nanoHUB-U Physics of Electronic Polymers/Lecture 3.4: Operating Mechanism of OPVs
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[Slide 1 L3.4] Hello and welcome back to The Physics of Electronic Polymers. In this lecture we'll talk about how we can think about the device operation of organic photovoltaics. And in particularly what happens and what five steps occur in order to convert light into electricity? In particular, we'd like to be able to explain what kind of balance we have between nanoscale phase separation. So how our two component mixture that we talked back in unit two is really influences how these devices operate. And second we should be able to sketch what a typical current density voltage curve looks like for an organic photovoltaic device. 

[Slide 2] So I'll remind you that typically when we have an organic photovoltaic device, we'll have two different materials. We'll typically have a small molecule that I'll have drawn in blue here, and we'll also have our liquid crystalline or semi-crystalline or rod-like red polymer here. And we'll mix these two together, and we'll form some kind of active layer. And the reason that we form these two materials in the same active layer will be explained here in just a second. But we'll slap those between an anode and a cathode, so that we can extract our current as well. So what happens when we go ahead and have OPVs and we shine light in and we can shine the light in this way, right. We'll go ahead and shine it in. We'll go ahead and have the light cause some kind of photo excitation either in the donor and the acceptor. Here I've drawn in the donor phase and I'll explain what that means in just a second. And we create again our hole and our electron, okay? And these are in the homo and the lumo bands respectively. And this creation of this electron whole pair, we'll call an exciton, occurs in organic materials because they have a relatively low dielectric constant. So this electron in this hole, they want to be free charges but they're bound together, okay? And there's a very high probability if they don't see some kind of interface to make them become unbound, some kind of thermodynamic energy to make then unbound, they'll just recombine and this excited electron will relax back down into its ground state. Okay, but what can happen is this exciton can move around in space and it diffuses in some kind of free three dimensional manner. It's charged neutral so the electric field has no influence on it. But if it diffuses and it reaches this electron acceptor interface, what can actually happen is the electron can lower its energy by moving into the blue phase and the hole keeps its energy by not going into the blue phase. And that's exactly what we like to do. Okay, so if we go ahead and have this exciton separation, then we look like a scenario where we look very much like our OFET where we just have a hole moving through the red phase and electron moving to the blue phase. And now we're just talking about charge mobility like we talked about in our field effect transistors. So really the key step here is this light absorption, exciton diffusion, exciton separation, and that's why we blend the red and the blue materials together because we need something that has this staggered difference. Between this LUMO level and this LUMO level to go ahead and give a thermodynamic driving force for charge transfer in these systems. 

[Slide 3] Now when we go ahead and do this, if we shine light on this guy, we'll get a curve that looks something like this, okay. And this curve right here in the dark looks just like a diode curve. So, a diode basically passes no current in one direction. Here we've shown it as the negative direction. And passes a lot of current as you move forward into the positive direction. That's in the dark. And the four are in alike condition, we'll see the exact same difference but now. At zero applied voltage, we'll call that the short circuit condition, we'll have some kind of current because it's created because we're shining electrons onto our, or shining light onto our system generating electrons in holes which then turns into current, right? And if we go ahead in and shine light, shine light, shine light and sweep our voltage forward.  What we'll get is one where our diode current exactly matches Our photogenerated current, and we'll get zero current at an applied voltage. And we'll call that our open circuit voltage, okay? 

[Slide 4] So now that we know what kind of curves we're expected to see, let's think about what actually controls these processes. So we have our exciton diffusion. Okay, remember, I said our exciton is about an electron-hole pair. It lives for about 300 picoseconds, which means that it can explore space for about 10 nanometers before it recombines, and it goes back to just relatively stable electron in its ground state. The Exciton is neutral. It has no impact whether we have an electric field or not. It's just going to move around in space, but it requires about 300 million-electron volts of energy to break it. So our offset between what the top of our LUMO band here is, and on top of the LUMO band in our blue phase needs to be about 0.3ev or greater. And if we have that we can separate our electron from our hole. So we're going to diffuse around, we're going to see this and if we do that, then we get our free charge carriers. 

[Slide 5] If we're able to get that free charge carrier to that donor acceptor interface, then we're able to see that charge transfer will occur, okay? The trick here is now we want to have a structure that has a whole lot of interface, because if we have a lot of interface, there's a high likelihood of having this exciton separation occur, right? We only get the material to diffuse for 10 nanometers. Thus, we want to have, basically, interface within 20 nanometers, so then I can go 10 nanometers one way or 10 nanometers the other way, in order to get high exciton separation. If that occurs then we 'll go ahead and be able to sweep our charge out. But the problem there is we can't have any islands, we have to have continuous domains so our charge can sweep all the way through, okay. So how this thing phase separates is remarkably important in terms of how our materials perform. With that, I thank you for your attention. We'll talk next time about how our mixing theory goes ahead and allows us to start interpreting, how we think about, if we mix one red material with one blue material, how we can think about going ahead and predicting what that final nanostructure will be, and trying to get to some idealized version of the active layer that we drew on slide 2. With that, I thank you for your attention and