Eco-Friendly Photoconductors

Photoconductors, which are found in cell phones, digital cameras and other consumer gadgets, are considered a green technology in performance because they convert light into electricity. However, to be considered truly green, the materials used to make superconductors must be eco-friendly, especially since they potentially could be widely used for producing solar energy.

Northwestern University researchers have now designed a high-performing photoconducting material that uses zinc oxide – an environmentally friendly inorganic compound – instead of lead sulfide, which is currently the best performing photoconductor material.

The new material converts light into electricity but, unlike conventional materials, zinc oxide has an environmentally benign chemistry, low-cost production, a high level of detectivity, mechanical flexibility and wavelength tunability (the ability to design the material to absorb the most important part of the solar spectrum).

This impressive package of features holds promise for the material’s use in large-area photovoltaic solar cells as well as in flexible electronics – even in clothing and newspapers. Conventional photoconducting materials are expensive to manufacture, making them unsuitable for widespread solar energy use, and they also are rigid, which makes them unsuitable for flexible electronics.

“One property of our hybrid material that is especially important for solar energy devices is its high level of detectivity – less light is needed to get a good strong and clear signal,” said lead researcher Samuel I. Stupp, board of trustees professor of materials science and engineering, chemistry and medicine, and director of the Institute for BioNanotechnology in Medicine at Northwestern. “This comes from the material’s highly ordered architecture, which helps transport the electrons efficiently.”

The material has a detectivity level comparable to amorphous silicon, which is widely used in large-area electronics applications, such as liquid-crystal displays.

Stupp and his research team designed a novel nanoscale architecture that places the inorganic component (zinc oxide) right next to the organic component, with this pattern alternating over and over, like pages in a book. The pages are packed very tightly.

Each organic page, which can be one of thousands of types of molecules, absorbs light, and an electron is transferred directly to the zinc oxide page, generating current. This works – and works extremely efficiently – because the organic and inorganic components are so close together.

To build a book, the researchers grow a precursor material to zinc oxide in the presence of self-assembling organic molecules. (The material can be grown on any metallic or conducting substrate.) The precursor, zinc hydroxide, is formed using electrodeposition and then thermally converted to zinc oxide. Each zinc oxide page is a nanometer thick, while the organic page is 1 to 2 nm thick, depending on the molecule being used.

The researchers demonstrate that they can build orderly books with high detectivity. But for their materials to be used for solar energy, Stupp said, they must build entire “macroscopic libraries” of these books. And, las with the books, the libraries must be highly ordered.

“Right now our library is a little disordered, but we are working on optimizing our materials for use in solar energy devices,” Stupp said.

The other researchers are Marina Sofos, Joshua Goldberger, David A. Stone, Jonathan E. Allen, Qing Ma, David J. Herman,Wei-Wen Tsai and Lincoln J. Lauhon, all from Northwestern.

For more information, visit: www.northwestern.edu