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NEWS OF THE WEEK
SCIENCE
April 1, 2002
Volume 80, Number 13
CENEAR 80 13 p. 15
ISSN 0009-2347
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HYBRID SOLAR CELL FABRICATED
Photovoltaic device employs inorganic-organic composite material

MICHAEL FREEMANTLE, C&EN LONDON

A 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 combine 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.

"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.

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