For a while, 2007 looked to be the year when organic photovoltaic (PV) technology would finally come into its own. Reports from leading research labs claimed record-setting breakthroughs in performance. Meanwhile, the U.S. Department of Energy (DOE) began welcoming investigators working on organic PV to compete for its mainstream solar-research grants, and venture capitalists invested tens of millions of dollars in organic PV development firms like Konarka Technologies, in Lowell, Mass., and Plextronics, in Pittsburgh.
Spurring all this interest was the promise of a much cheaper and more versatile source of solar power. Unlike traditional semiconductors such as silicon, this newer class of PV employs carbon-based plastics, dyes, and nanostructures and can be manufactured via a printing process that would be far cheaper than the high-temperature vacuum processing used for inorganics. Organic PV is also much more flexible and lighter in weight than inorganics, suggesting an enormous range of uses, including portable battery chargers and power-producing coatings for roofing shingles, tents, and vehicles.
Of course, such promising possibilities will only materialize once the technology becomes robust and powerful enough to be commercialized. That, too, seemed increasingly likely last year. In April, scientists from Wake Forest University, in Winston-Salem, N.C., announced a cell with a 6.1 percent energy-conversion efficiency--meaning that the cell captured as electricity 6.1 percent of the photon energy hitting it. The Wake Forest device represented a huge step up from a 4.8 percent efficient cell made by Konarka, until then considered organic PV's best in show. Three months later, Alan Heeger, the Nobel Prize–winning researcher at the University of California, Santa Barbara, who had pioneered organic electronics, trumpeted a device having a 6.5 percent efficiency.
What made those announcements so significant was that the numbers were just shy of the 7 percent mark that some developers peg as the threshold for successfully marketing organic PV for rooftop applications. While commercial inorganic PV panels now boast 10 to 20 percent efficiencies, organic PV developers are betting on finding markets at much lower performance levels, thanks to the technology's lower manufacturing cost.
But even as the news of higher efficiencies emerged, some researchers were raising doubts. They noted that the cells had not been rigorously verified by independent testing labs, unlike Konarka's 4.8 percent cell and a 5.4 percent cell announced in August by Plextronics. In November, 21 organic PV researchers in the United States and Europe published an editorial in the journal Materials Today bemoaning the ”significant number” of organic cells that had recently been reported with ”unrealistic and scientifically questionable” performance.
The signatories included Keith Emery, director of the PV certification lab at the DOE's National Renewable Energy Laboratory (NREL) in Golden, Colo. [see photo, ”In Dispute”]. Disagreements over solar-cell efficiencies aren't new, but in the past Emery had preferred a low-profile approach to resolving competing claims. Not this time. He says he felt compelled to raise the alarm over what he viewed as widespread disregard for standard test methods. ”Some members of the organic PV community seem to have not followed the knowledge gained by…other mainstream PV technologies,” Emery says. ”They haven't read the literature.”
Emery and others working with PV fret that a reputation for bogus reporting could erode the field's legitimacy and scare off investors. ”Truth in advertising is critical,” says University of Denver physicist Sean Shaheen, who contributed to an early organic PV cell in 2000 that kick-started today's race to market. ”The concern is that somebody starts investing money on a false claim and loses a lot of money, and therefore confidence in the field is shattered.”
You might think that measuring a solar cell's efficiency is simply a matter of shining sunlight on it and recording how much electricity comes out. But it's more complicated than that.
For starters, testers tend not to use actual sunlight to measure efficiency, because the amount of light can vary from day to day and place to place. Instead, they replicate sunlight using an artificial light source that they carefully calibrate and characterize. The key variables are the intensity of the illumination and its color balance. The standard level of illumination is 100 milliwatts per square centimeter (or 1000 watts per square meter, which is roughly equal to full sunlight hitting the Earth's surface). This, along with a precise definition of the cell's area, controls how much raw energy the device receives during testing.
Getting the color balance right is more about measuring how wrong it is, because no light source has the same spectrum as sunlight. To start, the tester measures the deviation of the light source from a standard solar spectrum. Then, to understand how that deviation will affect the test cell, he or she determines the experimental cell's sensitivity to different hues of light--that is, its spectral response [see photos, ”Up Close”]. The power output from the cell is measured one wavelength at a time, using monochromatic light generated by a spectrometer. Once the spectrum of the light source and the spectral response of the cell have been obtained, the tester can calculate what's called a spectral-mismatch factor, which expresses how much a given light source will under- or overestimate the cell's capacity to convert sunlight into electricity.