Organic solar cells have a singular advantage over their inorganic counterparts: They are much less expensive to produce. Unfortunately, their application has been held back by lagging energy conversion efficiencies. Now, a team of researchers from Lucent Technologies' Bell Laboratories and the University of Konstanz in Germany has reported an organic cell of doped pentacene that may lead to the development of efficient plastic photovoltaic cells. "In order to exploit the advantages of organic materials, efficiencies have to be increased," said Jan Hendrik Schön, a researcher at the institutions and co-author of the study, detailed in the Jan. 27 issue of Nature. "Up to now, the [photovoltaic] efficiencies were well below those of inorganic photovoltaic devices." Inorganic cells, such as those based on silicon or on thin films of CdTe or Cu(In,Ga)Se2, can display efficiencies greater than 15 percent. Organic cells, however, do not require the high deposition temperatures or complex solution processing of inorganic devices, and they can be deposited onto large flexible substrates. The researchers selected the organic semiconductor pentacene for the cell, which had shown promise in organic transistors. They grew 1-µm-thick pentacene crystals by horizontal physical vapor transport, immersed them in iodine or bromine solutions, and deposited front and back contacts by radio-frequency magnetron sputtering. Test results under air mass 1.5 global illumination exceeded the researchers' expectations. "We did not expect an increase of efficiency by five orders of magnitude," said Schön. "That was a nice surprise." They found that the cells displayed energy conversion efficiencies as high as 2.4 percent and internal quantum efficiencies up to 36 percent. Much work remains before commercialization. After 100 hours of operation, the organic cells display a 10 percent loss in efficiency, which the researchers believe is the result of interactions between the dopants and the contacts. Whether a viable doped pentacene cell actually emerges from the experiments may be secondary. The research into solar energy itself is important, said Schön: "Every progress in the field of photovoltaics, organic or inorganic, can be a step in the right direction."