in the process that plants use to capture energy through photosynthesis, says Marc Baldo, a professor of electrical engineering at MIT; in addition, organic light-emitting diodes use them to generate light. And, he says, it might be possible to manipulate these excitons on the nanoscale to improve the photovoltaic properties of disordered materials. Baldo heads a DOE-funded energy research center for excitonics, which includes researchers from MIT, Harvard University, and Brookhaven National Laboratory.
Ultimately, however, using sunlight to produce electricity will never supply enough of the energy we need: existing solar technologies, after all, produce power only during the day, and electricity can’t easily be stored. Instead, we must find a way to use sunlight to make fuels such as hydrogen, which can readily and cheaply be stored until they’re needed.
Learning how to efficiently make such fuels directly from the sun–a process called artificial photosynthesis, because the aim is to essentially mimic the natural process used by green plants–is “still 20 to 30 years down the road,” says Harry Gray, a chemist at Caltech and director of a solar-research collaboration that includes scientists from a number of universities. Although researchers, including some in his group, are getting “nice results” on certain aspects of artificial photosynthesis, lots of difficult problems remain to be solved. “It’s going to take a long time to get it together,” he says.
Silicon photovoltaics will be the dominant solar technology “for quite a while,” says Gray. “If all goes well, we will move into cheaper solar cells that are not single-crystal silicon, such as organic photovoltaics. But the transition [to cheaper photovoltaics] is not going to come all that fast.”
Will the stimulus bill facilitate that much-needed transition to more efficient technologies? Severin Borenstein, for one, is doubtful. Borenstein, the director of the University of California Energy Institute, says the problem with the stimulus funding is that when it comes to existing technologies, the DOE will need to pick which projects to support. “The worry is that the government will invest in the wrong technologies,” he says; picking technology winners is something that “historically it has not been very good at.”A far more effective way to promote the growth of renewable energy, he believes, is to put a price on carbon dioxide emissions through a carbon tax or a cap-and-trade scheme (see “Carbon Trading on the Cheap”). Either approach would provide market-based incentives for deploying renewables and would represent a more efficient and “technology-neutral” government policy. At the same time, he says it is important for the government to fund research into new renewable technologies.
From an economist’s perspective, Borenstein says, government subsidies are justified to address “market failures”: cases in which the market doesn’t allocate enough resources to the pursuit of socially desirable goals, such as reducing greenhouse-gas emissions. The government incentives then support efforts that are financially risky but are likely to provide a common benefit. In such a context, he says, the argument for public spending on research into new solar technologies is strong–but the case for subsidizing the current commercial technologies, particularly photovoltaics, is “really weak.” Existing photovoltaics are expensive even compared with other renewables such as wind and solar thermal, he says, and they won’t necessarily lead to cheaper technologies, either. “You’re obviously going to get [solar] panels put in, but is that going to generate something that will have a lasting benefit? Will it help you build a solar industry? I think the answer is probably not.”