Scientists at the Indian Institute of Science (IISc) have developed a super-flexible composite semiconductor material that may have possible applications in next-generation flexible or curved displays, foldable phones and wearable electronics.
Traditional semiconductor devices such as transistors, the building blocks of most electronic circuits used in the display industry are either made of amorphous silicon or amorphous oxides, both of which are not at all flexible and resistant to stress. Adding polymers to oxide semiconductors can increase their flexibility, but there is a limit to how much can be added without compromising the semiconductor’s performance, IISc noted.
In a current study published in the journal ‘Advanced Materials Technologies’, scientists from the leading institute’s materials engineering department have found a way to produce a composite containing a significant amount of polymer – up to 40 percent of the material’s weight – using a solution process technique, namely inkjet printing.
Interestingly, this approach allowed the semiconducting properties of the oxide semiconductor to remain unchanged by the addition of the polymer. The added large amount of polymer also made the composite semiconductor highly flexible and foldable without degrading its performance, Bengaluru-based IISc.
A composite semiconductor is made up of two materials a water insoluble polymer such as ethyl cellulose, which provides flexibility, and indium oxide, a semiconductor that provides excellent electronic transmission properties.
In designing the material, the researchers mixed a polymer with an oxide precursor in such a way that interconnected channels of oxide nanoparticles (around phase-separated polymer islands) were formed through which electrons could move from one end of the transistor (the source) to the other (discharge), ensuring constant current flow.
Polymer-rich islands helps stop cracks
The key to creating these interconnected pathways, the researchers found, was choosing the right kind of water-insoluble polymer that doesn’t mix with the oxide lattice when the oxide semiconductor is made. This ‘phase separation’ and formation of polymer-rich islands helps stop cracks, making it super flexible,” says Subho Dasgupta, associate professor in the Department of Materials Engineering and corresponding author of the study.
Semi-conductive or insulating materials
Semiconductor materials are usually produced using deposition techniques such as sputtering. Instead, Dasgupta’s team uses inkjet printing to deposit material on a variety of flexible substrates from plastics to paper. A polymeric material called Kapton was used in this study. Like words and images printed on paper, electronic components can be printed on any surface using special functional inks containing either electrically conductive, semi-conductive or insulating materials. However, there are challenges.
“Sometimes it is very difficult to obtain a continuous and homogeneous film. Therefore, we had to optimize certain protocols, for example preheating the printed semiconductor layer on the Kapton substrate before high-temperature annealing,” explains first author Mitta Divya, a former PhD. student in the Department of Materials Engineering and currently a postdoctoral fellow at King Abdullah University of Science and Technology (KAUST), Saudi Arabia.
Another challenge is ensuring the right environmental conditions under which the ink can be printed. “If the humidity is too low, you can’t print because the ink dries out in the nozzle,” says Mr. Dasgupta.
He adds that in the future, these printed semiconductors can be used to make fully printed and flexible TV screens, wearable devices and large electronic billboards, along with printed organic light emitting diode (OLED) display front ends. These printed semiconductors will be cheap and easy to manufacture, potentially revolutionizing the display industry.
His team has received a patent for their material and plans to test its durability and quality control on a device-to-device basis before it can be scaled up for mass production. They also plan to look for other polymers that can help design such flexible semiconductors, according to the statement.
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