Columbia Knowledge Series: 'Thin Film Electronics Lets Us Make Flexible, Transparent Devices'; Says Prof John Kymissis

What is thin film electronics, and why is it important for us to know about it?

Electronics are made using materials known as semiconductors. Silicon is by far the most popular semiconductor—you would have seen photos of people holding up silicon wafers. Most electronics today are made using single crystal material. So that silicon wafer is a large huge single crystal silicon, which is cut into plates, those plates are what we call wafers, and the transistors and  other devices that we use are built on top.

Thin film electronics is a different technique. Instead of starting with a single crystal, we instead start with a piece of plastic or a piece of glass, and we deposit semiconductors on top, and that’s called the thin film process. Using that approach, we can use a different type of substrate, we’re not limited to the not infinitely large and very rigid silicon wafers. We can make things that are flexible, transparent and that are on glass.

The display on your TV or your phone for example is made using thin film electronics. In your TV, they start with a large piece of glass and there are millions of transistors on that glass that control the display and also in your phone, there is usually a piece of plastic and electronics are built on that plastic, which is then laminated on top.

The fact that it’s made from plastic makes the phone lighter and also makes the display thinner which allows more space for the battery and other electronics.

Your research spans energy storage, sensors, communication, and sustainability-driven applications across sectors like healthcare, security, agriculture, and energy harvesting. Can you tell us what technological advancements or innovations in sustainability-focused applications are getting commercial interest?

One area we are really excited about is making large microphones. Sound, when it travels, has a wavelength which ranges from millimetres to meters, depending on the frequency. When we think of ultrasound range, tens of kilohertz, that’s a large enough space that we can actually see it using microphones, but the microphones have to be big enough. So we have a technique of building electronics on piezoelectric materials which allows us to make these very large microphones that can see sound and measure turbulence.

 

We’ve also used it with some other collaborators to make very tiny microphones that can fit inside the ear. The microphone in the ear — the goal there is to make a microphone which can work for a cochlear implant user. Normally, if you know anybody with a cochlear implant, they have a headset outside that they have to take off when they take a shower, or when they’re doing sports. In order to increase the period of the day when the implant works, the idea is to put the microphone inside the ear.

One of the other areas is in making medical sensors the size of the human body. We’ve shown that by measuring the blood flow in the skin and in the brain we can diagnose certain diseases. Epilepsy is a great example of this. We work with a group at Cornell University in mapping where epileptic seizures happen in the brain. Today that’s mostly done electrically, but our collaborators have shown that if you measure where the blood is flowing, you can then tell where the epileptic seizure focus is, and that’s important if you’re considering surgery for resolving the epileptic seizures.

We’ve shown that we can use thin film electronics to make a large enough sensor that measures using optical techniques, basically measuring the colour of the brain. We can see where the blood is flowing and from there, with a higher resolution, detect where the epileptic seizure is starting and that can inform the surgery.

How about thin film electronics in the energy sector?

When you make a photovoltaic with silicon, there is actually a lot of input in terms of energy input needed to make a cell. You usually don’t cross over for five or six years after you make the cell. So if you have a net zero target, silicon photovoltaics are not amazing  because you’re just paying back the carbon for a quarter of the life of the cell. With thin film electronics, with  organic materials and certain compound semiconductors, because you don’t have to melt the silicon and purify it, the crossover point can be much sooner, less than a year. That also allows us to reach a better level of carbon neutrality as one of the application areas. In the US, the number 2 solar producer actually makes thin film cells. These cells are light enough to be put on the roof of buildings without the need for new brackets to support the weight. They’re basically solar shingles which can be put on the surface. That’s already a commercially successful product.

One of the things you’re working on is the potential of wireless sensing networks for urban  ecological  monitoring. Can you tell us a little about how this can help planners to optimise urban green spaces?

We had a project that we call the Plant Spike. When you go to the store, plants have a little flag in them that tells you what the plant is. One of my students  had the idea to make a circuit board that would tell you the health of the plant, measuring hydration of the soil, the amount of sunlight it got, and then report these back. With the Plant Spike that you can measure water, drainage, you can quantify how good the site is, you can change the watering schedule or amend the soil, etc.

We did a second version of it, which also interfaces with a dendrometer. The dendrometer is like a belt you strap around the tree to measure its growth precisely. We were able to use the dendrometer correlating the measurements of the Plant Spike to map the trees on our campus. We were able to say which trees need to be watered more, things like that. With all that information you can try to optimise the deployment of those greens and get the most bang for the buck out of your space and from your investments.