What's New in Innovation?

Just One Word—Plastics

A ‘universal’ plastic coating could lead to lower cost, more flexible electronic devices.

plastic solar cell

Using polymers to develop new low work function materials, EFRC researchers created the first completely plastic solar cell, where not just the cell itself but also its electrodes are made of plastic.

Using polymers to develop new low work function materials, Energy Frontier Research Center (EFRC) researchers created the first completely plastic solar cell, where not just the cell itself but also its electrodes are made of plastic.

Science fiction writers give us a creative peek into thinking beyond the boundaries of our current ideas of technology. But as we all know, with the passage of time, some imagined science fiction possibilities also occasionally have a way of becoming realities.

One recent innovation to cross the border from science fiction to science fact is the idea of printed electronics that could result eventually in paper-thin computers, cell phones, and televisions. Similar in principle to the operation of a standard inkjet printer or the screen-printing of a t-shirt, the printed electronics process deposits functional electronic or optical inks on a surface in layers to produce an electronic device.

Just as the Gutenberg printing press flung open the door making books much more accessible, printed electronics may someday make a range of technologies, including large solar cells, affordable and widely available.

While products such as organic light-emitting diodes and organic photovoltaics are being manufactured as printed electronics through this method, plenty of other possible uses are waiting in the wings, mainly because key technological hurdles remain unresolved. Work funded partly by the Center for Interface Science: Solar Electric Materials (CISSEM) is opening up significant new avenues for electronic printing and is addressing these roadblocks. CISSEM—one of 46 EFRCs established by the DOE Office of Science in 2009—is led by the University of Arizona with the Georgia Institute of Technology (Georgia Tech), Princeton University, the University of Washington, and the National Renewable Energy Laboratory (NREL) as partners.

A team of CISSEM senior researchers at Georgia Tech and Princeton led by Professor Bernard Kippelen focused on the major sticking point of optoelectronic devices (electronic devices that convert electricity to light or vice versa): the need for an electrode or conductor with both a low “work function” and good air stability. While developing new low work function materials, the researchers hit on a major “first”: the creation of the world’s first completely plastic solar cell.

Work function is defined as the minimal amount of energy needed for an electron to be extracted from a material. In a solar cell, photons of light striking the cell create electrons and positively charged “holes” that move inside the solar cell and are extracted by different electrodes to produce electricity. High work function electrodes are required to extract holes while low work function electrodes collect electrons. The lower the work function of the electron-collecting electrode, the higher the power conversion efficiency of the solar cell will be.

A number of low work function metals already exist, including calcium, magnesium, lithium, and cesium. The problem is that when exposed to moisture and oxygen, these chemically reactive metals quickly become oxidized and stop functioning. To use these metals in today’s electronics, devices are built in laboratories where the environment is tightly controlled, reducing the chance for water and/or oxygen exposure. Then for long-term protection, the device is covered with a thick, rigid barrier such as glass. All these measures increase the cost, weight, and complexity of a device.

To get around the oxidation problem, researchers at Georgia Tech and Princeton tried a different approach. Some metals, such as aluminum and gold, are stable in oxygen and water but have a high work function. To lower the work function of these materials, the researchers applied a thin coating (approximately a few nanometer thick—10,000 times thinner than a human hair) of a polymer surface modifier.

Polymer surface modifiers do what their name implies—they alter the surface of whatever they cover, just as a Teflon coating makes frying pans non-stick. While the polymer coating approach has been used before, previous polymer surface modifiers were linked to the electrode surface through specific chemical interactions, making them feasible only for particular material combinations. With the current investigation, the researchers wanted something that would be applicable to a wide range of materials.

They settled on using an air-stable, water-based solution processed, commercially available polyethylenimine (PEI or PEIE) polymer, used for capturing carbon dioxide or used in biology for gene delivery. PEI “physisorbs,” meaning it physically adsorbs, sticks to a wide range of different materials, including metals, graphene, and even other polymers. By applying this polymer surface modifier, the scientists changed the stable, yet high work function conductor into an efficient, low work function electrode.

“These polymers are inexpensive, environmentally friendly, and compatible with existent roll-to-roll mass production techniques,” said Bernard Kippelen, CISSEM senior investigator and director of Georgia Tech’s Center for Organic Photonics and Electronics. “Replacing the reactive metals with stable conductors, including conducting polymers, completely changes the requirements of how electronics are manufactured and protected. Their use could pave the way for lower cost and more flexible devices.”

While this broadly applicable electronic printing technique represents an important advance and has the potential to lower considerably the financial and environmental costs of manufacturing low work function metal electrodes, there is still much work to be done. Preliminary tests looking at the long-term stability and device lifespan are promising. Future research will test the electrodes’ capacity to last the lifetime of a commercial product in real-life conditions. Also to be examined is precisely how the technique might eventually be scaled up for mass production.

Story by Dawn Adin, of the DOE Office of Science, Dawn.Adin@science.doe.gov. Reprinted with permission by the U.S. Department of Energy Office of Science. To read the entire article, go to: http://science.energy.gov/discovery-and-innovation/stories/2012/127033/.

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