Organic photovoltaics could be dirt cheap, but their efficiency is in dispute
Illustration: Greg Mably
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.
Photos: Left: National Renewable Energy Laboratory; Right: Ken Bennett/Wake Forest University
David Carroll [right] of Wake Forest University, led a team that last year reported having developed an organic solar cell with a record-setting 6.1 percent energy-conversion efficiency. Such technology could eventually be used to make power-generating coatings for cars, tents, and roof shingles. But some solar-cell experts, including Keith Emery of the National Renewable Energy Laboratory [left], contend that Carroll and other organic PV researchers are sidestepping proper verification of their devices.
Power output is determined by exposing the cell to the calibrated light source and measuring the current produced as a function of the voltage on the circuit. The optimal combination of voltage and current--corrected for spectral mismatch--represents the cell's maximum power output. Divide by the energy of illumination the cell received and you've got the cell's conversion efficiency.
Although such standard verification procedures have long been used for inorganic PV, it's easy to introduce errors when applying them to organic solar cells. That's because the unconventional semiconductors that make organic technology so exciting are also tricky to manipulate and measure. Most of the organic PV being worked on today--and drawing the greatest interest from investors and researchers--are called bulk-heterojunction cells, which Heeger and his collaborators invented in the early 1990s. They are composed of conducting polymers and carbon nanostructures called buckyballs that, in the right combinations, mimic the light-absorbing p-n junction of inorganic photovoltaics. (Another type of organic PV technology known as dye-sensitized or Grätzel cells displays higher efficiencies in the lab but has limited commercial appeal because of its liquid components.)
Most experimental organic PV cells are produced by pouring a solution of polymers and buckyballs onto a glass plate, spinning the plate to spread the solution into a film, heating it to drive off the water, and then sandwiching the resulting film between electrodes. Under illumination, the conducting polymer absorbs photons, kicking off electrons that are then attracted by the buckyballs and routed to an electrode. To optimize the transfer of charges from plastic to buckyball--and thus the device's efficiency--researchers continually seek improvements in materials, heat treatments, and other processing tricks. Another tack is to stack PV cells on top of one another. Heeger's group created such an architecture for its claimed 6.5 percent efficient device.
Organic cells behave differently under illumination than their inorganic cousins. Organic cells absorb mostly short-wavelength light at the blue end of the spectrum, while inorganic cells--including the reference cells used to calibrate the test lamps--absorb mostly red and infrared light. That makes spectral mismatch more pronounced when testing organic PV, because the reference cell and experimental cell have widely divergent spectral responses.
At the same time, organic semiconductors are fragile, making it hard to get a precise fix on the spectral mismatch. To measure a cell's spectral response accurately can take hours and thousands of readings across the spectrum, during which oxygen- and moisture-sensitive semiconductors, coatings, and electrodes can degrade.
Finally, organic PV researchers tend to produce very small cells--some less than a millimeter on a side. The larger the cell, the greater the chance of its containing uneven layers of film, which degrades performance. But diminutive cells are more likely to exhibit what solar-cell experts call perimeter effects--extra illumination that creeps in from beyond the edges of the test cell, thereby exaggerating its performance. Certification labs will test any size cell (Emery says NREL has tested cells as small as 0.008 cm2), but they give greater credence to cells that are 1 cm2 or larger, a size that Emery says ”is sort of the transition point where perimeter effects become less important.”
Shaheen and Emery estimate that uncorrected spectral mismatch and extra illumination can exaggerate the conversion efficiency of test cells by as much as 50 percent. And their rapid degradation under testing conditions makes the organic PV a moving target. The perceived inattention to these measurement artifacts is what's fueling skepticism. ”The community may think that there's 6 percent organic PV out there. I don't believe there is,” Emery says.
In a letter published in the December issue of Applied Physics Letters , Emery directly challenged the 6.1 percent efficiency claim from Wake Forest. The Wake Forest paper had described the group's use of temperature cycling to create polymer filaments in the bulk-heterojunction cells that enhanced their charge conduction and enabled thicker absorbing layers. The process purportedly boosted the cell's efficiency by nearly a fifth. Wake Forest's news release quoted project leader David Carroll, director of the university's nanotechnology center, as hoping to reach 10 percent efficiency in the coming year [see photo, ”In Dispute”]. Emery's letter, however, argued that the true efficiency of Carroll's cell might be closer to 3 percent.
Emery's critique hinges on a mathematical cross-check that PV researchers rely on to spot weird results. It repurposes the measurements taken to map a cell's spectral response, using them to predict the current expected from the cell under sunlight. In essence, researchers multiply the values measured under monochromatic light by that wavelength's contribution to the solar spectrum and then sum them.
Carroll and his colleagues did not present that cross-check in their paper, so Emery used the spectral-response data provided and made the calculation himself. To square the resulting predicted currents with the 6.1 percent efficiency claimed, the cells would need to be generating more than one charged electron for each photon absorbed--a result that Emery dismisses as ”not physically possible.” Noting that Carroll accounted for spectral mismatch, Emery concludes that perimeter effects inflated the results. ”The mistake is probably in the device area--which is not unexpected for a device with an area less than 10 mm2,” Emery wrote.
