IEEE Spectrum April, 2008 - 30

Solar-Cell
Squabble

organic photovoltaics could be dirt cheap,
but their efficiency is in dispute by Peter Fairley

or 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 recordsetting breakthroughs in performance. Meanwhile, the
U.S. Department of Energy (DOE) began welcoming investigators working on organic PV to compete for its mainstream solarresearch 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

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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.
April 2008 * iEEE SpEctrum * NA

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Table of Contents for the Digital Edition of IEEE Spectrum April, 2008