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Light sensor breakthrough could enhance solar cells and digital cameras

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June 30, 2009

The sensitivity of digital cameras like this CCD will be improved by the generation of mul...

The sensitivity of digital cameras like this CCD will be improved by the generation of multiple excitons per photon (image: NASA)

A new type of light sensor that acts like a pixel in a digital camera has been created by researchers at the University of Toronto. Scientists believe the sensor, which takes advantage of a phenomenon called multi-exciton generation (MEG), could lead to substantial advancements in the performance of a variety of electronic devices including digital cameras.

In solar cells and digital cameras, particles of light - known as photons - are absorbed in a semiconductor, such a silicon, and generate excited electrons, known as excitons. The semiconductor chip then measures the current that flows as a result. Normally, each photon is converted into at most one exciton. This impairs the efficiency of solar cells and it limits the sensitivity of digital cameras. When a scene is dimly lit, small portable cameras like those in laptops suffer from noise and grainy images because of the small number of excitons.

"Digital cameras are now universal, but they suffer from a major limitation: they take poor pictures under dim light. One reason for this is that the image sensor chips inside cameras collect, at most, one electron's worth of current for every photon (particle of light) that strikes the pixel," says Ted Sargent, professor in University of Toronto’s Department of Electrical and Computer Engineering. "Generating multiple excitons per photon could ultimately lead to better low-light pictures."

Until now, no group had collected an electrical current from a device that takes advantage of MEG. The breakthrough may greatly increase the power conversion efficiency of solar cells and image sensor chips.

"Multi-exciton generation breaks the conventional rules that bind traditional semiconductor devices," says Sargent. "This finding shows that it's more than a fascinating concept: the tangible benefits of multiple excitons can be seen in a light sensor's measured current."

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