Carroll, however, stands by his results. He says Emery's cross-check is flawed because the spectral response presented in the paper is that of a related but nonidentical cell. Why present power measurements for one cell and spectral response for another? According to Carroll, the temperature cycling that produced the 6.1 percent cells also left them incapable of withstanding the spectral-response tests. ”They came apart,” he says.
Carroll charges that his critics' complaints are just the politically motivated sniping of scientific competitors. He expresses the utmost respect for Emery as a scientist, but Carroll also considers Emery to be the collaborator of a competitor who, like Carroll, is vying for grant money from the U.S. Air Force Office of Scientific Research, which underwrites much of the basic research in organic PV. ”People are very aggressive,” Carroll says, ”and they're aggressive because of money.”
Emery insists that he simply calibrated test equipment for the researcher in question. ”We had absolutely nothing to do with designing the cells,” Emery says. ”I won't let anyone in my lab get involved in fabricating PV technology.” He adds that doing so would violate his lab's certification.
Photos: Left: National Renewable Energy Laboratory; Right: Plextronics
A technician [left] at the National Renewable Energy Laboratory measures the spectral response of a photovoltaic device, which indicates how sensitive it is to different hues of light. Industry leader Plextronics subjects its organic PV cells [right] to such independent evaluation, but others contend that the practice is expensive and time-consuming and that it stifles innovation.
Inorganic PV experts avoid such bickering through a culture of independent verification: prior to publishing, scientists and companies submit potentially record-breaking cells to international reference labs, including NREL, Germany's Fraunhofer Institute for Solar Energy Systems, and Japan's National Institute of Advanced Industrial Science and Technology, which offer such services at no cost to the researchers. The Materials Today editorial urged the organic PV community to adopt the same practice.
Easier said than done. The testing may be free, but preserving the fragile cells while they're en route and awaiting testing (a two- to eight-week queue at NREL) can cost several thousand dollars. At a minimum, the cells must be vacuum-packed. Better still is encapsulating the cells in glass or protective polymers--a sophisticated process that is beyond the expertise and resources of most academic labs.
Verification advocates recognize the added challenge but say that the interests of the community must prevail. ”We have sympathy, up to a point,” says Shawn Williams, vice president of technology for Plextronics, which does adhere to prepublication verification. Especially with the industry approaching commercialization, Williams says, record claims must be ”tempered” with certified results: ”It's about credibility. If people go out there and publish results that are not substantiated, then we or anyone else who's out there with real results get lost in the noise.”
Some influential academics agree. One is Niyazi Serdar Sariciftci, a physicist at Austria's Johannes Kepler University Linz who collaborated with Heeger on his early PV research and whose work since then has helped Konarka raise more than US $100 million to commercialize organic PV. ”In my opinion, every scientist has to go to an accredited lab and certify the claimed efficiency, particularly if the claim is for a world-record efficiency,” Sariciftci says. ”If it has never been reproduced objectively, it does not exist for science.”
That kind of talk strikes others as elitist and counterproductive. ”It took years to reproduce the Millikan oil-drop experiment. Therefore it didn't exist to scientists?” Carroll asks, referring to physicist Robert Millikan's famous 1910 experiment measuring the electron's charge and the resulting acrimonious debate that delayed Millikan's Nobel Prize by several years. Carroll says that a verify-then-publish system would thwart young researchers and other newcomers short on funds and influence. ”That's going to begin to narrow down the number of people who are allowed to work and publish,” he says. ”Is that really what you want happening at a time when we need to be exploring all options in solar energy?”
Heeger echoes Carroll's last sentiment. He and others who favor the publish-first, verify-later approach could even point to one of Heeger's own early reports on organic PV technology: a 1995 paper in Science reporting a cell that, according to the authors, delivered a conversion efficiency of 2.9 percent, seemingly shattering the 1 percent ceiling that had defined organic PV for a decade. Competitors quickly recognized that a calculation error had more than doubled the reported power output. Despite the mistake, the paper continues to be heavily cited as a seminal advance that pointed a path to the organic PV devices now edging toward commercial readiness.
Heeger never issued a retraction and still makes no apologies for the error. ”Numbers are indeed important,” he says. ”But scientific advances that demonstrate and enable quantum-step improvements are perhaps even more important.”
About the Author
PETER FAIRLEY frequently writes about energy for IEEE Spectrum. In ”Solar-Cell Squabble”, he covers an ongoing controversy among organic photovoltaic researchers. After interviewing people on all sides, Fairley concluded that ”discerning the real progress happening in solar power from the hype ain't easy.”
To Probe Further
The home page for the National Renewable Energy Laboratory’s device-performance group (http://www.nrel.gov/pv/measurements/device_performance.html) has useful information about how it measures photovoltaic cells.
Materials Today’s November editorial on organic PV testing, ”The Value of Values,” is available at http://dx.doi.org/10.1016/S1369-7021(07)70290-0.