Researchers develop a simple processing technique that could cut the cost of organic photovoltaics and wearable electronics
With a new technique for manufacturing single-layer organic polymer solar cells, scientists at UC Santa Barbara and three other universities might very well move organic photovoltaics into a whole new generation of wearable devices and enable small-scale distributed power generation.
The simple doping solution-based process involves briefly immersing organic semiconductor films in a solution at room temperature. This technique, which could replace a more complex approach that requires vacuum processing, has the potential to affect many device platforms, including organic printed electronics, sensors, photodetectors and light-emitting diodes. The researchers’ findings appear in the journal Nature Materials.
“Because the new process is simple to use, general in terms of applicability and should be configurable into mass productions, it has the potential to greatly accelerate the widespread implementation of plastic electronics, of which solar cells are one example,” said co-author Guillermo Bazan, director of UCSB’s Center for Polymers and Organic Solids. “One can see impacts in technologies ranging from light-emitting devices to transistors to transparent solar cells that can be incorporated into building design or greenhouses.”
Studied in many academic and industrial laboratories for two decades, organic solar cells have experienced a continuous and steady improvement in their power conversion efficiency with laboratory values reaching 13 percent compared to around 20 percent for commercial silicon-based cells. Though polymer-based cells are currently less efficient, they require less energy to produce than silicon cells and can be more easily recycled at the end of their lifetimes.
This new method, which provides a way of inducing p-type electrical doping in organic semiconductor films, offers a simpler alternative to the air-sensitive molybdenum oxide layers used in the most efficient polymer solar cells. Thin films of organic semiconductors and their blends are immersed in polyoxometalate solutions in nitromethane for a brief time — on the order of minutes. The geometry of these new devices is unique as the functions of hole and electron collection are built into the light-absorbing active layer, resulting in the simplest single-layer geometry with few interfaces.
“High-performing organic solar cells require a multiple layer device structure,” said co-author Thuc-Quyen Nguyen, a professor in UCSB’s Department of Chemistry and Biochemistry. “The realization of single-layer photovoltaics with our approach will simplify the device fabrication process and therefore should reduce the cost. The initial lifetime testing of these single layer devices is promising. This exciting development will help transform organic photovoltaics into a commercial technology.”
Organic solar cells are unique within the context of providing transparent, flexible and easy-to-fabricate energy-producing devices. These could result in a host of novel applications, such as energy-harvesting windows and films that enable zero-cost farming by creating greenhouses that support crops and produce energy at the same time.
Learn more: Solar Cell Game Changer
In science, sometimes the best discoveries come when you’re exploring something else entirely.
That’s the case with recent findings from the National Institute of Standards and Technology (NIST), where a research team has come up with a way to build safe, nontoxic gold wires onto flexible, thin plastic film. Their demonstration potentially clears the path for a host of wearable electronic devices that monitor our health.
The finding might overcome a basic issue confronting medical engineers: How to create electronics that are flexible enough to be worn comfortably on or even inside the human body—without exposing a person to harmful chemicals in the process—and will last long enough to be useful and convenient.
“Overall this could be a major step in wearable sensor research,” said NIST biomedical engineer Darwin Reyes-Hernandez.
Wearable electronics, as envisioned in this brief animation, would permit the wearer to monitor not only familiar vital signs, but a host of other biomarkers in the body – potentially catching the signs of disease well before symptoms appear.
Wearable health monitors are already commonplace; bracelet-style fitness trackers have escaped mere utility to become a full-on fashion trend. But the medical field has its eye on something more profound, known as personalized medicine. The long-term goal is to keep track of hundreds of real-time changes in our bodies—from fluctuations in the amount of potassium in sweat to the level of particular sugars or proteins in the bloodstream. These changes manifest themselves a bit differently in each person, and some of them could mark the onset of disease in ways not yet apparent to a doctor’s eye. Wearable electronics might help spot those problems early.
First, though, engineers need a way to build them so that they work dependably and safely—a tall order for the metals that make up their circuits and the flexible surfaces or “substrates” on which they are built.
Gold is a good option because it does not corrode, unlike most metals, and it has the added value of being nontoxic. But it’s also brittle. If you bend it, it tends to crack, potentially breaking completely— meaning thin gold wires might stop conducting electricity after a few twists of the body.
“Gold has been used to make wires that run across plastic surfaces, but until now the plastic has needed to be fairly rigid,” said Reyes-Hernandez. “You wouldn’t want it attached to you; it would be uncomfortable.”
