nanoHUB U Organic Electronic Devices/Lecture 4.5: Emerging Trends in OPV Devices
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[Slide 1] Hello. And welcome back to the nanoHUB-U course, Organic Electronic Devices. Over the past series of lectures, we've talked about how organic photovoltaic devices are made, how they're fabricated, how they're tested, and the physics behind the device operation. We've talked about how interlayers can be inserted into these polymer photovoltaics. And today, we'd like to conclude talking about organic photovoltaic devices with emerging trends in the field. So we'll talk about new and interesting device structures. And how these device structures can lead to high performance organic photovoltaic devices. And in fact what we'll see is that the state of the art inorganic photovoltaics is now approaching a level of that seen in inorganic system. So we're going to be in that 10 to 15 percent efficient power conversion efficiency window, So let's get started. 

[Slide 2]So today, the first thing we'll talk about is the concept of an inverted organic photovoltaic device. And I'll explain what that means in just a second here. We'll go on to justify why one would want to use an inverted device structure, and what are some of the benefits both in terms of the active layer nanostructure and in terms of device stability. And we'll finally conclude with something called tandem or multi-junction organic photovoltaic devices. In particular, we'll have emphasis on polymer-based tandem devices. So by the conclusion of today's lecture, what we'll hope to be able to do is to define what I mean when I say an inverted organic electronic device and describe how that's distinctly different from a regular architecture. We'll discuss the potential advantages and name a few of them in very specific detail, and explain what we mean by tandem or multi-junction solar cell or multiple-junction solar cell. And why one would want to use these, and how we can design molecules specifically for tandem three junction or four junction organic photovoltaic devices. 

[Slide 3] So we know that in our standard our regular or our normal-- we use all those terms interchangeably in the device architecture. We're going to start with some kind of electrode, ITO. This is going to serve as our anode. We'll have some kind of hole transport layer like we talked about last time. A lot of times that will be Pedot PSS like we talked about. We'll then have our semiconductor blend, and we'll cap it off with some kind of reflective metal. Occasionally, we'll put a cathodic modifier in there as well. But in general, we may or may not use that. But in this case, the light is going to come in through the ITO because it's on our transparent conductor, and this is going to serve as our anode. So that's a regular device structure, and that's one we've been using most frequently in this course. When we talk about an inverted structure. The thing that will change is the polarity of the electrodes. In other words, we're going to make our transparent contact, our ITO contact. That's now going to become the cathode. So we're going to pick new modifying layers, such that electrons wish to flow to the ITO contact and holes wish to flow to the other contact, the reflective metal contact. So in a typical example here from 2006, we see that we have ITO, and then this TIOx layer, you know, we recalled from last time that generally TiOx is an electron transport layer. Well then put in our semiconducting blend, have our hole transport layer of Pedot PSS, and we'll put some reflective metal as a top contact. Often, that will be either silver or gold, based on their work functions relative to the transport levels of the organic semiconductors. So now you can see why this is now called an inverted structure because if I look at it in the exact same way, the light comes in from the same side but now our anode and our cathode are reversed. So that's where the term inverted comes from. 

[Slide 4] And what we'll see, that these devices has been around from 2006 but in one of the earliest examples, we can see whether the devices in the dark, and here we have current density in a log plot versus voltage. For both the normal and the inverted structure, we see almost the exact same behavior. And why is that? That's because the diode in organic photovoltaic cells is really controlled by that semiconducting active layer blend. In this case, it was P3HT and PCBM. So no matter what the metal contacts are, the device operates the same way because it's inherently controlled by the materials that absorb the light and transport the charge. Once we illuminate the device here on the left-hand plot, now we have current density versus voltage but current density is on a linear scale. We'll see that very similarly here, the dark is the inverted device structure and the open circles are the regular structure, we'll see that the VOC remains the same. The fill factor improved slightly for the inverted device. And the short circuit current is lost a little bit in the inverted solar cell. And that's very typical. In general, the inverted solar cells will have slightly lower short circuit current density than the regular solar cells. But you will, in general, see a trend in the fill factor. And you can see that what that turns out to mean is that that the power conversion efficiency, the PCE of the inverted device is slightly lower than that of the regular device. So then we have this inverted structure, it's-- I'm telling you it is emerging trend, why would we ever want to use it if the power conversion efficiency is lower? We'll there's two primary advantages. And the first has to do with the microstructure and nanostructure of the semiconducting active layer. And what that has to do with it is that a lot of hole-transporting polymers, especially when they're mixed with fullerene derivatives which is commonly happening in the polymer solar cells. So if we have P3HT or some of our low-band gap polymers, they tend to like to face separate to the upper surface. So the air liquid surface during coding as opposed to the organic substrate surface during coding. So inherently, the top contact is going to be rich in hole-transporting material. If that's the case, we'd like to put that hole-transporting semiconductor next to our hole-transporting electrode. So that's advantage one. And that's why our fill factor increases. 

