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Creating transparent electrical conductors

Scientists at Stanford University as well as the Department of Energy’s SLAC National Accelerator laboratory in the United States have developed a method for synthesising flexible, transparent electrical conductors. These conductors produce record breaking performance levels for various applications such as solar cells, displays and other devices by spreading the polymers, like butter on toast, onto clear surfaces with a tiny blade.

The technique, first reported on the 29th of October in the advance online edition of Proceedings of the National Academy of Sciences, has already been used to create patterned electrodes for touch sensors and organic solar cells. With further development and research this technique could become the tool in which manufacturing transparent conductors could be taken to a larger scale.

According to Zhenan Bao, a Stanford professor and member of SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC, the record high conductivity levels were reached by simply fine turning the coating process with a material known by people for years. This shows that high performance levels could be achieved by controlling the assembly and structure of the materials at a molecular level.

Clear Conductors for Flexible Gadgets

Professors Bao’s team developed this “solution shearing technique” working in conjunction with Michael Toney and Stefan Mannsfeld, as well as scientists from the SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), whose teams utilised X-rays to examine the structures and properties of the finished films.

Transparent conductors are used where it’s important to get light in or out of a device, such as in solar cells, electromagnetic shielding, antistatic layers and lighting displays. Today these conductors are mostly made with indium tin oxide, or ITO. But ITO is expensive to work with, and it isn’t compatible with the flexible displays being developed for a new generation of TV screens, computers and other electronics.

“There’s a huge drive in industry to create things that are transparent, inexpensive, conduct well and are made from abundant materials,” said Mannsfeld, who is now a professor at the Dresden University of Technology. As a result scientists have been constantly looking and exploring various alternatives. For the purposes of this study, scientists made use of PEDOT:PESS1, a conductive blend of two polymers which appear transparent when dried. It is also more cost effective and flexible than ITO2. This process has the potential to revolutionise the electronics industry and usher in the age of inexpensive, transparent conductors for electronics across the board.

PEDOT:PSS or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate is a polymer mixture of two ionomers. One component in this mixture is made up of sodium polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonatedand carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) or PEDOT is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.
Indium tin oxide (ITO) is a ternary composition of indium,tin and oxygen in varying proportions. Depending on the oxygen content, it can either be described as a ceramic or alloy. Indium tin oxide is one of the most widely used transparent conducting oxides because of its two chief properties, its electrical conductivity and optical transparency, as well as the ease with which it can be deposited as a thin film.

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The above post has been re-posted from information provided by SLAC National Accelerator Laboratory. Note: Information has been edited for content and length.

Source: Worfolk et al., Proceedings of the National Academy of Sciences, 29 October 2015(10.1073/pnas.1509958112)

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