From home appliances and TVs to cars and phones, digital electronics have become pervasive in nearly all conveniences of life. But they come at an environmental cost. Less than a quarter of the 50 million tons of electronic waste produced every year gets recycled.
Engineers at Duke University now report the world’s first fully recyclable electronics printed using mild temperatures and only water instead of hazardous chemicals. This technique promises electronics that have minimal environmental and carbon footprint.
The technology, reported in the journal Nano Letters, could lead to low-cost electronic devices from which all the materials can be reclaimed and reused. “These results provide evidence that a relatively complex device—a thin-film transistor—can be fully printed in a process that involves nothing but water,” says Aaron Franklin, an electrical and computer engineering professor at Duke.
Transistors are the building blocks of digital logic circuits and computer chips. Traditional processes to make them are complex and need clean rooms with expensive equipment. To print transistors, researchers use inks made of various electronic materials suspended in harsh solvents. Printing a transistor involves putting down layers of different inks on top each other. “These chemicals have deleterious environmental impact and require large amounts of water and energy for waste processing,” Franklin says.
An ever-increasing number of electronic sensors are today getting distributed in all types of environments: the so-called Internet of Things (IoT). “As our hunger for data continues to grow,” he says, “so will the need for fabricating these sensing networks using more environmentally friendly processes.”
Large industries such as displays also rely on thin-film transistors made with processes that emit greenhouse gases, he adds. “If those transistors were able to be printed from a water-only process with recyclable materials it would be transformative.”
But getting rid of harsh chemicals in the manufacturing process is easier said than done. The Duke team makes transistors using three carbon-based inks. Some of these carbon nanomaterials tend to clump together, and it is difficult to print them in a uniform, dense layer needed for good electrical traits. To keep them from clumping, water-based inks still require detergent-like additives. But then removing the additives at the end requires more harsh solvents and high temperatures, burning more energy.
Franklin and his colleagues developed a cyclical process in which they print a layer using water-based ink, rinse with water, dry at a relatively low temperature of up to 70°C and printed on again. That’s the lowest temperature to date for realizing fully printed thin-film transistors, he says.
The team has shown in past work that nearly all of the carbon nanomaterials used in printing can be recovered and reused without loss in performance. And while the new process uses a lot of water, it is a fraction of the water needed to deal with the toxic chemicals used in traditional fabrication methods.
Now that they have demonstrated that a route to sustainable, recyclable printed electronics is possible, the researchers plan to focus on improving performance and reliability, and getting devices ready for applications.
Source: Shiheng Lu et al. All-Carbon Thin-Film Transistors Using Water-Only Printing. Nano Letters, 2023.
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