nanoHUB-U Physics of Electronic Polymers/Lecture 5.6: Organic Electrochemical Transistors (OECTs)
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[Slide 1 L5.6] Hello and welcome back to The Physics of Electronic Polymers. In this lecture we're going to move a step forward and move away from what we've talked about in terms transistors that use a field effect to induce a signal in charge transport and instead, talk about organic electrochemical transistors or OECTs. And the reason we like these devices is because they've really been on the leading edge in terms In terms of bio responsiveness and biomonitoring of flexible electronic devices. So what we'd like to be able to do by the end of this lecture is compare and contrast the operating mechanism of OECTs to our organic field effect transistors or OFETs as we talked about before. And also be able to explain in a very basic level why this may be very useful in terms of bio sensing both in real time and outside the body as well. 

[Slide 2] So when we have a OECT, it looks very much like an organic field effect transistor. It has a source and a drain. In between it, it has a semiconducting or conducting polymer, okay? And on top of it, it has some kind of a electrolyte, which will serve like our oxide layer. An organic field effect transistor. And then of course with that we have some kind of gate which we symbolized by g here. And this is a general schematic device so in terms of how the device looks, it looks very similar to an organic field effect transistor. The key difference here is that instead of having an oxide layer gate, the semiconducting polymer. We're going to have some kind of electrolytic material here. So that's why it's called an electrochemical transistor because there's going to be some kind of electrochemical interaction between the electrolyte and the semiconducting polymer. This is a side view schematic if we look down from the top view, we can see our metal contact, our source and our drain and across them, across this channel width here, is the PEDOT:PSS, which is our semiconducting polymer that'll go ahead and modulate the charge. And how much charge it passes will be directly related to what the electrochemical state of the polymer is, which we can modulate via the gate and what's in our electrolyte solution. And this, what's in our electrolyte solution, will be something. It oftentimes will be something that's related to a biological species. 

[Slide 3] So if we look at this, when we plot our drain current, our id versus our drain voltage. Usually when we increase the magnitude of VD what we saw on our organic field effect transistors is that we would actually see the current rise. So usually this arrow will be pointed this way up. And we would have a higher drain current at the same drain voltage with a higher gate voltage but here it's opposite. Actually as we increase the gate voltage we're going to decrease the current. And that's because what we're doing. Now is instead of inducing charge into the semiconducting material, we're actually going ahead and pushing some of the IAS problem, the electrolyte, into our semiconducting material which actually takes a waste for a charge. So we can't pass as much charge in this way. Now when we do our ID versus our VG our gate voltage plot now. This looks completely different. Now usually we have our VG have the same sign as VD. Now they have opposite signs. And instead of measuring some kind of mobility here what we'll measure is what we call the transconductance or g sub m. And those transconductance is really just the delta of the ID divided by the delta of the VG. So if we go ahead and measured that, we can see that our transconductance that has units of something like conductivity here, siemens, here we're in millisiemens. But we can reach two or three millisiemens when we use our PEDOT:PSS device. And really, the difference in our conductivity are conductance is what's going to allow us to go ahead and have a signal for a biological response. The cool thing about OECTs, with respect to polymers is that if we make one we can have a pristine material right here and we have a whole lot of gold contacts here. 

[Slide 4] And our semiconducting polymer on top and we're going to go ahead and crumble it all up. And when we crumple this all up We can do that and then we can unfold it. So on a piece of plastic, it's a semi-conducting piece of polymer. The goal is relatively flexible in this case. So we can manipulate the material in a rather facile manner. But when we do that, the cool thing that happens is before crumpling, we see these lines and then after crumpling, we see the dash lines and you can see they actually overlay each other both in terms of the IDVD and the IDVG plot. And what that means is that if we monitor the transconductance before in red and after this crumpling, and you can see here it's very well crumpled, but normalized transconductance is almost identical. And in fact the time response is about the same as well. So not only can we have this high conductivity and this high transconductance with this whole flexible, foldable, bendable, crumpleable, if you will. Device, we actually see the same behavior. And we go ahead and see that the time response, so how quickly we can get that signal out, is almost exactly the same. So here we're able to go ahead and use a material, use something on a flexible substrate with a flexible polymer, to get to a very manageable organic electronic device that you can very easily picture going inside of one's body. 

[Slide 5] Now, what we see in this exact paper here out of 2013, is this idea that when we use these kind of materials, you can use a lot of different things for OECTs. You can use inorganic materials, you can use classic organic semiconductors like P3HT, and you can use PEDOT:PSS. And it turns out that when you use PEDOT:PSS and you optimize a device structure like they did back in 2013. You can actually see some of the highest performance one has ever seen for any kind of OECT inorganic or organic. So not only do we have a system that has great electronic performance and has great mechanical properties, but think about it we're talking about materials that in general the active layers composed of just carbon and hydrogen. So now you have something that's very similar to what's actually inside your body. There's not just this whole idea that you might have some kind of negative response by putting some kind of gallium, lead, or some kind of other material that's not native to the body inside of your body at high concentrations. So now we have a system where what really set up to not just monitor things and monitor things in a really good manner but do it in a biocompatible way. So next time we'll show how we can really use these materials, and some of the really killer new applications that these electronically active polymers have been used in in biosensing. So with that, I thank you for your attention, and I look forward to seeing you next time on The Physics of Electronic Polymers.