Scientists from Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have shown they can make flexible, transparent electrical conductors with record-high performance for use in solar cells, displays and other devices by spreading polymers on a clear surface with a tiny blade, like a knife spreading butter on toast.
The technique, reported Oct. 29 in the advance online edition of Proceedings of the National Academy of Sciences, has already been used to make patterned electrodes for touch sensors and organic solar cells, and with further development could be a tool for manufacturing transparent conductors on a large scale.
"We were able to achieve record-high conductivity with a material people have known for years, just by tuning the coating process," said Zhenan Bao, a Stanford professor and member of SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC, who led the study. "That shows there's a lot of room for achieving high performance through controlling the assembly and structure of materials at the molecular scale."
Clear Conductors for Flexible Gadgets
Bao's group developed this "solution shearing" technique in close collaboration with Michael Toney and Stefan Mannsfeld, staff scientists at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL), whose teams used X-rays to look at 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.
So, scientists have been exploring alternatives. In this study they turned to PEDOT:PSS, a conductive blend of two polymers that turns transparent as it dries. It's also much cheaper and more flexible than ITO.
Tuning the Process Yields Record Results
A matchbook-sized silicon blade spread a thin layer of PEDOT:PSS on various surfaces -- including glass, silicon, and PET, a clear polyester resin used in beverage containers -- at speeds up to 6 meters per minute. By varying the speed and adjusting the temperature of this process, the researchers were able to produce see-through films of various thickness and conductivity, and also get the PEDOT and PSS polymers to separate into layers, which increased the film's conductivity even more. The best films they made beat the existing conductivity record for PEDOT:PSS.
http://www.sciencedaily.com/releases/2015/11/151103141314.htm