[]Second advantage has to do with the idea that the top contacts we use in regular structure solar cells are generally something like aluminum. And you know very well that aluminum oxidizes relatively easily in regular conditions, in ambient conditions exposed to oxygen and water vapor from the air. But you also note that silver and gold don't oxidize that easily, right? So that tells us that over the course of the device's lifetime, we would anticipate that inverted solar cells would actually have a longer device stability without encapsulation than the regular structure would have. And that turns out to be true and I'll show you that in just a second. So there's this idea that both the inverted structures, nanoscale morphology within the device is more efficient. And the idea that we'll have longer device lifetimes as well is very important. 

[Slide 5] And here we see-- this is a cartoon representation. But here is exactly what we're talking about in terms of nanostructure. Here we see that these yellow balls here, those represent a fullerene derivative, and the blue lines represent the polymer material, that's why they blue squiggles again. But you'll see what happens in these inverted devices, or really, any kind of an active layer device, is that the polymer tends to face separate to that top electrode. So if that top electrode is hole-transporting. Then that's where we want to be at, right? Because generally our polymers are P-type material and generally the fullerenes are n-type material. And what we see is that we can actually verify this with experiments. So here is some work out of 2009, and what we're plotting here is the ratio of the weight fraction of PCBM, divided by the weight fraction of P3HT. So our n-type material divided by our P-type material. And that's in the upper plot. The lower plot here is a different derivative of PCBM divided by our P3HT. And what you'll see is no matter how we cast it. Whether we do with a fast-grow technique, we've fast-grown with an anneal. We slow-grow it or that we slow-grow it with anneal, the PCBM to P3HT ratio, PCBM is always more prevalent on the bottom contact. Bottom contact versus top contact. Bottom contact, top contact. Bottom contact, top contact. Bottom contact, top contact. The bottom contact always has more PCBM in it, so it just makes sense that we'd want our bottom contact to be our electron-collecting material. Thus, we want to have this inverted device structure. 

[Slide 6] The other thing we'll look about is device lifetime. So here's another P3HT PCBM inverted solar cell, this is from 2009. And we can see that what happens is if we just take the device, and we leave it unannealed in our inverted structure, that we'll see some increase in VOC, some increase in short circuit current, and this leads to increase in efficiency. But here what we're seeing is that as we expose it to time, so that X axis is time exposed to ambient conditions, we see that our efficiency on our inverted devices increases with time. And if we take another strategy, it will increase with time, and then in our third strategy, it will increase in a different manner with time. But the big takeaway point here is that now we're seeing our solar cell work better as it's exposed to ambient conditions. And eventually reaches a value of our regular device structure when it's freshly made. But our freshly-- but our regular device structure efficiency will decrease with time. So now were seeing a completely different paradigm where the exposure to oxygen is a good thing for these devices. And if we want to think about coating a large area of solar cells with devices, we really need this oxygen exposure to not hurt the device in practical applications. Even if we sealed our organic solar cells with quite a bit of insulation to prevent any water or oxygen coming in so we put a nice barrier for those materials down, first of all, the more we have to put down, the larger the likelihood of it scattering light and then not being absorbed by our solar cell. The second problem is even if we put a lot of that down, there's going to be eventually be some kind of hole generated in that protective barrier whether it be from nature, the elements, some kind if wildlife looking at our solar cells, and we'd really want our solar cells to work well, in that time in between when we can fix the device. So right now, we see that this oxygen exposure is actually a good thing, for these inverted device structures. Having said all that, we can also talk about what we'll call a tandem cell. And really the reason we talk about tandem cells after inverted device structures, a lot of tandem cells are based on this inverted device structure. 

