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A. Paul Alivisatos and colleagues at the University of California, Berkeley and Lawrence Berkeley National Laboratory created the hybrid solar cell (see image), which consists of a 200-nanometer-thick semiconducting layer sandwiched between two electrodes. Specifically, the layer contains rods of semiconducting cadmium selenide measuring just seven nanometers in diameter and 60 nanometers in length mixed into a semiconducting plastic material known as P3HT. The nanorods are chemically pure clusters of less than 100,000 atoms that produce electrons (and accompanying electron holes) when exposed to light of a specific wavelength. The electrons and holes then flow through the layer to the electrodes, creating a current. The scientists report a power conversion efficiency of 6.9 percent for light with a wavelength of 515 nanometers and 1.7 percent for sunlight. "All solar cells using plastic semiconductors have been stuck at 2 percent efficiency, but we have that much at the beginning of our research," study co-author Wendy U. Huynh notes. "I think we can do so much better than plastic electronics."
A number of features of the solar cell already hint at feasible improvements. If the nanorods--which are a jumbled mess in the prototype--can be packed closer together and oriented perpendicular to the electrodes, electron transfer to the electrodes would be more efficient. Also, altering the diameter of the rods while the length remains constant changes their absorption spectrum, which means that they can be tuned to absorb different colors that span the spectrum of sunlight.
If researchers can improve their efficiency by a factor of 10, the new cells could challenge existing, yet expensive, high-efficiency silicon-based devices. For one thing, they are much easier to manufacture because they don't require sophisticated processing. They could also be applied to a wider variety of surfaces. According to co-author Janke J. Dittmer, "the beauty of this is that you could put solar cells directly on plastic, which has unlimited flexibility."