Cricket cream provides recipe for photocells

Northwestern University researchers have turned to a material used in baby powder and the opaque sun block favoured by cricketers to build a more efficient form of photodetector that is a lot safer to make and use than the current top performer, lead sulphide.

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

Each organic segment – which can be one of thousands of different types of molecules – absorbs light, and an electron is transferred directly to the zinc oxide page, generating current.

To build the layer of alternating molecules, the researchers grow a precursor material to zinc oxide in the presence of self-assembling organic molecules. 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 one to two nanometers thick, depending on the molecule being used.

In the Nature Materials paper the researchers demonstrate that they can build orderly books with high detectivity. But in order for their materials to be used for commercial solar-energy production, Stupp says, they must build entire “macroscopic libraries” of these books. And, like the books, the libraries must be highly ordered.

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

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