[Slide 7] And the reason we'd like to go to inverted solar cells and this tandem device structure is the idea that solar cells need to absorb light, and the more light you can absorb, the better your solar cell will perform. And there's two ways you can do that. The first way is to create new molecules, that absorb over the entire visible and infrared spectrum of light. But as we've seen, a lot of the molecules we make are very tailored to a certain region of the electromagnetic spectrum. You can make wide absorbing materials over a lot of different wavelengths but it's difficult and are hard to come by making them absorb a lot of light and transport charge and mix well with a fullerene derivative, becomes very challenging. However, there's a whole category of polymers, some that absorb well in the blue, some that absorb well in the red, some that absorb well in the infrared. So the idea behind this is to tailor a polymer to a specific part of the spectrum and we have a lot of those. And then just use these as individual sub-cells. And then stack those sub-cells on top of one another. So in this way, the first cell absorbs the blue. The cell above it absorbs the red. And the cell above it absorbs the infrared. If that's the case, then all the light will be absorbed and we can get more light out without having to go through a whole new molecular design process. We can just use polymers that we already know about. And this has been done. And in one of the most efficient example that's been done in this geometry. So we'll start with our ITO contact as we always do. We'll add zinc oxide and this will be our electron transport layer. We'll then have a polymer blend of P3HT, so this, remember band gap is around of 1.9 EV and a fullerene derivative. We'll then have our hole transport layer. And that's sub-cell one. And it will start sub-cell two and that will have its electron transport layer. It's blend of polymer and fullerene. Now, we're going to have a low band gap polymer. Then we're going to have our hole transport layer moly oxide, and our hole collecting electrode silver. And that's between that zinc oxide and that moly oxide silver layer that's our sub-cell two. And the idea behind this is, is that we're going to have two different absorption spectrums, right? We're going to have one where we're characteristic of P3HT and that's this first big hump here. So that's sub-cell one's absorption spectrum. And you can see it dies off right around 650 which we've seen for P3HT before. And then we'll also have sub-cell two. And it will absorb all the way out to roughly 900. So you can see that between 300 and 900, almost the entirety of the optical electronic spectrum is going to be absorbed. And that means that we're going to capture more of those photons. And how does that translate to higher efficiencies now is really the question. 

[Slide 8] Well the great news is that this tandem cell I just showed you was one of the highest-performing solar cells, at least in terms of polymer solar cells ever recorded. And it was roughly 10.6 percent efficient. This is one of the first times that we saw polymer solar cells get above the 10 percent efficient mark. This is work coming out of Yang Yang's laboratory at UCLA. But there's a few things that we need to point out, right? So when we look at this, we see that the first sub-cell, P3HT and this fullerene derivative ICBA. Hence, an open circuit voltage is around 0.84, a short circuit current around 10.3 milliamps per centimeter squared, a really good fill factor of around 71. So we get a power conversion efficiency around 6. And this is pretty normal. When we look at the individual sub-cell of the low band gap polymer and PCBM. We see that we lose a little bit in VOC but we increase the short circuit current density quite a bit. Why does that happen? That's because now we have a lower band gap polymer. There's more photons out there in the infrared. So it can absorb more of the photons, so it increases the short circuit current density relative if P3HT was a light-absorbent material. You'll note here that there's two different fullerene derivatives. One of them is C61 and one of them is C60 or 71, I'm sorry C61 and C71. And thus the C71 has a higher short circuit current density. And that's because C71 absorbs more light than C61. So you get that added benefit as well. So those are the individual cells. Now what happens if we stack them together? Well when we stack them together, we see something very interesting happen. And what we see very interesting to happen is that we don't see a whole lot of return on our JSC but we see a huge return on our VOC and why is that? 