Reyes-Hernandez doesn’t work on wearable electronics. His field is microfluidics, the study of tiny quantities of liquid and their flow, typically through narrow, thin channels. One day he was exploring a commercially available porous polyester membrane—it feels like ordinary plastic wrap, only a lot lighter and thinner—to see if its tiny holes could make it useful for separating different fluid components. He patterned some gold electrodes onto the membrane to create a simple device that would help with separations. While sitting at his desk, he twisted the plastic a few times and noticed the electrodes, which covered numerous pores as they crisscrossed the surface, still conducted electricity. This wasn’t the case with nonporous membranes.
“Apparently the pores keep the gold from cracking as dramatically as usual,” he said. “The cracks are so tiny that the gold still conducts well after bending.”
Reyes-Hernandez said the porous membrane’s electrodes show even higher conductivity than their counterparts on rigid surfaces, an unexpected benefit that he cannot explain as yet. The next steps, he said, will be to test changes in conductivity over the long term after many bends and twists, and also to build some sort of sensor out of the electrode-coated membrane to explore its real-world usability.
“This thin membrane could fit into very small places,” he said, “and its flexibility and high conductivity make it a very special material, almost one of a kind.”
A new, ultrathin film that is both transparent and highly conductive to electric current has been produced by a cheap and simple method devised by an international team of nanomaterials researchers from the University of Illinois at Chicago and Korea University.
The film is also bendable and stretchable, offering potential applications in roll-up touchscreen displays, wearable electronics, flexible solar cells and electronic skin. The results are reported in Advanced Functional Materials.
The new film is made of fused silver nanowires, and is produced by spraying the nanowire particles through a tiny jet nozzle at supersonic speed. The result is a film with nearly the electrical conductivity of silver-plate — and the transparency of glass, says senior author Alexander Yarin, UIC Distinguished Professor of Mechanical Engineering.
“The silver nanowire is a particle, but very long and thin,” Yarin said. The nanowires measure about 20 microns long, so four laid end-to-end would span the width of a human hair. But their diameter is a thousand times smaller — and significantly smaller than the wavelength of visible light, which minimizes light scattering.
The researchers suspended the nanowire particles in water and propelled them by air through a de Laval nozzle, which has the same geometry as a jet engine, but is only a few millimeters in diameter.
“The liquid needs to be atomized so it evaporates in flight,” Yarin said. When the nanowires strike the surface they are being applied to at supersonic speed, they fuse together, as their kinetic energy is converted to heat.
“The ideal speed is 400 meters per second,” Yarin said. “If the energy is too high, say 600 meters per second, it cuts the wires. If too low, as at 200 meters per second, there’s not enough heat to fuse the wires.”
The researchers applied the nanowires to flexible plastic films and to three-dimensional objects. “The surface shape doesn’t matter,” Yarin said.
The transparent flexible film can be bent repeatedly and stretched to seven times its original length and still work, said Sam Yoon, the corresponding author of the study and a professor of mechanical engineering at Korea University.
Earlier this year, Yarin and Yoon and their colleagues produced a transparent conducting film by electroplating a mat of tangled nanofiber with copper. Compared to that film, the self-fused silver nanowire film offers better scalability and production rate, Yoon said.
“It should be easier and cheaper to fabricate, as it’s a one-step versus a two-step process,” said Yarin. “You can do it roll-to-roll on an industrial line, continuously.”
The consumer marketplace is flooded with a lively assortment of smart wearable electronics that do everything from monitor vital signs, fitness or sun exposure to play music, charge other electronics or even purify the air around you — all wirelessly.
Now, a team of University of Wisconsin—Madison engineers has created the world’s fastest stretchable, wearable integrated circuits, an advance that could drive the Internet of Things and a much more connected, high-speed wireless world.
Led by Zhenqiang “Jack” Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW–Madison, the researchers published details of these powerful, highly efficient integrated circuits today, May 27, 2016, in the journal Advanced Functional Materials.
The advance is a platform for manufacturers seeking to expand the capabilities and applications of wearable electronics — including those with biomedical applications — particularly as they strive to develop devices that take advantage of a new generation of wireless broadband technologies referred to as 5G.
With wavelength sizes between a millimeter and a meter, microwave radio frequencies are electromagnetic waves that use frequencies in the .3 gigahertz to 300 gigahertz range. That falls directly in the 5G range.