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  • Not just for your nose

Photonics Spectra
Jun 2009
Zinc oxide in photodetectors demonstrates low cost, tunability and more.

Anne L. Fischer, Senior Editor,

What you put on your nose to protect you from the sun at the beach also can be beneficial for photodetectors, say researchers at Northwestern University in Chicago.

Making photodetectors with crystalline silicon chips suffers from a range of limitations. In the quest to find a better method, the team thinks it has found one. It has developed a material that uses zinc oxide – commonly used to prevent sunburn – to convert light into electricity.

In a study published online by Nature Materials, Samuel I. Stupp, professor and lead researcher, reported that the zinc material has a very high level of detectivity resulting from its highly ordered architecture. He used the analogy of a book to describe how his team placed the inorganic component (zinc oxide) next to the organic component, as if each were a separate page. The organic “page,” which can be made up of one of thousands of types of molecules, absorbs light, and an electron is then transferred to the inorganic page, generating current.

Cheap and easy

The setup begins by electrodepositing the hybrid organic/zinc hydroxide material onto an electrode, where the zinc hydroxide material is converted to zinc oxide. The researchers’ goal was to use rapid, reproducible steps to inexpensively and easily create highly ordered, nanostructured optoelectronic active matrices.

Researchers at Northwestern University are moving photodetectors a little closer to green by substituting zinc oxide for lead or silicon.

The advantages, besides lack of toxicity, include cost, level of detectivity and tunability. Zinc is inexpensive and abundant, whereas silicon is not. The researchers found that the zinc oxide material in the initial unoptimized devices was a few orders of magnitude lower than that reported for lead sulfide nanoparticles; however, it has one of the best-reported detectivity values for hybrid organic/inorganic, purely organic and amorphous silicon systems. They expect to see significant enhancements in detectivity with appropriate device engineering.

And, finally, the investigators found almost unlimited tunability of the hybrid system and compared it with photodetectors using lead sulfide nanoparticles, where wavelength tunability is achieved solely by controlling nanoparticle size.

The advantages of using the benign material in small handheld gadgets are many, including reducing the level of hazardous materials people must handle in the manufacturing process right through to disposal.

In the future, zinc oxide also may be used in large photovoltaic solar cells or in flexible electronics. To scale up to photovoltaic solar use, however, the little books that investigators are building today will have to be highly ordered and turned into multivolume libraries, which right now is one of the challenges, Stupp said.

Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
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