[Slide 9] So, under the assumption that the fill factor of the two devices are relatively similar. So if I stacked up my P3HT, ICBA with my C61, PC61 BM and my low band gap polymer, we'll see that the fill factors are roughly 71 and roughly 66 or 65. So those are roughly the same. But under the assumption that the fill factors are roughly the same between the two cells, then the short circuit current density of the tandem device when are hooked together, will be limited by the minimum short circuit current density that we see in one of the two cells. So that just tells me, that if I have a charged neutral cell that I can't take out more holes from one of the cells than I can take out electrons from another one of the cells, right? The currents have to be balanced. And we see that that's the case here, right? So when we have these guys stacked, the maximum we can expect would be 17.8, but our P3HT cells are limited at 10.3. So then when we make the tandem cells, we see that we can't get above 10.3 but we're pretty close to the JSC of the P3HT cell. So that's now a limitation. Why would we ever want to go to this tandem device geometry? Well it turns out that the open circuit voltage, of the tandem cell will be equal to, again in this fill factor approximation limit that the VOC of the tandem cell would be equal to the VOC of the first cell plus the VOC of the second cell. So that's just rewriting these equations here, that means that our efficiency of our tandem cell will just be the short circuit current density of the minimum times this fill factor that's roughly the same for the two times VOC one plus VOC two all over this PN. And that's exactly what we see here, right, 0.84 plus 0.7 will be 1.54, here we get 1.53, all right? And this should be 1.52. Here we get 1.51. So the fill factors there, almost the same and this boost up are power conversion efficiency from 6 or 7 percent up above 10 percent, because we have this added effect associated with the VOC values. And this is really the true power of the tandem solar cells, this idea of VOC increasing as long as you can have a decent JSC throughout your device. So it doesn't had to just be a tandem cell, a two device structure. It can be a multi-junction structure. 

[Slide 10] And in fact, the same group, Yang Yang's group at UCLA has taken this to the next level, right? And now we have three different polymers, right? We have P3HT. We have a low band gap polymer and then we have an even lower band gap polymer. So here the band gap is 1.4, 1.58 and 1.9. If you now stack these three complementary polymers up against one another, what we'll see is that first of all, we have a huge VOC. We're out past 2 volts for our VOC. And when we make this triple junction solar cell, we see that our VOC is roughly 2.3. Our short circuit current density is limited only 7.6, right? That's really probably most limited by that P3HT layer. Our fill factor is OK, but this huge VOC leads to a power conversion efficiency of 11.6. That's over multiple devices, that's the average. So what we can see there is that by increasing this VOC, by keep stacking up the materials, stacking up these devices, we can get a huge payback on our power conversion efficiency, assuming that our short circuit current density doesn't fall off too quickly. And this is one of the highest reported power conversion efficiencies for polymer solar cells today, and really starts to push the limit of where these materials and devices can be used relative to their inorganic counterparts which are roughly somewhere on the order of 15 to 20 percent for your standard silicon solar cell. 

[Slide 11] So with that, what I hope I've shown you over the last few minutes within the conclusion of organic-- photovoltaic device section is the difference between a regular or a normal device architecture in a polymer solar cell, and thus, a concept of the inverted architecture. After we understand the inverted architecture, and what implications that has, we can move it onto this multiple junction, this tandem solar cell kind of geometry and stack one sub-cell on top of one another. And by stacking these sub-cells, as long as we can keep the short circuit current density high, we can really boost up the VOC of the device and that's the overall efficiency. In that case, we can approach some of the highest-performing polymer solar cells ever recorded. So with that, I'll mention that next time, we'll shift our attention now away from taking incoming light and getting electricity out to input an electricity and getting light out. And those are called organic light-emitting devices. Those are actually the highest commercial breakthrough success for organic electronic materials to date. But we'll talk about the implications of that here on the next installment of the course. Well with that, I thank you for your attention. And I look forward to seeing you next time on the nanoHUB-U course, Organic Electronic Devices.