Spray-on photocells harvest infrared

The sun bombards the earth with 10,000 times more energy each day than humans produce using other power sources, including fossil fuel, nuclear and hydroelectric. Key to harnessing this power is boosting the efficiency of solar energy collectors.

Researchers from the University of Toronto have found a way to cheaply and easily harvest the infrared portion of the sun's spectrum of lightwaves with a paint-like material that can be sprayed on large surfaces. Similar materials have been developed for visible light. Infrared lightwaves include heat, are longer than visible lightwaves and make up about half of the sun's lightwaves.

The researchers' material can be used in solar cells. It can also be used as a heat detector for medical diagnostics and infrared cameras.

Living tissue transmits infrared light to depths of 10 centimeters; researchers are working on cancer detectors that use infrared light as a diagnostic tool. Heat sensors are also used in existing infrared cameras to allow pictures to be taken in the dark. The new material has the potential to make infrared cameras as inexpensive and convenient as today's digital cameras, according to the researchers.

The material is a mix of lead sulfur nanocrystals and semiconducting polymer, or plastic. The researchers make solar cells by adding a solvent to the mixture and spraying it on a surface. The solvent evaporates to leave a film of the material. The researchers can tailor the range of wavelengths the material absorbs within a range of 800 nanometers to 2,000 nanometers by changing the size of the nanocrystals. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms.

The material could be ready for commercial use within three to five years, according to the researchers. The work appeared in the January 9, 2005 issue of Nature Materials.