NEWS OF THE WEEK SCIENCE
HYBRID SOLAR CELL FABRICATED Photovoltaic device employs inorganic-organic composite material
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NEW SOLAR CELL USES A
semiconducting film of cadmium selenide nanorods dispersed in an organic polymer. The hybrid cells could find use in intermediate applications that require more solar power conversion efficiency than conventional solar panels but less than the highly efficient, expensive cells made for satellites, according to the group at the University of California, Lawrence Berkeley National Laboratory, that developed the device. Chemistry professor A. Paul Alivisatos, graduate student Wendy U. Huynh, and postdoctoral researcher Janke J. Dittmer fabricated the photovoltaic device by spin-casting a solution of CdSe nanocrystals and the conjugated polymer poly-3(hexylthiophene) onto an indium tin oxide glass substrate coated with a transparent conducting polymer electrode [Science, 295, 2425 (2002)}. An aluminum electrode was then deposited on top of the semiconducting film. "There is a great deal of interest in a new generation of solar cells that can be 'painted on/ as opposed to deposited by vacuum deposition," Alivisatos tells C&EN. "This could help to reduce the cost of fabrication. Previous devices of this type have been plastic solar cells based on polymers. Those cells are limited partly by the fact that the mobility of electrons in the semiconductor polymers is very low" In the hybrid cell, CdSe is used as the electron transport material and the conjugated polymer as the hole transport material. "Potentially our cells will comHTTP://PUBS.ACS.ORG/CEN
bine the superior electrical characteristics of inorganics with the ease of processing of organics," Alivisatos tells C&EN. The group showed that the power conversion efficiency of the devices using monochromatic light is 6.9%, one of the highest reported for plastic photovoltaic devices, the authors note. Using the full solar spectrum, however, the conversion efficiency of the new hybrid cells is only 1.7%, while organic solar cells made up of polymers that can be processed from solution have reported efficiencies up to 2.5%. Conventional inorganic devices routinely exhibit efficiencies of 10%, and the most advanced can achieve up to 30% efficiency
SURFACING The length and arrangement of CdSe nanorods, shown in transmission electron micrograph image, can be tailored to achieve optimal absorption of incident light in the solar cell.
"The hybrid solar cell is exciting, cheap technology if they can get the efficiency up to 10%, which I think they will in time," comments Keith W J. Barnham, physics professor at Imperial College, London, and an expert on high-efficiency solar cells. Alivisatos points out that the methods for producing very high efficiency cells today are too expensive and do not scale well for making large areas. "In our cell, many of the tricks used to boost efficiency in high-end cells can be employed while keeping the production methods simple and the costs low," he says. The group is now investigating methods for improving the conversion efficiency of the cells. -MICHAEL FREEMANTLE
BUSINESS
Payday Blues At The Top Two Chemical Firms t may be difficult for the average employee to feel their pain, but DuPont CEO Charles 0. Holliday Jr. and Dow Chemical CEO Michael D. Parker did not receive 2001 bonuses. The economic slowdown was to blame for both the decline in earnings at the two firms and the take-home pay of the two executives. According to the shareholder proxy statement DuPont filed with the Securities & Exchange Commission, Holliday waived a 2001 bonus and received only a 4% salary increase to about $1.1 million. He received a bonus of $1.7 million for 2000. Parker Holliday also asked "that he receive no salary increase in 2002, and no adjustment will be made," according to the proxy. But Holliday did receive 540,000
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stock options in February, "to further strengthen the link between Mr. Holliday's compensation and increased value to company stockholders." Those options could be worth $36 million if they appreciate 10% a year until they expire in 2011. While Parker fared somewhat better— Dow's proxy statement shows he got a 36% pay raise to $960,000 in 2001—he did not receive a bonus in 2001 either. His bonus in 2000—his first year as CEO—was $930,000. Parker received stock options as well. The Holliday 125,100 he received were 40% fewer than he received in 2000. They could be worth $6.8 million by 2011 .-MARC REISCH
C&EN
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