The Indian Institute of Science (IISc)‘s Department of Materials Engineering has created a highly flexible composite semiconductor material that holds potential for various applications in next-generation technologies such as flexible or curved displays, foldable phones, and wearable electronics.

Traditional semiconductor devices, such as transistors, are either made of amorphous silicon or amorphous oxides, both of which are inflexible and not strain tolerant at all. Adding polymers to the oxide semiconductors may increase their flexibility, but there is a limit to how much can be added without compromising the semiconductor’s performance, said the researchers.

The current study, published in Advanced Materials Technologies, found a way to fabricate a composite containing a significant amount of polymer — up to 40 per cent of the material weight — using a solution-process technique, specifically inkjet printing.

In contrast, previous studies have reported only up to 1-2 per cent polymer addition. Additionally, the approach enabled the semiconducting properties of the oxide semiconductor to remain unaltered with the polymer addition. The large quantity of polymer also made the composite semiconductor highly flexible and foldable without deteriorating its performance.

Composition of composite semiconductor

The composite semiconductor is made up of two materials: a water-insoluble polymer such as ethyl cellulose that provides flexibility, and indium oxide, a semiconductor that brings electronic transport properties.

According to the study, the key to forming these connected pathways is the choice of the right kind of water-insoluble polymer that does not mix with the oxide lattice when the oxide semiconductor is being fabricated. “This ‘phase separation’ and the formation of polymer-rich islands help in crack arrest, making it super flexible,” said Subho Dasgupta, Associate Professor in the Department of Materials Engineering, and corresponding author of the study.

Semiconductor materials are usually fabricated using deposition techniques such as sputtering. Instead, the team used inkjet printing to deposit their material onto various flexible substrates ranging from plastics to paper.

“Sometimes it is very difficult to get a continuous and homogeneous film. Therefore, we had to optimise certain protocols, for example, preheating the printed semiconductor layer on the Kapton substrate prior to high-temperature annealing,” explains first author Mitta Divya, former PhD student at the Department of Materials Engineering and currently a postdoc at King Abdullah University of Science and Technology (KAUST), Saudi Arabia.

Similarly, ensuring the right environmental conditions under which the ink can be printed is another challenge. “If the humidity is too low, you can’t print, because the ink dries up within the nozzle,” says Subho Dasgupta.

Going forward, such printed semiconductors can be used to fabricate fully printed and flexible television screens, wearables, and large electronic billboards alongside printed organic light emitting diode (OLED) display front-ends.

Moreover, these printed semiconductors will be low-cost and easy to manufacture, which could potentially revolutionise the display industry. The team of researchers has obtained a patent for their material and plans to test its shelf-life and quality control from device to device before it can be scaled up for mass production. They also plan to look for other polymers that can help design such flexible semiconductors.