Just One Word—Plastics

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. Photo courtesy of Virginie Drujon-Kippelen

Science fiction writers give us a creative peek into thinking beyond the boundaries of our current ideas of technology, with only their imagination limiting the scientific possibilities. But as we all know, with the passage of time, some of these 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, and hence knowledge, much more accessible, printed electronics may someday make a wide range of technologies, including large solar cells, affordable and widely available.

While products such as organic light-emitting diodes and organic photovoltaics are currently being manufactured as printed electronics through this method, plenty of other possible uses are still 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 starting to open up significant new avenues for electronic printing and is addressing these roadblocks. CISSEM—one of 46 Energy Frontier Research Center (EFRC) 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 decided to focus on the major sticking point of optoelectronic (electronic devices that convert electricity to light or vice versa) devices: the need for an electrode or conductor with both a low “work function” and good air stability. In the process of 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.

“
These polymers are inexpensive, environmentally friendly, and compatible with existent roll-to-roll mass production techniques. . . . Their use could pave the way for lower cost and more flexible devices.”
Bernard Kippelen
CISSEM

A number of low work function metals already exist, including such elements as calcium, magnesium, lithium, and cesium. The problem is that when exposed to moisture and oxygen even in small amounts, 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 or an expensive encapsulation layers. All these measures increase the cost, weight, and complexity of a device.

To get around the oxidation problem, the researchers at Georgia Tech and Princeton decided to try a different approach. Some metals, such as aluminum and gold, are very stable in oxygen and water but have a high work function. In an attempt to lower the work function of these materials, the researchers decided to apply 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 exactly 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, which is used for capturing carbon dioxide or used in biology for gene delivery. PEI “physisorbs,” meaning it physically adsorbs, or 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."

Enlarge Photo

Bernard Kippelen and members of his research team with their completely plastic solar cell. Photo courtesy of the Georgia Institute of Technology

Bernard Kippelen and members of his research team with their completely plastic solar cell.

To assess the utility of the new method, Kippelen’s postdoctoral researcher Yinhua Zhou—along with colleagues from the research groups of Professors Seth Marder, Jean-Luc Brédas, and Samuel Graham at Georgia Tech, and Professor Antoine Kahn at Princeton—incorporated their newly formed low-work function polymer modified electrodes into various electronic devices, including organic light-emitting diodes, organic field-effect transistors, and organic solar cells. They found that their new material performed the same as the lowest work function metal contacts currently used to manufacture these devices, but with much improved environmental stability.

In an even more exciting test—one that pushed the envelope just a little further and suggested an even wider potential for the technique—researchers coated a commonly available conducting polymer, PEDOT:PSS, with PEIE to lower the conducting polymer’s work function. With this material, they built the very first completely plastic organic solar cell, where all the parts, including the anode and cathode, are polymers. When it comes to solar energy, the development of all-plastic solar cells could significantly lower manufacturing costs.

"This discovery of new low-work function polymer interlayer materials is both surprising and quite important, and it has caused us to rethink all that we thought we understood about how to achieve low work function in interlayer materials for thin film solar cells," says Neal Armstrong, Director of CISSEM. “We believe that it will enable a variety of new organic solar cell platforms, and it creates a new direction for basic research underpinning the efficient and selective harvesting of solar-derived electrical charges.”

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.

Still, the new CISSEM technique offers much promise for the emerging technology of organic printed electronics. By breaking down barriers to affordable flexible devices, the researchers have given us a glimpse, once again, of science fiction becoming science reality.

—Dawn Adin, DOE Office of Science, [email protected]

Some material has been adapted with permission from the website of Georgia Institute for Technology http://www.gatech.edu/newsroom/release.html?nid=124901

Funding

DOE Office of Science, Office of Basic Energy Sciences

National Science Foundation

Office of Naval Research

Publications

Yinhua Zhou, Canek Fuentes-Hernandez, Jaewon Shim, Jens Meyer, Anthony J. Giordano, Hong Li, Paul Winget, Theodoros Papadopoulos, Hyeunseok Cheun, Jungbae Kim, Mathieu Fenoll, Amir Dindar, Wojciech Haske, Ehsan Najafabadi, Talha M. Khan, Hossein Sojoudi, Stephen Barlow, Samuel Graham, Jean-Luc Brédas, Seth R. Marder, Antoine Kahn, and Bernard Kippelen, “A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics.” Science 336, 327 (2012). [DOI: 10.1126/science.1218829]

Just One Word—Plastics

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