Paper-based electronics have primarily been limited to use in printed organic electronics. While this is promising for commodity applications such as packaging tags and toys, the speed of organic semiconductors is not suitable for most radio-frequency applications. Among the uses for which paper-based electronic devices have been heretofore unsuitable is connecting to the cloud over Bluetooth frequencies for the Internet of Things (IoT), smart sensors, and other smart applications.
Researchers at the University of Texas at Austin (UT-Austin) are reporting this week at the International Electron Devices Meeting that graphene and molybdenum disulfide (MoS2), with their extraordinary conductivity, can enable paper-based electronics to achieve the frequency required to make them fit for IoT and smart sensor applications. The researchers claim that this work represents the first time that high-performance two-dimensional (2D) transistors have been demonstrated on a paper substrate.
Despite the fact that paper has been shown to be applicable to printed organic electronics in the past, it has not traditionally been manufactured with electronics in mind. As a result, regular sheet of office paper is not well suited to high-performance transistor manufacturing.
To overcome this main issue, the UT-Austin researchers, led by Deji Akinwande, first had to address the roughness of the paper surface, which is detrimental to the flow of current.
Another obstacle that Akinwande and his colleague, Saungeun Park, needed to overcome was paper’s high absorption of water. This water absorption makes it impossible to put paper through standard electronic fabrication processes that involve water and chemicals.
Another challenge of working with paper was its propensity to go up in flames when exposed to high heat, which imposes constraints on the maximum processing and operating temperature.
“After much research, we addressed these problems with a singular solution which is basically a thin-film coating of polymer on top of the paper,” said Akinwande. “With this coating layer, the paper becomes very smooth, no longer absorbs water or chemicals, and the temperature capability is slightly improved which is sufficient for device fabrication.”
Paper is arguably the cheapest substrate for electronics, about 10-100 times cheaper than plastics and far cheaper than semiconductor substrates. However, graphene and molybdenum disulfide are still relatively expensive and represent the main cost bottleneck for the paper-based electronics.
While the price of graphene and other related 2D materials are expected to go down as manufacturing techniques improve, Akinwande concedes that these materials will never be as cheap as paper. “The paper-based 2D technology will likely be most suitable for applications where functionality and performance are more important than just cost alone,” he added.
Currently, the UT-Austin researchers have demonstrated high-speed but simple transistor devices. The next steps will be to realize complex circuits such as wireless radio systems on paper. Akinwande believes this will be an important advance towards practical system-level demonstrations that can advance the research from the lab towards commercialization.
In order for the technology to reach the level of commercialization, several fabrication techniques will need to be adopted in order to scale up to a roll-to-roll manufacturing process, according to Akinwande. This includes nano-imprint lithography for making small devices, in-line metrology for quality control, roll-to-roll growth of 2D materials and subsequent transfer onto plastics.
“All of these manufacturing processes are either mature or in advanced stages of R&D worldwide,” said Akinwande. “At UT-Austin, we in fact have a nanomanufacturing center (NASCENT) focused on the scale-up of novel nanotechnology such as flexible/paper electronics.”